SCIENTIFIC AND HAZARDS ASSESSMENT
OF THE
SOUFRIERE HILLS VOLCANO
MONTSERRAT
MONTSERRAT VOLCANO OBSERVATORY
12 MARCH 1999
ANNEX ON HEALTH RISK OF VOLCANIC ASH
CONTRIBUTED BY DR P J BAXTER
SCIENTIFIC AND HAZARDS ASSESSMENT OF THE SOUFRIERE HILLS
VOLCANO, MONTSERRAT: REPORT OF A MEETING HELD IN TRINIDAD, 24-
25 FEBRUARY 1999
Executive Summary
A Growth of the lava dome at the Soufriere Hills Volcano ceased in early March 1998
when the magmatic eruption stopped. There has been residual activity and instability
over the last year, but no sign of the magmatic eruption starting again. The post-
eruptive residual volcanic activity has included occasional collapse of the lava dome
to generate pyroclastic flows, vigorous venting of ash and gas sometimes
accompanied by small explosions and the generation of small but mobile pyroclastic
flows, occasional swarms of volcano-tectonic earthquakes, and continued ground
deformation on the eastern flanks of the volcano. There is also a large amount of
loose volcanic debris and ash on the slopes of the volcano which has been
remobilised to form volcanic mudflows during periods of heavy rain.
B The residual activity over the last year is thought to have been caused by the
continued release and ascent of volcanic gases from the source of the magma deeper
in the earth. The rise of pressurised gas makes the dome unstable, is associated with
the continued low level earthquake activity and is responsible for minor explosions,
some pyroclastic flows and the ash venting. Such activity is known at other
volcanoes as an eruption is ending and may continue for many years. However, the
scale and magnitude of this activity and the associated hazards are expected to
diminish with time.
C The residual activity together with the occurrence of volcanic mudflows during
intense rainfall and the substantial amounts of volcanic ash deposits will continue to
pose a number of hazards in southern Montserrat.
D The Tar River valley, Plymouth and the north east flanks of the volcano are
particularly dangerous. Further collapses of the lava dome and pyroclastic flows
generated during periods of ash venting threaten major valleys on the flanks of the
volcano. However, the reduction in dome volume since 3 July 1998 and the change
in its morphology and configuration within the crater, led the meeting to conclude
that the potential for big gravitational collapses (and hence large pyroclastic flows) is
now much reduced. The collapses which could be generated during periods of ash
venting are most likely to be confined to the Tar River valley and to Gage‟s Valley-
Fort Ghaut, although the possibility of avalanches in other directions cannot be
completely ruled out. Because it is thought unlikely that pyroclastic flows will
extend as far as the largest pyroclastic flows of the eruption, the areas already
inundated by flows define a logical zone of high hazard (fig 1). Within this zone
pyroclastic flows, which can be generated with no warning, could be lethal.
E It is now a year since magmatic activity stopped and so some of the areas in the
current exclusion zone are no longer considered under direct threat, provided that a
magmatic eruption does not restart. However, many of these areas have substantial
amounts of fine volcanic ash and further minor ash falls are likely. The health risks
to potential residents from volcanic ash in these areas should be subject to expert
medical assessment (see annex provided by Dr P J Baxter).
F The meeting reconsidered the issue of the likelihood of no further magmatic eruption
in the next 6 months, (previously assessed at about 95% probability in the July 1998
study) and, on the strength of the evidence gathered in the intervening period,
concluded that there was no significant new information or basis for revising this
estimate.
G The results of the July 1998 elicitation on the chances of magmatic activity resuming
over the longer term were also reviewed, and no changes were proposed to the
estimates provided previously (i.e. 15% probability of reactivation within 5 years;
25% probability within10 to 30 years).
H The present assessment, taking account of the possibility of both magmatic and non-
magmatic activity, indicates a slight reduction in overall risk compared with the July
1998 appraisal. The results of the latter indicated that the risk levels in the populated
areas of Montserrat were no worse than those to which populations on other
Caribbean islands with dormant volcanoes have been historically exposed. The risks
in the currently populated areas of Montserrat remain LOW to NEGLIGIBLE on the
CMO‟s Risk Scale, with IRPA values (annualised individual risk exposure) ranging
from about 1 in 2,500 in parts of Area 4 marginal to the Belham valley, to less than 1
in 200,000 for Area 1. However, the increase in population numbers on Montserrat
in recent months suggests that the societal risk of multiple casualties in the event of
renewed magmatic eruption may be slightly increased.
I The threat to marginal areas of the current exclusion zone along the north side of the
Belham valley has come down as a consequence of the reduction, since July 1998, of
the volume of material comprising the dome edifice in the crater and the change in
its morphology. Under the present circumstances of no magmatic activity, the
chances of major flows or surges affecting even the margins of these occupied areas
are lessened, and the risk to individual life and limb is now assessed to be MINIMAL
(IRPA ~ 1 in 200,000) for people residing on the north side of the Belham valley.
J A risk analysis has been undertaken for partial re-occupation of areas south of the
Belham River (Iles Bay; Cork Hill; St George‟s Hill; Richmond Hill and Fox‟s Bay)
on the basis that, for the near future, only limited numbers of returning residents
would be involved in each case. Under present conditions at the volcano (i.e. no
magmatic activity), the annualised individual risk of fatality for residents in one of
these areas from dome collapse is estimated to be of order 1:50,000, or VERY LOW
on the CMO‟s scale. However, if magmatic activity were renewed, the risk exposure
for individuals would immediately become MODERATE (IRPA ~ 1 in 250) requiring
an immediate decision regarding re-evacuation.
K St. George‟s Hill has been identified as a possible vantage point for tourists and other
visitors to view the volcano and the effects on Plymouth: given the relatively short
timescale for which they would be present there, and their presumed mobility by
taxi, the annualised individual risk of fatality must be considerably lower than that of
any permanent resident (above reported to be about 1:50,000 or VERY LOW on the
CMO‟s scale), while non-magmatic conditions exist.
L The risk for workers undertaking remedial work in and around the Bramble Airport
area under present non-magmatic conditions has been assessed from a model based
on 25 workers operating 8 hours per day, 7 days a week, for a range of scenarios.
The two end members of this range are: 1] If measures are taken to monitor the
volcano very closely and there is assumed to be a 90% chance of successfully
detecting the start of a dome collapse and giving immediate warning, it is estimated
that the individual fatality risk exposure (IRPA) for alerted workers at or near
Bramble airport would be about 1:16,000 p.a. 2] For the worst case scenario in
which the probability of a big collapse is treated very conservatively AND no
effective warning of sudden collapse is given, the IRPA for workers may reach 1:650
p.a. (i.e. comparable to offshore oil workers in the North Sea). A best estimate risk
value would fall between these extremes, but requires further scientific input.
M For flight operations at Bramble airport, a revised risk analysis in which the
possibility of magmatic reactivation occurring sometime within the next 30 years is
taken into account (not just the present non-magmatic conditions) has been
completed. This indicates that if ground activities were implemented well away
from the airstrip (as discussed in more detail in the September 1998 report), the
reduced danger of large dome collapses produces a moderate drop in overall risk
exposure: over the long term, the risk to a group of people going onto the airstrip for
a very short time to board or leave a flight, or to perform ground support duties, is
estimated to be fractionally lower than that for permanent residents living on the
margins of Area 4, for example. For the individual going to the airstrip for a short
time only, the estimated IRPA is about 1 in 7000, which exposure is similar to
someone living full-time in Area 3 (Woodlands), for instance.
N Heat dissipation in a recently extruded dome is very slow and, given the relatively
large volume of material involved in the 1995-98 eruption, it is considered likely
that, if further major collapses do not occur, many decades (or even a century or two)
will have to elapse before the heat energy contained within the dome is sufficiently
reduced to effectively remove all hazard potential from that source.
O Volcanic mudflows are generated only during periods of intense rainfall and
dangerous situations can be easily identified. The situation at the Belham River
crossing is no different to many parts of the world where periods of intense rainfall
can cause mudflows and floods and populations routinely cope with the situation. In
terms of risk to individuals from dome collapse pyroclastic flows and surges when
crossing the river, the limited exposure time and small numbers involved at any one
time suggest the risk levels should be no worse than for residents living continuously
in a re-occupied Cork Hill, for instance.
P Continued careful monitoring of the volcano is required to help minimise exposure
to residual hazards and to ensure identification of any signs of resumption of
magmatic eruption.
Q MVO is confident that renewed magma ascent can be detected and that escalation to
dangerous levels of activity can be recognised. In some circumstances, escalation
might take place in a very short period of time (a matter of hours). However, any
warning helps to significantly reduce the risk exposure of the population below that
estimated for an unprepared society, and risks will be continuously reassessed by the
MVO.
Introduction
1. A meeting of MVO scientists took place at the Seismic Research Unit of the University
of the West Indies, Trinidad, on 24, 25 and 26 February 1999 for the purpose of
evaluation of the status of the Soufriere Hills Volcano. The meeting was commissioned
by HMG to take place approximately six months after the previous evaluation on July
1998.
2. The assessment of July 1998 recorded that the dome of the Soufriere Hills Volcano had
stopped growing in early March 1998. Since that time there is no evidence of any
further magma ascent from depth to the Earth's surface. Thus the primary magmatic
eruption has stopped. However, there is residual volcanic activity which still causes
hazards in southern Montserrat.
3. This report evaluates the residual volcanic activity in the period following magmatic
eruption. The activity since July 1998 is summarised, the potential hazards associated
with this activity are identified and the implications of this assessment for the
vulnerability of different areas on Montserrat are given. The hazards to specific places
are considered in the context of this analysis as requested by DFID. An update of the
formal risk analysis for Montserrat is presented.
4. In the following report it is very important for the reader and user to understand the
terminology that scientists use to describe the state of a volcano. From a scientific point
of view an eruption can be said to have stopped when magma (molten rock) is no longer
rising to the earth's surface. However, there can still be activity at the surface even when
magma is no longer rising and this activity can be impressive and hazardous. In this
report such post-eruption phenomena are described as residual volcanic activity.
Following a major volcanic eruption like the Soufriere Hills in 1995-1998 the residual
volcanic activity can last for a considerable period of time (often months or years) before
gradually returning to the background low level activity of a dormant volcano.
Activity July 1998 to February 1999
5. The residual post-eruption activity has included several collapses of the andesite lava
dome accompanied by occasional minor explosions, vigorous ash venting and gas
release. The collapse of 3 July 1998 formed a major pyroclastic flow down the Tar
River valley. On 12 November a deep canyon-like depression on the southern side of
the dome developed. The canyon was enlarged by further collapses, which extended the
deep canyon to the WNW and made the depression open both to the west and east.
About 30% of the dome has been removed. The new canyon cut the dome in two and
the precipitous walls continue to be very unstable.
6. The July 3 1998 collapse revealed vents on the floor of the new depression from which
ash, steam and volcanic gases discharged. The vents are aligned along approximately
east-west fractures. There have been several periods of strong ash venting and gas
discharge over the last year. Some episodes were initiated by explosions and collapses
of parts of the dome to form pyroclastic flows. Some of the more vigorous explosions
and ash venting has formed fine-grained but highly mobile pyroclastic flows which have
flowed down the flanks of the volcano. On November 12 a large collapse of the dome
caused pyroclastic flows to go down the Gages valley into Plymouth as well as down the
Tar River and White River. Temperatures of over 350ºC were measured in the deposits,
demonstrating that the interior of the volcano is still very hot and capable of producing
very dangerous flows with no warning.
7. Seismic activity has consisted largely of volcano-tectonic earthquakes and some signals
from rockfalls and pyroclastic flows. There are very few hybrid earthquakes, which
were the dominant seismicity during active dome growth. Volcano-tectonic earthquake
swarms accompany the major episodes of collapse and ash venting. Swarms can both
precede and follow major dome collapses and explosions that initiate ash venting
periods. The earthquakes have been located at depths of 1 to 3 km below sea level. The
activity waxes and wanes over periods of a few weeks with very low activity and periods
of enhanced activity such as October through to December 1998.
8. Ground deformation may still be occurring on the eastern flanks of the volcano. A new
fracture has also been observed on a road on the eastern flanks. The continued ground
deformation on the eastern flanks should be watched carefully for any signs of larger
scale instability.
9. The fluxes of the volcanic gases sulphur dioxide and chlorine have been monitored.
Although the rise of magma has stopped, the SO2 flux has remained high with typical
values between 500 and 1500 tons per day. The gas flux fluctuates substantially partly
reflecting uncertainties in the measurements and partly reflecting real fluctuation. The
periods of high flux correlate with the periods of vigorous ash venting. The ratio of
sulphur to chlorine gases has increased by more than an order of magnitude in
comparison with measurements made in 1996.
10. The scientific group agreed that the following explanation is consistent with the
observations of these residual activities. The considerable gas fluxes and high ratio of
sulphur to chlorine imply a relatively deep source for the volcanic gas. The eruption is
thought to have been caused originally by a different kind of magma known as basalt
being intruded at depths greater than about 5 or 6 km and pushing out the andesite
magma. The basalt is now thought to be cooling and solidifying at depth and releasing
gases which rise along deep fractures to the surface. These gases increase the pressure
in the interior of the volcano which is released in explosions and vigorous venting of ash
and gas. The fractures feeding the pressurised gases to the surface are oriented east-west
and have developed underneath the dome and destabilised it.
11. Residual activity, particularly continued gas release and earthquakes related to post-
eruption adjustments of a disturbed volcano, is typical after a major volcanic eruption.
Such activity can continue for many months to years. However in general such activity
diminishes with time as the volcanic system settles. In the case of the Soufriere Hills the
residual activity has already lasted a year and the gas fluxes have not yet shown the
decline that has been observed on other volcanoes.
12. The magmatic eruption emplaced about 100 million cubic metres of loose ash and
volcanic debris on the flanks of the volcano. During the hurricane season in 1998 there
were intense periods of rainfall, in particular during the passage of Hurricane Georges.
Volcanic mudflows were formed in the hurricane and discharged large amounts of debris
into the lower reaches of the major river valleys. In particular mudflow debris inundated
Plymouth further burying and damaging buildings in the centre of town, buried the
Belham river bridge and spread debris in the vicinity of Bramble airport. In the case of
the Belham river much of the debris in the catchment area has already been swept to the
lower reaches of the valley and into the sea at Old Road Bay. However, there still
remain large amounts of loose material on the upper flanks and valleys of other
catchments.
13. Despite heavy rainfall over the last several months thick ash fall deposits still drape
much of the western flanks of the volcano with the ash thickening substantially from
Cork Hill to Plymouth.
Residual Hazards
14. Pyroclastic flows remain a major hazard. The dome is precipitous and unstable,
particularly along the newly formed east-west canyon. The interior of the dome is also
still very hot and is likely to remain so, potentially for decades. The modifications in the
dome and reduction in its size have changed the degree of hazard around the volcano
since the July 1998 assessment. The dome has been consistently collapsing into the new
east-west canyon, making collapses into this feature the favoured area for future
instability. Pyroclastic flows therefore can be expected down the Tar River and Gages
valley with the Plymouth area being very vulnerable. The dome has reduced in size so
the capacity to cause very large pyroclastic flows is decreasing. For example the
probability of a collapse three times larger than the reference collapse of 21 September
1997 has reduced greatly and is considered negligible in some directions, thus reducing
the hazard. There remains a large mass of dome material on the northern side over the
valleys to the north-east towards Bramble airport and towards the Belham valley.
However, this material has remained stable for over a year. With time it is expected that
continued cooling, consolidation, degradation and collapse of the dome will reduce the
hazard further.
15. The most vigorous ash venting episodes are initiated by explosions and can form another
type of pyroclastic flow. These flows are mobile and hot and can travel down several
valleys simultaneously. As is the case for pyroclastic flows from dome collapse these
flows can occur with no warning. Particular attention is drawn to the vulnerability of
Plymouth.
16. The boundaries of the exclusion zone have been considered during this assessment in the
light of the reduction in the hazard now that a year has passed since magmatic activity
stopped. As long as magmatic activity does not start again then the scientific consensus
is that pyroclastic flows are very unlikely to extend beyond the largest flows that
occurred during the eruption. Thus an objective and conservative boundary for a zone of
high hazard can be defined as the limit of areas already inundated by pyroclastic flows
and surges. The accompanying map (Figure 1) shows the boundary of the zone based on
this criterion with the boundary defined as 100 m beyond the extent of deposits in areas
of gentle relief and 50 m above the limit of surge clouds in areas of rugged relief. The
group recommend that the area defined by this boundary is used as a guide for
identifying a revised exclusion zone.
17. Volcanic mudflows will continue to occur during periods of intense rain. Mudflows
should not be a concern provided the susceptible valleys are avoided during periods of
high rainfall. The situation at the Belham River Crossing is no different to many other
parts of the world where high rainfall causes flash floods and mudflows. The
populations in countries like Iceland and Chile routinely deal with such situations.
18. It is likely that minor ash falls will occur from time to time as further ash venting
episodes are expected. Due to the prevailing wind directions at Montserrat the
likelihood of ashfall increases significantly as the western flanks of the volcano are
approached. There are still large amounts of fallen ash in some of those areas south of
the Belham valley which are now regarded as at low risk from other volcanic hazards.
This report refers to the July 1998 report where the issues of the health effects of the ash
are considered and the requirements for clean-up and monitoring are stated. A brief
annex to the current report has been contributed by Dr P J Baxter
Probabilistic assessment of current volcanic hazards
19. The objective of the present risk assessment is to update earlier calculations of the
potential loss of life from direct volcanic hazards and make adjustments which arise
from changes in perceived likelihoods of occurrence for the various identified threats.
The timeframe for the assessment exercise covers the next six months, as in previous
assessments. The outlook for general eruptive activity levels over longer defined future
time periods (i.e. 5 years; 30 years) remains unchanged from the last assessment in July
1998. (The present analysis does not include estimates of numbers of injured persons -
which, for emergency planning purposes, might be inferred from fatality numbers by
medical specialists and volcano emergency specialists - or long-term health risks from
ash).
20. For the present risk assessment, some modifications have been made to previous
versions of the map defining areas of population clusters (Figure 2). The boundaries of
the Population Area 4 have now been redrawn to enclose only Old Towne, Salem and
Frith, where formerly it extended across the Belham River to encompass the coastal strip
down the western side of Garibaldi Hill (in previous assessments, there were assumed to
be no residents on the south side of the river, anyway). Five new (un-numbered)
subdivisions are introduced on Figure 2: Iles Bay; Cork Hill and Devlins; St George‟s
Hill and Weekes; Richmond Hill, and Fox‟s Bay, to allow estimation of risk exposure in
these areas if limited re-occupation is contemplated. A separate sub-area for Bramble
Airport is also included.
21. In order to take account in the risk calculations of the changing population numbers on
Montserrat, the most recent population total (about 4,400 persons, according to the
Statistics Department of the GoM in February 1999) has been used to provide updated
estimates of populations in the four currently-occupied geographical zones 1 - 4 (see
also Figure 2), with the corresponding partitioning for risk assessment modelling
purposes as indicated on Table 1. These approximate figures reflect the slight increase
in overall numbers when compared with those used in the earlier assessments, but
individual area totals have no real claim to absolute accuracy.
Assessment: December 1997 April 1998 July 1998 February 1999
Popn. Zone 1 1088 1355 1403 1700
Popn. Zone 2 2248 1199 1286 1600
Popn. Zone 3 619 493 544 750
Popn. Zone 4 134 92 105 350
Totals 4088 3139 3338 4400
Table 1: Numbers of persons in each Population Zone used for risk analysis calculations,
compared with values used in the three previous assessments.
22. It is required to consider risk exposure levels if partial re-occupation of some areas south
of the Belham River were allowed in the near future. For risk modelling purposes,
limited numbers of returning residents are assumed to be involved in each case, with
upper limit population numbers for the time being of 50; 200; 100; 100; 50 into Iles Bay;
Cork Hill; St George‟s Hill; Richmond Hill and Fox‟s Bay, respectively. These numbers
are arbitrarily assumed to be drawn mainly from Population Zones 1 and 2, and from
people returning to Montserrat from abroad, and are intended for guidance only.
23. The hazard and risk assessment approach and methodology follows that described in
detail in the December 1997 report, subsequently validated by the UK Government‟s
Chief Scientific Adviser‟s consultative group. The approach has been to assemble all
plausible volcanic scenarios for the Soufriere Hills volcano on a logic-tree framework
(Figure 3), with branching to accommodate a hierarchy of related hazards. A Monte
Carlo technique is then used to re-sample the probability distributions for each branch of
the logic-tree, for the given population distribution, to determine the corresponding
levels of risk exposure in different population areas, arising from such hazards. Within
the Monte Carlo simulations, statistical distributions have been used, wherever
appropriate, to represent the spread of scientific uncertainty associated with individual
factors, estimates of frequency of occurrence, or other inputs.
24. These risk analyses are based on the current scientific assessment of potential volcanic
hazards. If volcanic activity changes, a significant event occurs, or important new
information arises (e.g. from monitoring), then the assessed probabilities of occurrence
of hazardous events may also change. Regular updates of risk analyses therefore
contribute an essential element to the on-going process of hazard management in
Montserrat.
25. The meeting considered all the scientific evidence which had been gathered throughout
the crisis and, in particular, that which had accumulated since the last meeting in July
1998. A review of the relative probabilities of important scenarios was undertaken, and
the principal changes are summarized on the event-tree of Figure 3. Because of the
requirement to estimate risk in specific areas south of the Belham River, such as
Richmond Hill and Fox‟s Bay, a pair of branches have been added to the event tree
(Figure 3), comprising representative instances of two magnitudes of dome collapse out
of the Gage‟s sector of the volcano.
26. On this occasion, the changes to the probability tree comprise reductions to the assessed
probabilities of dome collapses in key directions, reflecting the reduction in dome
volume since 3 July 1998 and the change in its morphology and configuration within the
crater. The total volume of dome material in the crater, as at February 1999, was
reported by MVO to be 77 x 106 m3, of which two-thirds is contained in three heaps on
the north side of the crater, the remainder sitting over the former Galway‟s wall area.
The triggering of dome collapses by external events, such as a major nearby regional
earthquake, was discussed by the meeting, and this was identified as the most likely
cause of significant avalanching from the stagnant dome remnants. However, it has to
be recognised also that considerable uncertainty surrounds the setting off of dynamic
avalanching processes from big domes containing high heat energy contents and high,
possibly non-uniform internal pressurisation. Initiation of failure may not necessarily
take place at the point on the dome where simple gravitational instability is greatest.
Thus, although collapses into the 3 July 1998 scar are considered most likely, the
possibility of an outburst in a different direction cannot be precluded.
27. Given these circumstances, the meeting concluded that the potential for a really big
gravitational collapse, such as a 3x reference event, or larger, (and hence associated
large pyroclastic flows) affecting the Belham River valley is now reduced to a negligible
level, and a conditional probability of occurrence in the next six months not exceeding
10-6 was adopted. The present dome configuration and local topography above Tuitt‟s
Ghaut suggests that, if provoked by a large earthquake, for instance, such a collapse
towards Bramble airport has a slightly greater chance of being triggered, for which the
conditional probability of occurrence is set at about 10-5 in the same interval. The
possibility of a big collapse out of the Gages area of the crater, perhaps involving some
pre-eruptive material as well (as in the Boxing Day 1997 event), is also considered
conceivable, and incorporated conservatively in the model at a conditional probability of
10-3.
28. Smaller collapses, represented in magnitude by the 1x reference event, if they occur, are
likely to be confined to the Tar River valley and to Gage‟s valley-Fort Ghaut, although
for the reasons given in para 24 above, the possibility of avalanches of this magnitude
spilling out in other directions cannot be completely precluded. The meeting felt there
was about a 5% chance of a dome collapse of this magnitude in the next six months, so
in the risk model, the probability of a 1x reference collapse down Gage‟s is ascribed a
likelihood of about 3.3% in the next six months, twice that for one in the Bramble
direction (1.6% probability), and about seven times that for a similar event occurring
into the upper Belham River valley (0.5% probability).
29. The meeting determined that the assessed likelihood of magmatic reactivation of the
volcano in the next six months (see lower branches on Figure 3) should remain
unchanged from the last elicitation, at a 5% chance in the next six months. The chances
of a major life-threatening magmatic eruption, which could impact outside the present
exclusion zone in the next six months (assessed last time at about 0.7%), also remains
unaltered: the equivalent annual probability is just over 1%, which is close to the long-
term average occurrence rate for an Eastern Caribbean volcanic island, as indicated by
historical experience. In other words, the threat from the volcano in its present state to
the currently populated areas of Montserrat continues to appear no greater than that
posed to other West Indian populations living near a volcano in repose.
30. Very low probability extreme volcanic threats from renewed magmatic activity (e.g.
fountain collapse, Plinian explosion) are again incorporated in the risk model (although
not shown on the summary event tree on Figure 3) and are implemented in the
quantitative risk analysis, which follows.
31. The meeting also reconsidered possible hazard levels over longer timescales, following
up on the initial presentation and discussion at the last assessment of the results of the
MVO survey of dome-building eruptions (MVO Open File Report 98/15). No
compelling arguments were proffered for changing the earlier decisions: over a timescale
of about 5 years, the synthesis of opinions had indicated about a 1 in 7 chance of
magmatic reactivation (slightly lower than would be inferred from the global survey of
other dome-building eruptions); for the longer term, to about 30 years, there was a clear
expectation of an increase in the likelihood of a repeat eruption (about 1 in 4 chance),
reflecting the Soufriere Hills volcano‟s recent history of repeated volcano-seismic crises,
as well as an apparent global propensity to such renewal intervals at other dome-building
volcanoes.
Simulations of casualties from volcanic hazards
32. A number of simulations have been run with various combinations of population
distributions to establish which factors control contemporary risk exposures for people
living on Montserrat, and what the degree of that risk is currently. Other simulations
have been conducted, under assumptions of no return to magmatic eruption, to
investigate risks associated with proposals to re-occupy certain areas south of the
Belham River.
33. The present quantitative risk assessment of the threats posed by volcanic activity in
Montserrat is based on a series of inputs from the various meetings of scientists,
including the most recent held in Trinidad on 24-25 February 1999. Because of time
constraints, the opportunity has not existed to report the present results back in detail to
the group for appraisal of the volcanological balance, further feedback or additional
iterations of the modelling procedure.
34. For the current population scenario, results are given in Table 2 for the probability levels
for various numbers of fatalities, together with the corresponding results from the earlier
assessments for the overall risks associated with both magmatic and non-magmatic
hazards from the volcano. These are shown also as a probability of exceedance curves
on Figure 4. However, these simulations are for a “fixed” population, in place at the
time of the risk event, and takes no account of any warning or mitigation measures
which might be achieved by recognising and acting upon the early onset of a hazardous
situation. For this reason, the results of the simulations may produce very conservative
estimates for potential casualty figures, and can be taken as upper bounds for the real
risk exposure.
Assessment N=1 N=5 N = 10 N = 50
December 1997 9 - 13 % 2.5 - 5 % 1.0 - 1.5 % 0.3 - 1.0 %
April 1998 4% 1.2 % 0.8 % 0.33 %
July 1998 0.8 % 0.4 % 0.4 % 0.2 %
February 1999 0.2% 0.2% 0.2% 0.2%
Table 2: Probability of N or more fatalities (in six months) for the present situation (and
population distributions given in Table 1), compared with the three previous
assessments.
35. This analysis indicates that, in the current situation, there is a chance of about 1 in 500
(0.2%) for one or more fatalities from any sort of eruptive activity on Montserrat in the
next six months, with a corresponding likelihood of a 99.8% probability of not incurring
any further fatalities. The chances of suffering 5 or more casualties (i.e. so-called
“mass” casualties, as defined by medical considerations) in the same period are now
assessed a factor 2x lower than in July 1998. The decrease in risk of small numbers of
casualties arises because the current interpretation of hazards effectively precludes the
possibility of dome collapses reaching marginal parts of populated areas, such as Salem.
36. In terms of larger numbers of casualties on Figure 4 (i.e. 50 fatalities or more), there is a
slight increase in the calculated risk levels compared with earlier analyses. As explained
in earlier assessment reports, these risks are controlled mainly by the fountain collapse
surge scenarios, and the uncertainties in predicting their effects at significant distances
from the volcano. Thus, any increase in population numbers (such as that indicated on
Table 1) gives rise to increased overall risk exposure to low probability events.
However, the risk level of multiple fatalities estimated by the model ignores any
mitigation provided by early warning of magmatic reactivation and, at a chance of
occurrence of about 1 in 1000 p.a., is comparable with the historical risk exposure on
other West Indian volcanic islands (for unprepared populations, with no effective
monitoring in place).
37. Annualised individual risk categories for the different areas, under the current risk
analysis assumptions, are given in Table 3. For an individual in one of the areas north of
Lawyer's Mountain, the annualised individual risk for death by eruptive activity would
be classified as MINIMAL to NEGLIGIBLE under the CMO‟s scale. People in Salem
full-time are judged to be accepting a LOW level of personal risk. For each of these
cases there is a concomitant risk of injury (as opposed to fatality) which may involve a
significantly greater risk exposure in probability terms.
Individual risk on Area 4 Area 3 Area 2 Area 1
CMO's scale
All volcanic hazards LOW LOW MINIMAL MINIMAL/
NEGLIGIBLE
Non-magmatic activity MINIMAL NEGLIGIBLE NEGLIGIBLE NEGLIGIBLE
only
Table 3: Categories of annualised individual risk of death by volcanic action in each
populated area, for a) all volcanic hazards, and b) for residual hazards only.
(The CMO's scale is reproduced in Appendix 3).
38. These risk categories have been derived on the basis of an unprepared population
immobilised in the areas identified on Table 3 at the time of the hazardous event
occurring. Because they do not incorporate the significant mitigation which could be
achieved by effective early warning from monitoring activities (see below), they
therefore represent worst case scenarios. The true risk exposure would be somewhat
lower, depending on the lead time of any warning provided, and the responsiveness of
the people involved.
39. For the situation where there is no further magmatic activity, the risk exposures are
substantially lower (see Table 3): the northern margins of the Belham Valley (i.e.
southern parts of Salem, Old Towne and Friths) are in the MINIMAL risk exposure
category, while the risk to areas north of Salem would be categorised NEGLIGIBLE.
40. A common way to illustrate societal risk (as opposed to individual risk) is to show how
many fatalities can be expected at set probability levels. Table 4 shows the numbers of
deaths for probability exceedance levels of 1 in 100, 1 in 500, 1 in 1,000 and 1 in 10,000.
As discussed before, the two lowest probability expected casualty figures are similar to
the long-term historic rate of disastrous eruptions in the region, and do not take account
of modern monitoring and mitigation capabilities.
Probability 1 in 10,000 1 in 1,000 1 in 500 1 in 100
No. fatalities: All
volcanic hazards 200 100 10 <1
Non-magmatic
Activity only <1 0 0 0
Table 4: Expected number of deaths at given probability exceedance levels for
present population.
41. An additional set of risk analyses have been undertaken for an hypothetical situation in
which there is partial re-occupation of areas south of the Belham River (Iles Bay; Cork
Hill; St George‟s Hill; Richmond Hill and Fox‟s Bay). It is plausible to assume that, for
the near future at least, only limited numbers of returning residents would be involved in
each case (for present purposes upper limits to population numbers of 50; 200; 100; 100;
50 have been assumed for the identified areas, respectively). The threat being
considered in this instance is a pyroclastic flow from dome collapse, occurring in
different directions for which relative probabilities of occurrence have been assigned by
consensus (see above).
42. Under present conditions at the volcano (i.e. no magmatic activity), the results of the
south of Belham River simulation are shown on Figure 5: in the next six months there
appears to be about a 1 in 250 chance of suffering one or more casualties if about 500
people return to live there full-time. There is certain to be some significant conservatism
in this societal risk estimate, given the assumptions of the model, and a slightly less
pessimistic perspective is provided by considering the equivalent individual risk
exposure: the annualised risk of fatality is indicated to be of order 1 in 50,000, or VERY
LOW on the CMO‟s scale. Of course, this exposure would change significantly if
magmatic eruptive activity were to resume, and the individual risk would then revert to
MODERATE (IRPA ~ 1 in 250) requiring an immediate decision regarding re-
evacuation of the area.
Specific issues raised by DFID
43. The scientific group was asked to consider a number of specific issues. These are now
addressed within the context of the more general hazards assessment above.
Hazards and risk at Belham River Crossing
44. As indicated in paragraph M, the main identified hazardous conditions for the river
crossing are mudflows, and these only occur here after a period of intense rainfall. A
large amount of volcanic debris in the catchment area of the Belham River has already
been washed down onto the golf course and out to sea. As further debris is transported
down the river the capacity of the river system to cause dangerous mudflows will
diminish. Education of the public who use the crossing and warning signs are likely to
be the most effective way of mitigating this particular hazard.
45. In terms of risk to individuals from a dome collapse pyroclastic flow or surge when
crossing the river, the consensus view is that under present conditions there is a
negligible chance of occurrence of a collapse big enough to reach as far as the crossing.
Taken together with the limited exposure time and small numbers involved at any one
time, it is suggested the risk level from such a hazard can be discounted, unless
magmatic activity resumes.
Hazards and risk in Cork Hill
46. The boundary of the newly defined hazardous area (see Figure 1) passes through parts of
Cork Hill as a consequence of applying the criteria described in section 14 to the
pyroclastic flow deposit of 25 June 1997. Now that the magmatic eruption has stopped
the probability that a dome collapse large enough to reach the same distance as June 25
has diminished significantly, but cannot be entirely ruled out. Controlled access to other
areas south of the Belham valley through Cork Hill can be envisaged.
47. A quantitative risk analysis has been undertaken of the exposure of a re-settled
population of about 200 people in Cork Hill, under present non-magmatic activity
conditions, assuming a threat from a dome collapse sending a flow in the direction of the
Belham valley. The model simulation indicates a less than 1 in 1,000 chance of
suffering a fatality in Cork Hill in the next six months (see Figure 5) from this hazard,
and a negligible risk (less than 1 in 1,000,000) of multiple casualties. The individual
risk exposure would be classified as VERY LOW in this general area.
Hazards and risk in Iles Bay, Richmond Hill and Fox’s Bay
48. These areas now fall outside the zone of high hazard identified on the accompanying
map. Richmond Hill and Fox‟s Bay still have substantial deposits of ash and the health
aspects of these deposits will need careful consideration (see annex by Dr P J Baxter).
There is also likely to be an on-going low-level hazard for the Richmond Hill and Fox‟s
Bay areas from acid rain, precipitated from the gas-rich plume which the volcano is
currently producing; this may well produce conditions of accelerated corrosion in
properties downwind from the volcano.
49. In respect of direct volcanic hazards, the limitation on achievable pyroclastic flow run-
out distances inferred for present non-magmatic conditions at the volcano presents a
very low risk overall for casualties in any of these areas: providing activity remains non-
magmatic, the modelling suggests only about a 1 in 10,000 chance of a single fatality in
all three combined in the next six months (see Figure 5), with an equivalent
NEGLIGIBLE individual risk exposure in terms of the CMO‟s scale.
Hazards and risk on St George’s Hill
50. St. George‟s Hill has been identified as a possible vantage point for tourists and other
visitors to view the volcano and the effects on Plymouth. This area falls outside the
newly defined zone of high hazard and is accessible through Cork Hill. In the July 1998
assessment it was stated that tourists should be regarded no differently to other citizens
of Montserrat; however, given the relatively short timescale for which they would be
present there, and their presumed mobility by taxi, the annualised individual risk of
fatality for a short-term visitor must be considerably lower than that of any permanent
resident living in the same locality.
51. In terms of volcanic threats to the St. George‟s Hill vicinity, there is some possibility of
parts of this area being affected by dome collapse flows either from the north-western
sector of the dome (i.e. one which would also impact the Belham Valley) or from
collapses descending Gage‟s valley. Both eventualities have been included in the risk
model, indicating a 1 in 500 chance of receiving one or more casualties in six months for
a resident population of 100 people, with an upper limit to individual risk of about
1:50,000 p.a. (VERY LOW on the CMO‟s scale), while non-magmatic conditions exist.
Hazards and risk at Bramble airport
52. There has already been detailed evaluation of the situation with respect to hazard and
risk at Bramble airport in the report of September 1998. The latest appraisal of
conditions finds that the probability of a collapse that could reach Bramble airport has
reduced.
53. The risk for workers undertaking remedial work in and around the Bramble Airport area
under present non-magmatic conditions has been assessed with a model based on 25
workers operating 8 hours per day, 7 days a week. Three scenarios are considered: 1]
for the worst case scenario, when no effective warning of sudden collapse is possible
AND the probability of a big collapse in the direction of the airport may be as high as
5% in six months (a conservative value decided by the group of experts in Trinidad), the
potential loss of life risk exposure is as shown on the upper curve of Figure 6 - in this
case, the IRPA for an individual worker may reach 1:650 p.a. (i.e. comparable to
offshore oil workers in the North Sea). 2] If it is assumed that there is no activity within
the crater and that dome collapse occurs only because of an external trigger (such as a
big regional earthquake - i.e. with a much lower probability of occurrence than in the
worst case scenario above), AND if measures are in place to monitor the volcano such
that there is perhaps a 50% chance of successfully detecting the start of collapse for
immediate warning, it is estimated that the IRPA for alerted workers would be much
lower: about 1:10,000 p.a. (see also curve 2 on Figure 6). 3] If means were found to
increase the probability of giving prompt warning of a triggered collapse to 90%, the risk
exposure would be further reduced and an IRPA value as low as 1:16,000 p.a. might be
achieved. In all these cases, uncertainty in all the main contributory factors has been
incorporated by including suitable distributional spreads in the model. A robust risk
estimate for this situation is likely to fall somewhere between the extremes of these
illustrative scenarios, but requires further scientific input and analysis to establish.
54. On Figure 6, these risk estimates for remedial workers at Bramble under non-magmatic
conditions are also compared to the risk exposure of the wider population, as it is
currently distributed on island.
55. A revised risk assessment for using Bramble Airport, adopting the latest scientific
inferences on dome collapse potential, has been undertaken for two of the alternative
operational scenarios considered in the September 1998 report: 1) passenger handling
and support services (e.g. ATC) are all established in a new facility away from the
airport, and the runway is only visited for flight support, and passenger and baggage
transfer; and 2) all activities are fully restored at the airport itself. It should be noted
that in these cases, both magmatic and non-magmatic hazards have been taken into
account in the simulations.
56. For the scenario in which external operations are implemented well away from the
airstrip, a more substantial reduction in risk is suggested by the analysis (Figure 7): over
the long term, the risk to a group of people going to the airstrip for a short time to board
or leave a flight and to perform ground support is possibly marginally lower than that for
permanent residents on the margins of Area 4, say. For the individual going repeatedly
to the airstrip on short visits as one of a group of airport workers, the estimated IRPA is
about 1 in 7000, which is a similar exposure to someone living full-time in Area 3
(Woodlands), for instance.
57. The perceived reduction in hazard of a large dome collapse produces a small reduction
in overall risk exposure (see Figure 7) for the second alternative (all activities at
airfield), with a more marked reduction in the potential for mass casualties; in these
circumstances, the risk exposure of the airport “population” is still almost an order of
magnitude higher than for the rest of the population of Montserrat as a whole
(notwithstanding the limited exposure time at Bramble), and the individual risk is
MODERATE, at about 1 in 750 p.a. For comparison purposes, this IRPA is slightly
lower than that estimated for someone returning to live full-time south of the Belham
River (IRPA ~1 in 250 - see also the July 1998 assessment report), mainly because the
total exposure time is much less for airport activities, but higher than for a resident of
Area 4.
58. This revised analysis, and comparisons of results with other situations in Montserrat, is
based on the scenario in which the possibility of magmatic reactivation occurring
sometime within the next 30 years is taken into account, not just the present non-
magmatic conditions.
Vulnerability of property in area 4 (Salem, Friths and Old Towne)
59. Provided magmatic activity does not resume the level of hazard in this area to property is
very low and has returned to that which existed prior to the eruption. However if the
magmatic eruption were to resume then this area would again become a vulnerable area.
The chances of a new eruption will increase as a period of 30 years since the 1995-1998
eruption approaches. The probability of another eruption in 30 years time was assessed
as 25% in the July 1998 assessment. Damage to property has been previously
considered in an appendix to the December 1997 assessment report.
60. The 1997 assessment used a methodology to assess probalistically the risk of damage
and collapse to buildings under heavy tephra loading and pyroclastic particle impact. A
number of eruption scenarios were envisaged ranging from explosive eruptive events of
10 times the reference explosive eruption of 17 September 1996 to 30 times the
reference event and with varying wind directions. Due consideration was given to
estimates of the proportion of houses with different construction characteristics within
the housing stock of the various population zones. With an event of 10 times the
reference event and an E-W wind it was estimated that some 26% of the housing stock
would be vulnerable to roof collapse but this was very largely comprised of houses with
two roof types.
Type B: 0.56 mm grade 24 steel sheet, supported by battens of 75x50 mm
spaced at 1.350 mm, which in turn are supported by rafters of 150x75 mm,
spaced at 900 mm and spanning 3 metres
Type C: 0.56 mm grade 24 steel sheet, supported y battens of 50x25 mm spaced
at 1.350 mm, which in turn are supported by rafters of 100x50 mm, spaced at 500
mm and spanning 3 metres
During the magmatic eruption of the Soufriere Hills Volcano, no explosive event
approaching a 10 times reference event has been experienced and consequently there has
been minimal damage due to roof collapse in area 4 to date.
61. However, the strong eruptive activity of August and October 1997 took place as a
sequence of about 80 moderate explosions and it is conceivable that, in any future
eruption, similar vigorous magma production might generate a single, much more
intense explosion, the products of which could impact Area 4. Also, in any future
volcanic eruption of the Soufriere Hills volcano, a violent lateral surge explosion, like
that on Boxing Day 1997 but occurring under different conditions in the crater and
directed to the northwest, might directly affect buildings in some part of Salem.
Time-scale for reduction in hazards
62. There is a general expectation that the residual volcanic activity will diminish with time.
On other volcanoes residual activity after a major eruption commonly lasts from months
to a few years. There is however no robust method to estimate how long the residual
activity at Soufriere Hills will last. Further collapses of the dome will inevitably reduce
the hazard due to the decrease of the size of dome and its capacity to cause large
pyroclastic flows. However, heat dissipation in a recently extruded dome is very slow
and, given the relatively large volume of material involved in the 1995-98 eruption, it is
considered likely that, if further major collapses do not occur, several decades (or even a
century or two) will have to elapse before the heat energy contained within the dome is
reduced sufficiently to effectively remove all hazard potential from that source.
Effect of monitoring on risk exposure levels
63. The fact that the Soufriere Hills volcano is intensively monitored will have an impact on
the actual risk levels to which the population are exposed. It is implicit in the analysis
models just described that the population distribution is static at the time the threat
materialises, and that no precautionary action (eg evacuation) takes place. These are,
therefore, akin to worst case scenarios in terms of population preparedness and
mitigation. However, it is thought very likely (especially in the present circumstances of
quiescence) that the scientific team will recognise precursors to any new escalation of
magmatic activity. As a consequence, the critical risk which should determine decision-
making is the conditional probability of a hazardous event occurring, without any
effective warning. In real terms, this risk can be a significantly lower value than that
simulated for an unwarned, unresponsive population, and may be as much as an order of
magnitude smaller than that implied by the present model. There is, therefore, a
significant margin of potential conservatism in these risk estimates which disregards any
input from the monitoring. However, in some circumstances, the lead time for such
warnings may be somewhat shorter than the time needed to take full protective
measures, and this should be factored into decision-making.
Monitoring Requirements
64 In the current period of residual volcanic activity there is a requirement to continue
careful monitoring of the Soufriere Hills Volcano. The scientific group are concerned
that important decisions have not yet been made about the future management, staffing
and structure of the MVO despite the fact that the two year contract that has supported
the MVO ends on 31 March 1999. The scientific group are also concerned that there has
been little progress in building a purpose built observatory in sight of the volcano.
Greater urgency in promulgating the Observatory building is recommended and in the
meantime the group recommend that a forward observation post is constructed.
ANNEX
Health risk of ash (Contributed by Dr P J Baxter)
In the current phase of activity ash falls may arise intermittently as a result of dome
collapse with or without venting, or every few weeks in a major venting episode. An
analysis of ash samples from three recent events showed that the composition of the ash
continues to be dominated by a fine, respirable fraction containing 15-25% cristobalite.
Although the ash falls are less frequent and more modest in size than those during the
magmatic period, air monitoring in Salem and Old Towne using the already established
network of sites and DustTrak instruments should be continued in order to provide a
record of exposure to the population, especially in the dry season. Any accumulations of
ash on roads and surfaces around houses should continue to be removed as part of a
regular clean up programme as begun in August 1998. Residents and outdoor workers
in these areas should follow previous official advice on routinely preventing exposure to
ash.
A full assessment of the health risk of the ash deposits would be needed before
consideration is given to reoccupation of any of the areas south of the Belham River.
Although a clean-up operation might be contemplated in the least impacted areas, an
unknown factor at present is the amount of fine ash being resuspended by the prevailing
winds from the thick layers of ash on the slopes of the volcano and any other deposits
upwind. An air monitoring study would be required to assess this problem as fine ash
can be present in the air at unacceptable levels yet remain invisible to the naked eye.
Large amounts of ash remain on the upper slopes of the volcano. Clean-up operations in
the low-lying areas might not be effective because of the action of rainwater carrying ash
downhill, thereby depositing it along the verges of roads and around houses where it
would become trapped in grass and other vegetation. Resuspension of the fine ash in
these deposits would occur once they were dry and impacted by human activity. A
preliminary assessment is therefore needed to establish the extent to which the ash lying
on the volcano can be remobilised by the wind or rain, including the role of revegetation
of the ground in stabilising the existing deposits.
Even ashfalls leaving thin deposits on the ground can, if repeated regularly enough,
adversely affect air quality in areas of human activity. It may therefore be inadvisable to
pursue redevelopment in areas south of the Belham Valley whilst ashfalls are likely to
continue. A health impact assessment of the continuing ashfall activity should be
undertaken in collaboration with volcanologists at the MVO.
Appendix 1
TERMS OF REFERENCE FOR
MONTSERRAT: UPDATE OF HAZARD AND RISK ASSESSMENT OF THE
SOUFRIERE HILLS VOLCANO; 24-25 FEBRUARY: TRINIDAD
The meeting will update all aspects of the July 1998 report and in addition to the general re-
assessment of the status of the volcano and risk assessments will address the following
specific items:
a) Previous Area 4 (Salem, Frith and Old Towne). Levels of risk to property (as
distinct from the population).
b) Belham Valley River Bridge. Risk to people, vehicles crossing. Consideration of
how risk could be minimised.
c) Bramble Airport. Update of September 1998 risk analysis. Consideration of how to
implement protection measures, and minimise risk to workers constructing initial
protection measures.
d) Risk to Iles Bay area.
e) Risk to Cork Hill area.
f) Risk to Richmond Hill area.
g) Risk to Fox‟s Bay area.
h) Analysis of potential time-scale for reduction in these levels of risk (as the volume of
dome collapse and knowledge/predictive confidence increase).
i) Risk to St George‟s Hill
A short (2/3 page) summary of the conclusions will be prepared at the end of the meeting,
which will issue simultaneously to the Governor, GoM, FCO and DFID. A comprehensive
report including the detailed analysis should issue before 12 March 1999.
Appendix 2
SCIENTIFIC AND HAZARD ASSESSMENT: TRINIDAD, 24-25 FEBRUARY 1999
LIST OF PARTICIPANTS
Chairman
Professor R S J Sparks FRS* Chairman (Dept. of Geology, Bristol University, UK)
Invited Scientists
Dr W P Aspinall* (Aspinall and Associates, UK)
Mr L Lynch* (Seismic Research Unit, University of West Indies,
Trinidad)
Mr R A E Robertson* (Interim Director, MVO/Seismic Research Unit,
University of West Indies, Trinidad)
Dr K Rowley* (Independent scientist and LANDATA Associates,
Trinidad)
Dr J B Shepherd (Seismic Research Unit, University of West Indies,
Trinidad)
Dr S R Young* (British Geological Survey, Edinburgh, UK)
Convenor
Dr A J Reedman (British Geological Survey, Nottingham, UK)
Denotes delegates that participated in the formal elicitation of scientific opinions
Appendix 3
CHIEF MEDICAL OFFICER’S RISK SCALE
Negligible: an adverse event occurring at a frequency below one per million. This would be
of little concern for ordinary living if the issue was an environmental one, or the
consequence of a health care intervention. It should be noted, however, that this does not
mean that the event is not important – it almost certainly will be to the individual – nor that
it is not possible to reduce the risk even further. Other words which can be used in this
context are „remote‟ or „insignificant‟. If the word „safe‟ is to be used it must be seen to
mean negligible, but should not import no, or zero, risk.
Minimal: a risk of an adverse event occurring in the range of between one in a million and
one in 100,000, and that the conduct of normal life is not generally affected as long as
reasonable precautions are taken. The possibility of a risk is thus clearly noted and could be
described as „acceptable‟ or „very small‟. But what is acceptable to one individual may not
be to another.
Very low: a risk of between one in 100,000 and one in 10,000, and thus begins to describe
an event, or a consequence of a health care procedure, occurring more frequently.
Low: a risk of between one in 10,000 and one in 1,000. Once again this would fit into many
clinical procedures and environmental hazards. Other words which might be used include
„reasonable‟, „tolerable‟ and „small‟. Many risks fall into this very broad category.
Moderate: a risk of between one in 1,000 and one in 100. It would cover a wide range of
procedures, treatment and environmental events.
High: fairly regular events that would occur at a rate greater than one in 100. They may
also be described as „frequent‟, „significant‟ or „serious‟. It may be appropriate further to
subdivide this category.
Unknown: when the level of risk is unknown or unquantifiable. This is not uncommon in
the early stages of an environmental concern or the beginning of a newly recognised disease
process (such as the beginning of the HIV epidemic).
Reference: On the State of Public Health: the Annual Report of the Chief Medical Officer
of the Department of Health for the Year 1995. London: HMSO. 1996.