Sustainability in Built Environment
The Air We Breathe
- Indoor Air Quality and Natural Ventilation
Dr Yuguo Li
Department of Mechanical Engineering
The University of Hong Kong
IAQ Information Centre;
IAQ Management Group
What is sustainability?
“Sustainable development is development that meets the
needs of the present without compromising the ability of
future generations to meet their own needs”.
The United Nations World Commission on Environment and Development,
The Brundtland Report, 1987
Environmental Impact = Population
Affluence per person Technology
Proposed by Paul Ehrlich and John Holden
Ambient air quality
City Population (Million) TSP (mg/m 3)
Dalian 3 185
Shanghai 13 246
Beijing 11 377
Stockholm 1.5 9
Melbourne 3 35
Hong Kong 6 95
Note: Data from Air Quality in Hong Kong, 1999, EPD, HK
and 2000 World Development Indicators, World Bank
Indoor air quality is more important than
ambient air quality
Natural ventilation can be an effective and
energy-efficient ventilation and cooling
Discussion - What can we do?
Our basic needs
Healthy and comfortable - Thermal, visual,
acoustical, tongue, air quality
We spend more than 90% of our time
Indoor air quality is defined as the nature of
indoor air that affects the health and well-
being of occupants
Chemical balance of human body
The energy supply of life provided by
chemical reaction of carbohydrates, fats and
proteins with oxygen and produces CO2 and
Carbohydrates from food, can be stored in
We extract oxygen from breathing air.
Oxygen cannot be stored in our body.
Do you know?
For normal conditions and a sedentary person:
Takes about 15-40 breaths per minute or 22 k to 57 k breaths per
During each air change, about 1 litre of air is replaced
We breathe about 10 to 45 m3 of air per day. How big is out bed
room? 3 X 5 X 3 = 45 m3?
Other way to find out how much oxygen is needed? – We need
1500 to 4000 kcal per day. As one litre of oxygen is needed to
produce 4.8 kcal, we need 300-800 litres of oxygen per day, I.e.
1.5 to 4 m3 of air.
Not all the oxygen is extracted from the air inhaled with each
breath and exhaled air contains 16% residual oxygen.
Simplistic views: Good air quality means freedom from
unwanted odours, and restriction of air contaminant
concentrations below the levels at which irritation or toxic
effects manifest themselves.
Cost of poor indoor air quality costs Australians A$12
billion per year – CSIRO
There is a need for compromise between good IAQ and
cost of achieving good IAQ
ASHRAE 62-1999: Acceptable IAQ means 80% or more
people do not express dissatisfaction.
Indoor Air Quality
Typical indoor spaces
Homes, offices, public spaces
Main indoor pollutants
Two approaches to control IAQ
Use better materials
Cleaning and filtration
Questions so far -
Is IAQ different from thermal comfort?
Is ventilation different from air
Indoor air and outdoor air, which one is
What are the factors affecting indoor air
Why individual control?
1. To handle personal difference and it may be difficult to
ensure everyone is thermally comfortable.
2. Most occupants prefer some degree of individual
Desired Control Over Office Environment
23% No Control
Why provide HVAC?
Fresh air delivery
Removal or dilution of pollutants
For thermal comfort
Removal or addition of heat loads
Comfortable living or working conditions
Certainty of the thermal environment
Other issues - productivity, cost, flexibility, risk,
energy efficiency, environment...
with the ”old” HVAC and natural ventilation?
• ”Old” HVAC
• Energy consuming
• Sick building syndrome
• Minimum ventilation rate
• Mostly centralised
• Natural ventilation
• Do not work at extreme conditions
• Difficult to control
• Difficult to predict and design
Air conditioning – use 48% of energy in
and 30% in residential buildings
30% of offices in HK are sick buildings
Energy = CO2 + $
Poor indoor air quality = poor productivity
Fanger’s study showed 6% improvement in
Three Periods in Indoor Environment Design
1900 1973 199?
Natural Design Mech Design Hybrid/Integrated
Wind causes a positive pressure on
the windward and negative on the
leeward, thus air flows into the
windward opening and out of the
+ - leeward one.
Difference in air density induced
by temperature or humidity
differences drives vertical air
Wind assists (??) stack
q q q 2
Flow rate (m3/s)
qs - flow rate due to stack alone (m3/s)
qw - flow rate due to wind alone (m3/s)
Questions so far -
With ASHRAE 62-1999, we need 10 litre
per second outdoor air per person, do we
need really so high natural ventilation flow
rate as shown earlier?
Do you always turn on your air conditioner
Is natural ventilation always good?
What Natural Ventilation Can
Fresh air, dilute or remove pollutants
Ventilation for IAQ
Remove heat, provide thermally comfortable air
Ventilation for Cooling
Connect us to natural environment
Energy saving by 10-50%
Better indoor air quality in winter, spring
Adaptive thermal comfort
Connect users to environment
Improved productivity if designed properly
Better air quality at roof
level in urban areas
Experimentally proved by
Hunt and Holford (1998)
Features in Arabic
Direct winds into building
evaporative cooling - cool
At night, acts as
Questions so far -
In order to use more natural ventilation, is it
useful to improve ambient air quality?
What are the differences between outdoor
air and fresh air?
Please list three example natural ventilation
Grong School, Norway
BRE Energy Building, UK
Manly Lab, Sydney
Questions so far -
Is it possible to use natural ventilation in an
What are the roles that architects, engineers,
users, developers and users play in
sustainable building design?
Indoor air quality can be more significant than outdoor
Indoor air quality is related to our health, our
productivity and out living
Ventilation is different from air-conditioning
Natural ventilation – a natural choice, but not the only
“Air” is just one element of the environment.