by Abraham Marinelarena
The need for a clean environment is rooted in the control of infections in hospitals. Microbiologist
and surgeons for the past hundred years have established that bacteria cause wound infections. It
therefore followed that the elimination of bacteria from the hospital and, in particular, the operating
room would prevent infection. This was the scientific basis for the first cleanrooms.
The control of infection was approached using antiseptic techniques to clean instruments, wounds and
surgeon’s hands. To prevent airborne infection the same antiseptic solution was sprayed into the air.
Latter aseptic techniques such as sterilization of wound dressings and instruments and the use of
surgical gloves, masks and gowns were adopted together with antiseptic techniques to further reduce
Although antiseptic and aseptic techniques helped to control infection, the principal omission was
positive ventilation by filtered air. It was found that proper ventilation substantially reduced airborne
infection of people. But, it is only towards the end of second world war that ventilation in hospitals
was adopted for contamination control.
The development of the first cleanrooms for industrial manufacturing started during the second world
war in the United States and England in an attempt to improve the quality and reliability of
instrumentation used in guns, tanks and aircraft. It was realized that the cleanliness of the production
environment had to be improved or such items would malfunction. It therefore followed that airborne
dispersion of large quantities of particles by machines and people had to be removed by large
quantities of filtered air.
What is a Cleanroom ?
Airborne particles occur in nature as pollen, bacteria, miscellaneous living and dead organisms,
windblown dust, and sea spray. Industry generates particles from combustion processes, chemical
vapors, and friction in manufacturing equipment. People in the work space generate particles in the
form of skin flakes, lint, cosmetics, and respiratory emissions. Today, many manufacturing processes
require that spaces be designed to control particulate and microbial contamination while maintaining
reasonable installation and operating costs.
Federal Standard 209E defines a cleanroom as a room in which the concentration of airborne particles
is controlled to specified limits. British Standard 5295 defines a cleanroom as a room with control of
particulate contamination, constructed and used in such a way as to minimize the introduction,
generation and retention of particles inside the room and in which the temperature, humidity, air flow
patterns, air motion and pressure are controlled.
The industry differentiates between the cleanliness of rooms by referring to Class Numbers. The
methods most easily understood and universally applied is the one suggested by the Federal Standard
209E in which the number of particles equal to or greater than 0.5 microns measured in a cubic foot of
air designates the class number. The table below shows the relationship between Class Numbers and
the volume of particles of a various sizes in a cubic foot of air. For example, a class 100,000
cleanroom limits the concentration of airborne particles equal to or greater than 0.5 microns to
100,000 particles in a cubic foot of air.
Class 0.1µµ 0.2µµ 0.3µµ 0.5µµ 5µµ
S1 inch-lb part/m3 part/ft3 part/m3 part/ft3 part/m3 part/ft3 part/m3 part/ft3 part/m3 part/ft3
M1 350 9.9 75.0 2.14 30.9 0.875 10.0 0.283
M1.5 1 1240 35.0 265 7.50 106 3.00 35.3 1.00
M2 3500 99.1 757 21.4 309 8.75 100 2.83
M2.5 10 12400 350 2650 75.0 1060 30.0 353 10.0
M3 35000 991 7570 214 3090 87.5 1000 28.3
M3.5 100 26500 750 10600 300 3530 100
M4 75700 1240 30900 875 10000 283
M4.5 1,000 35300 1000 247 7.00
M5 100000 2830 618 17.5
M5.5 10,000 353000 10000 2470 70.0
M6 1000000 28300 6180 175
M6.5 100,000 3530000 100000 24700 700
M7 1000000 283000 61800 1750
Table 1. Cleanroom Classifications
The application of cleanrooms has increased and diversified as technology advances. The required
standard of cleanliness of a room is dependent on the task performed in it. The more susceptible the
product is to contamination the stricter the standard. The table below shows a selection of products
that are now being made in cleanrooms or require contamination control facilities.
ELECTRONICS Computers, TV-tubes, Magnetic Tapes
SEMICONDUCTORS Integrated Circuits
MICROMECHANICS Compact Disc Players, Miniature Bearings,
OPTICS Lenses, Photographic Film, Laser Equipment
BIOTECHNOLOGY Antibiotics, Generic Engineering
PHARMACY Sterile Pharmaceuticals, Sterile Disposable
MEDICAL DEVICES Heart Valves, Cardiac by-pass Systems
FOOD AND DRINK Brewery Production, Unsterilized Food and Drinks
HOSPITALS Immunodeficiency Therapy, Isolation of Contagious
Patients, Operating Rooms
Types of Cleanrooms
Cleanrooms have evolved into two major types which are differentiated by their method of
ventilation. These are turbulent air flow and laminar air flow cleanrooms.
Turbulent Air Flow
The general method of ventilation used in turbulent air flow cleanrooms is similar to that found in
buildings such as offices, schools, malls, manufacturing plants, auditoriums, shops, etc. The air is
supplied by an air conditioning system through diffusers in the ceiling. However, a cleanroom differs
from an ordinary ventilated room in three ways. These are increased air supply, the use of high
efficiency filters and room pressurization. Figure 1 shows the typical air flow patterns in a turbulent
air flow cleanroom.
The increased air supply is an important aspect of particle control. A typical turbulent air flow
cleanroom would have at least 10 air changes per hour and likely to have between 20 and 60. This
additional air supply is mainly provided to dilute to an acceptable concentration the contamination
produced in the room.
High efficiency filters are used to filter the supply air into a cleanroom to ensure the removal of small
particles. The high efficiency filters used in cleanrooms are installed at the point of air discharge into
Room pressurization is mainly provided to ensure that untreated air does not pass from dirtier
adjacent areas into the cleanroom. The cleanroom is positively pressurized with respect to these
dirtier areas. This is done by extracting less air from the room than is supplied to it.
Laminar Air Flow
Laminar air flow is used when low airborne concentrations of particles or bacteria are required. This
air flow pattern is in one direction, usually horizontal or vertical at a uniform speed of between 60 to
90 ft/min. and throughout the entire space. Figure 2 shows the typical air flow patterns in a laminar air
The air velocity is sufficient to remove relatively large particles before they settle onto surfaces. Any
contaminant released into the air can therefore be immediately removed by this laminar flow of air,
whereas the turbulent air flow ventilated system relies on mixing and dilution to remove
In an empty room with no obstructions to the airflow, contamination is removed faster by air
velocities much lower than those mentioned above. However in practical situations there are
obstructions and people moving in the space. Obstructions will cause the laminar air flow to be
turned into turbulent air flow around the obstructions. Higher contamination concentrations will be
established in the turbulent areas. Therefore, it has been found that the cleanliness of a laminar air
flow cleanroom is directly proportional to the air velocity. Air changes per unit of time are related to
the volume of the room and are many times greater than those supplied to a turbulent air flow
Because air flow is such an important aspect of particle control, the design of a clean room requires
careful consideration to air motion and airflow patterns. Depending on the degree of cleanliness
required, it is common for air systems to deliver considerably more air than would be needed solely to
meet temperature and humidity design points. Table 2 shows the relationship between cleanroom
airflow requirements versus cleanroom class as specified under Federal Standard 209E. The curves
show the range between idea, standard and compromised conditions.
AIR CHANGES PER HOUR
1 10 100 1000 10000 100000
Ideal ------------- Standard - . - . - . Compromised
Table 2: Air Changes v. Room Class
Airborne particles can be organic or inorganic. Most contamination control problems concern the
total gross contamination within the air. Particles of different sizes behave differently as air moves
through a room. For example, in an eight foot high room, a particle in the 50 micron range might take
60 seconds to settle, while a 1 micron particle might take 15 hours to settle.
Before any methods of contamination control of airborne particles can be applied, a decision must be
made as to how critical this particulate matter is to the process or product. The quantity of the
particles of a given size that might be present within the area must be considered. The source of the
contamination is divided into external sources and internal sources.
For any given space, there exists the external influence of gross atmospheric contamination. These
sources include air pollution in general and dust storms.
External contamination is brought in primarily through the air conditioning system. Also, external
contamination can infiltrate through building doors, windows and cracks. The external contamination
is controlled primarily by the type of filtration used and space pressurization.
People and the production process are some of the greatest sources of internal contamination. People
in the work space generate particles in the form of skin flakes, lint, cosmetics, and respiratory
emissions. Industry generates particles from combustion processes, chemical vapors, soldering
fumes, and cleaning agents. Other sources of internal contamination is generated through the activity
of manufacturing equipment.
Room Construction and Operation
Although airflow design is critical, it alone does not guarantee that cleanroom conditions will be met.
Construction finishes, personnel and garments, materials and equipment, and building entrances and
exits are other sources of particulate contamination that must also be controlled.
Room construction and material finishes are an important part of cleanroom design. Room
construction is important to provide an enclosure that will house a process to exclude outside
contaminants and that the material finishes will not contribute to particle generation in the space.
Walls, floors, ceiling tiles, lighting fixtures, doors, and windows are construction materials that must
be carefully selected to meet cleanroom standards.
People must wear garments to minimize the release of particles into the space. The type of garments
depend on the level of cleanliness required by a process. Smocks, coveralls, gloves, and head and
shoe covers are clothing accessories commonly used in clean spaces.
Materials and equipment must be cleaned before entering the cleanroom. Room entrances such as air
locks and pass-throughs are used to maintain pressure differentials and reduced contaminants. Also,
air showers are used to remove contaminants from personnel before entering the clean space.
The ability of a filter to remove particles from the air is reflected by its efficiency rating. The
American Society of Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE) has
developed a standard for measuring filter effectiveness. The standard describes test procedures to
classified filters in terms of arrestance and efficiency.
Arrestance is the amount of dust removed by the filter, usually represented as a percentage. Since
large particles make up most of the weight in an air sample, a filter could remove a fairly high
percentage of those particles while having no effect on the numerous small particles in the sample.
Thus, filters with an arrestance of 90% have little application in cleanrooms.
Efficiency measures the ability of the filter to remove the fine particles. ASHRAE efficiencies of
between 10% and 40% should remove 20% to 40% of the 1 micron particles in the air, but hardly any
of the 0.3 to 0.5 micron particles. ASHRAE efficiencies of 80% to 95% can remove 50% to 70% of
the 0.3 micron particles.
HEPA and ULPA Filters
A HEPA filter, i.e. high efficiency particulate air filters is defined by its particle removal efficiency
and its air flow rate. A HEPA filter is rated by its efficiency in removing small particles from air and
has a minimum efficiency of 99.97%. These high efficiency filters are usually designed to remove
particles of 0.3 microns and larger.
A ULPA filter, i.e. ultra low penetration air is a filter that has efficiencies higher than those of a
standard HEPA filter. An ULPA filter will have an efficiency greater than 99.99%. These filters are
constructed and will function the same way as a HEPA filters. They differ in that the filter medium
that is used has a higher proportion of smaller fibers and is hence more efficient.
An agent often used to test high efficiency filters is composed of atomized droplets of hot di-octyl-
phthalate (DOP). DOP has a fairly consistent average particle size of about 0.2 microns. High
efficiency filters used in cleanrooms are subjected to a DOP penetration test to determine the
percentage of particles passing through the filter.
HVAC Design Considerations
Temperature and Humidity
Most cleanroom require year-round cooling as a result of the fan energy associated with high
cleanroom airflow as well as the heat generated by the process, people, and lighting within the
facility. Temperature control is required to provide stable conditions for materials, instruments, and
personnel comfort. Human comfort requirements typically call for temperatures in the range of 72F
to 75 F, since workers frequently wear cleanroom garments over street clothes.
Humidity control is necessary to prevent corrosion, condensation on work surfaces, eliminate static
electricity, and provide personnel comfort. The human comfort zone is generally in the range of 30%
to 70% relative humidity.
A cleanroom facility may consist of multiple rooms with different requirements for contamination
control. Rooms in a clean facility should be maintained at static pressures higher than atmospheric to
prevent infiltration by wind. Positive differential pressures should be maintained between the rooms
to ensure air flows from the cleanest space to the least clean space. The only exception to using a
positive differential pressure is when dealing with specific hazardous materials where governmental
agencies require the room to be at a negative pressure.
Ventilation and Make Up Air
Ventilation and makeup air volumes are dictated by the amount required to maintain indoor air
quality, replace process exhaust and for building pressurization. This provides assurance that carbon
dioxide and oxygen remain in balance, that formaldehyde and other vapors given off by building
materials and furniture are diluted, and that air changes occur with sufficient frequency to minimize
the chance for high concentration of airborne pollutants within the building.
A cleanroom design requires careful consideration of its intended use, permissible particle
concentration, location, manufacturing process and of course cost. The design and specification of a
cleanroom requires close coordination between the many departments impacted by it and the design
The technology is readily available to design as clean a room as desired, provided the Owner is
willing to justify the associated cost. Close coordination between the Owner and its design team will
guarantee the best and most cost effective results.
Abraham Marinelarena is a mechanical engineer with Bath Engineering Group. He can be
contacted at 915/534-9110.