February 10, 2009
MA2SI Focused Clean Air in Surgical, Laboratory, and Industrial
By E. R. Crutcher
The application of focused clean air has been shown to reduce the incidence of
infections in surgical units and to improve contamination control in laboratory
environments and manufacturing facilities. The use of MA2SI focused clean air
has also resulted in significantly lower operating costs and more reliable
outcomes in these environments. MA2SI units have been used very successfully
in hospital; clinic, laboratory, and satellite assemble facilities. A review of
particle mobility and transport mechanisms as well as air volume behavior is
briefly presented along with case histories and documentation to support the
advantages of the MA2SI design and operating principles.
The most cost effective approach to clean and/or sterile environmental control is to
consider the most critical area of concern and work outward. This is actually the opposite
of the approach used in the design of most clean rooms and surgical theaters. The current
approach is to create a “clean” room in which the activity that requires cleanliness will be
carried out. The current approach guaranties that the filthiest location in the facility will
be the very location where the greatest cleanliness is required. The area of greatest
concern is the area of greatest activity and, as a result, the area with the highest rate of
The new approach is to create the highest concentration of clean air at the location of the
greatest concern. This location would be the site of the surgical wound in a surgical
theater or the most sensitive unit of hardware in an industrial or laboratory environment.
This approach is the result of understanding how particles, including spores, bacteria, and
viruses, move or are transported through an environment.
TRANSPORT OF CONTAMINANTS
Particles of all types move by two primary paths, either through the air or they are
transported by surfaces. They often move by both methods so neither path can be
ignored. The physical laws that describe each path allow precautions to be taken to
minimize the transport of detrimental contaminants. The dominant forces that control the
movement of particles in an airstream are drag and gravity if the particles are much larger
than the molecular mean free path in the air (>1.0 micrometers) and diffusion if their
diameter is on the same order of magnitude as the molecular mean free path (1.0 to 0.01
February 10, 2009
micrometers). These two size regimes need to be considered separately. The behavior of
airstreams carrying particles is described by the Navier—Stokes equations
Stoke’s Equation and Airborne Particles
The particles that are controlled by drag and gravity have a residence time in the air that
is controlled by their bulk density, bulk diameter, projected surface area, air flow
patterns, and a correction factor that is typically greater than 1. A common equation that
is used to predict the behavior of a particle in air is the famous Stoke’s Equation1,2:
(6πη)νr = r3π(4/3) (ρo - ρf)g (1)
r = particle radius in cm
η = viscosity of the medium (180 X 10-6 poises for air)
ν = velocity of the particle in cm/sec
ρo = density of the particle in g/cc
ρf = specific gravity of the fluid (approximately 1.2 X 10-3 for air)
g = acceleration due to gravity (981 cm/sec2)
In terms of particle velocity at equilibrium the equation becomes:
ν = r2(2g/9η)(ρo - ρf) (2)
As the diameter of the particle becomes small the velocity at which the particle falls
becomes small. A perfect sphere of water one hundred micrometers in diameter falling in
free space though air reaches a maximum velocity of 0.68 miles per hour. At ten
micrometers in diameter that droplet reaches a maximum velocity of 0.0068 miles per
hour. One sneeze creates hundreds of thousands of these droplets. A single bubble burst
or a drop hitting a surface can create tens of these particles. Particles that aren’t perfect
spheres tend to fall more slowly. Corrections for the Reynold’s Number or
Cunningham’s Correction are just a few of the correction factors required to make
Stoke’s Law work in the real world3.
How does this relate? The sedimentation velocities of particles as large as 0.5
millimeters are slow compared to the velocities of air streams in the vicinity of any heat
source in a room, including human beings. In an operating room the room soon becomes
a soup of particles for the patient, the surgeon, and everyone else in the room. In a
cleanroom every activity adds to the particle load in the intimate vicinity of the object
being assembled or modified. The millions of skin flakes released by each and every
person in the room every minute contributes to the airborne particle load in the room.
Some of these particles sediment into the wound, or onto sensitive surfaces, but they
don’t sediment by Stoke’s equation. The value of Stoke’s equation is not in the
calculation of what settles, but rather in understanding why things don’t settle as might be
February 10, 2009
Particles that are small tend to be deposited on surfaces by diffusion. Diffusion occurs
when small particles are so close to surfaces that the number of gas molecules in the air
striking the particle driving it toward the surface exceeds the number driving it away
from the surface. This works for particles near large surfaces or near the surface of other
particles. This size range includes all of the viruses and most of the bacteria. If the
particle count for particles larger than half a micrometer is one million per cubic foot in a
10 by 10 by 8 foot room then one forth of the total surface area in the room is on the
surface of particles. One forth of the deposited viruses and bacteria in that volume will
deposit on the surface of particles. That is assuming that the particles were free of
bacteria and viruses to begin with. That assumption is wrong. Many of the particles are
in fact skin flakes coming from people in the room and these are often contaminated with
viable viruses or bacteria. Particles from the surgical mask region of people working in
this area are certainly contaminated with viable organisms. This is a very simplistic
model but the point is valid, a significant percentage of the surface area in an
uncontrolled environment is on the airborne particles in that environment and many of
these particles carry viable organisms. This fact is well documented in the literature.
Stoke’s equation explains why particles in any part of the room may migrate to any other
part of the room. In a cleanroom a contaminating activity in any part of the room can
contaminate all other areas of the room if the activity is not actively isolated from the
room air flow5.
Navier—Stoke’s Equations and the Movement of Airborne
Focused clean air overcomes many of these particle settling
and diffusion problems by creating a positive pressure of
clean air over the critical surface (see Figure #1). Particles
are effectively diverted from a zone maintained at a relatively
positive pressure by a constant stream of clean air. But the
purpose of creating the clean zone is so that a sensitive
surface can be exposed and activities carried out on that
surface. The source of the focused clean air must be far
enough away from the surface to allow such activities with Figure #1: Erosion of the Clean
out being so far removed as to be compromised and become Air Zone Due to the Navier—
ineffectual. This distance must provide space for adequate Stoke’s Equations
illumination, a clear visual path, and an unobstructed
working space. These principles have been understood in the Clean air from the ULPA filter is slowed
development of cleanroom protocols but are often by the air outside the filter area. This
impractical or ineffectual as a result of standard cleanroom creates a mixing zone with a pressure
slightly higher than that in the clean air
design. Air is a compressible fluid and is very difficult to flow zone. The mixing zone air with
control over even moderate distances. The source of the some of the particles from air outside the
focused clean air can not be very far away from the sensitive clean air flow zone encroaches into the
site due to the principles described by the Navier—Stokes boundary of the clean air zone. This
equations6. A simpler way of modeling this condition is to process continues with distance from the
filter face until the clean air zone is lost,
consider the Venturi effect7 or Bernoulli’s principle8. The being replaced by a region of enhanced
Venturi effect states that if there is an increase in the velocity cleanliness.
of a stream of flow then the outward pressure head decreases.
February 10, 2009
Conversely, if there is a decrease in the velocity of a stream of flow then the outward
pressure increases. The flow from a HEPA filter encounters resistance from the
stationary air at the boundary of the flow field. This resistance creates turbulence,
reduces the velocity of the boundary layer, and creates a positive pressure toward the
center of the flow of air in the clean air stream. The result is that the boundary layer
becomes mixed with particles from the room air through turbulence and these diluted
particles are then carried by the increase in pressure deeper into the clean air volume.
This process continues as the clean air stream movers further and further from the source.
At some distance, typically greater than the diagonal dimension of the filter face, this
becomes a region of enhanced cleanliness. When the air flow from the filter face is no
longer distinguishable the level of cleanliness approaches the average level for the room.
Control of the clean air stream is essential to the control of the airborne particle exposure
to the sensitive surface. But this is only one of the particle transport mechanisms
operating in an indoor environment.
Mechanical Transport of Particles
One of the most obvious methods of transporting contaminants into a clean environment
is by physically carrying in the contaminants on ones self or on other objects carried into
the room. It is also responsible for the transfer of particles from one part of the
cleanroom to another. This mechanism is the reason for the rigorous cleaning that items
must go through to enter into a clean environment. Once objects are in the clean
environment they can accumulate particles from surfaces and from the air and become
recontaminated9,10. One classic example or recontamination is touching ones face with a
sterile, gloved hand. Another is the accumulation of particles on surfaces away from the
critical area which are then collected by tools, trays, or hands that come in contact with
the surface and then return to the critical area. Continuous cleaning and personal
discipline are the only ways to control the mechanical transport of contaminants.
CONTROLLING THE AIRSTREAM
As alluded to above, the only way to control airborne particles is to control the airstream.
There are two aspects of clean air control. The first is absolute filtration of particles from
the air and the second is the prevention of recontamination. Absolute filtration is a
relative term dependent on the particle size of interest. HEPA and ULPA filters are very
efficient for particle sizes down to fractions of a micrometer in diameter. These filters,
provided they are leak free and well sealed at the edges, are a reliable source of high
quality essentially particle free air. Leaks or seal failures can significantly degrade their
performance11. Preventing, or more properly, controlling the recontamination of the air
stream requires an understanding of how air volumes interact with one another as well as
how they are influenced by the movement of objects within their volume.
HEPA and ULPA Filtration
HEPA filters used in cleanroom environments are generally rated for 99.99% efficiency
at a particle diameter of 0.3 micrometers. ULPA filters are rated for 99.9995% efficiency
at a particle diameter of 0.12 micrometers. MA2SI uses ULPA filters in most
applications. This provides reliable clean and sterile air at the filter face provided the
February 10, 2009
face of the filter starts clean. The efficiency of the filter can actually increase as the filter
begins to accumulate particles from the air, but as this occurs the pressure drop across the
filter begins to increase and the velocity of clean air at the filter face may decrease if the
volume of air passing through the filter is allowed to decrease due to that backpressure.
Prefilters help reduce the rate at which the backpressure builds and so extend the useful
life of the ULPA filter. If the filters are not monitored and are replaced as needed, then
the filter can form leaks that destroy the integrity of the filtration system11. Filter
maintenance is the same for any air filtration system. Design and operational protocols
are required to maintain the quality of the air created by the filters.
Encroaching Air Volumes and Contaminants
The environment in a cleanroom is often considered as a single air volume in which
activities take place that compromise cleanliness. A more useful model is to consider the
cleanroom as a number of interacting air volumes, each volume being characterized by its
unique flow pattern and interaction with objects in the space. There are dozens of these
volumes. One example is the boundary region between two adjacent filters. Each filter
has a active area and a boundary represented by the edge of the filter and the edge-sealing
device. A “dead air” boundary space results between the active filter face area of two
filters. This boundary space can house a tunneling airflow that can and will draw
particles from outside the filter face into the space between the two filters and
periodically inject those particles into what is suppose to be a clean air flow (Figure #2).
Figure #2: Tunneling of Particles Between Filters in a Down-Flow Cleanroom
A heat source or other disturbance to the flow can inject particles into the region between rows of
HEPA filters. If this area is flat and more than 2 centimeters wide a vortex can form that will carry
these particles for many meters. The particles will gradually rain out onto other surfaces in the room.
February 10, 2009
Additional air volumes result from the presence of heat sources in the room. All heat
sources tend to generate particles that are then caught up into the convective flow of air
rising as a result of the thermal gradient. Each heat source creates a convective air
volume that will interact with the clean air volume. The heat source may be a lamp, a
laser, a heater, a cleanroom operator, a patient, or a surgeon, etc. All of these and more
create a thermal gradient and introduce particles into the air volume moved by that
gradient that then degrade the clean air volume. These air volumes behave in predictable
ways following well documented patterns. The convective flow from the human body
tends to follow the body due to the Coanda effect12,13. This volume becomes detached
above the body where it mixes with other volumes in the
The face mask, if not self ventilating, is another source of
a unique air volume14. The mask is a filter if it is not
securely sealed to the face and allows unfiltered
ventilation into the cleanroom. The typical cleanroom
mask is far superior to the typical surgical mask in that it
works with the full hood to essentially cover all exposed
skin except the eyes and part of the forehead. The typical
surgical mask leaves most of the face and neck exposed
along with ventilation gaps that carry more and more of
the tidal flow as the mask begins to collect condensed
water vapor from the breath. Tests conducted in surgical Figure #3: Contaminant Flow
theaters have shown the increase in viable bacteria with from a Person in a Down-Flow
time as a result of the air volume escaping the mask. Cleanroom
The more these interfering air volumes can be isolated In a conventional or full down-flow
from the clean air volume the better the control over the cleanroom particles from people in the
clean air volume. This is where the MA2SI focused room are entrained in the air impinging
on the patient. This is the result of the
clean air system really stands out. By providing clean air convective flow, Coanda effect, and the
with a single filter near the area of greatest concern a Bernolli effect.
level of cleanliness can be maintained in a restricted area
that is superior to that provided by a conventional or even
a full laminar down-flow cleanroom15. There are design
configurations that can moderate the effect of many of
these interfering air volumes, but people remain a
problem and have always been identified as the single
greatest source of contamination in most cleanroom
environments. Figures #3 and #4 illustrate the difference
in how these air volumes behave in a conventional down-
flow cleanroom and a cleanroom with the MA2SI unit.
Figure #4: Focused Clean Air
Focused clean air creates a barrier zone
that minimizes entrainment of particles
from the room, even those from people
6 in close proximity.
February 10, 2009
A study funded by NASA was published in 1974 on the topic of cleanroom technology
applied to surgery16. This study and other similar studies concluded that cleanroom
technology applied to surgery reduced the frequency of infections17,18,19. In this study
they included a primitive form of the focused clean air concept. Current knowledge of air
flow patterns in the vicinity of people and a better appreciation of fluid dynamics has lead
to the MA2SI focused clean air approach but the role of clean air in the in the hospital
surgical theater has been well established over the years. The failure to appreciate “point
of use” filtration has had severe consequences20. The MA2SI focused clean air approach
significantly improves on the concept of “point of use”. By moving the clean air source
directly to the patient the patient’s air volume in the vicinity of the wound is further
protected from the environment in general.
Microlab Northwest began using the focused clean air approach in sample preparation in
1995 using an early model of what would become the MA2SI model E100. It replaced a
horizontal laminar flow clean work station with a laboratory footprint of 35 square feet.
The E100 resulted in a laboratory footprint of 14 square feet when in use and of less than
4 square feet if needed. This represented a significant savings in the cost of laboratory
floor space. The clean environment was required because of the contamination sensitive
nature of much of the work done in this laboratory. Many of the samples consist of just a
few particles that need to be isolated and analyzed. An uncontrolled environment would
not allow this type of work to be conducted with confidence. The MA2SI E100 has
demonstrated its reliability and durability in this laboratory environment over many
A recent paper by Marshall T. Partington, M.D., F.A.C.S., of Redmond, Washington
reports on the application of MA2SI focused clean air in his clinic over a period of nearly
two years22. The perioperative infection rate in that facility was less than 1%.
Satellite Assembly Area
The Boeing Company has successfully used the E100 in a number of critical satellite
assembly areas to protect hardware and for critical assembly tasks. Most of these
applications have been in conventional cleanrooms where additional protection was
The MA2SI focused clean air concept is continuing to evolve. Applications for mobile
field hospitals and for emergency response facilities are being developed. These designs
overcome many of the shortcomings sited in a recent paper on microbial control in
mobile hospital operating rooms24. Plans are underway for the development of test
chamber to improve and discover new applications for the focused clean air technology.
February 10, 2009
New applications and continued improvements in the existing product line are expected
in the near future.
The focused air approach is the only approach that minimizes the effect of people near
the surface requiring the greatest cleanliness while maintaining a high degree of both
physical and visual access. The MA2SI units are designed with cleanliness in mind. The
exterior is designed to minimize surface area and optimize cleanability. There are no
hidden corners or crevices, projections, or sharp corners. The air intake to the system is a
functioning part of the air handling system on the portable units and is used to direct
activity generated contaminants through the prefilters and then through the ULPA. The
prefilters may include solvent or acid gas filters if required. They are easily maintained
with minimal disruption of the environment when maintenance is required. MA2SI units
have been used in surgical theaters, clinics, analytical laboratories, satellite assembly
facilities, and manufacturing environments. These units have performed well in every
case. A superior level of cleanliness control, including control of microorganisms, can be
achieved using MA2SI focused clean air systems to augment existing facilities, upgrade
older facilities, or as portable units to convert areas with limited environmental control
into stations with a significant degree of cleanliness. The MA2SI approach of focus
clean air presents a dramatic, cost effective improvement over past cleanroom design
workarounds and expensive laminar flow cleanrooms for many applications. For surgical
theaters, it puts state-of-art facilities easily within the budget of small rural hospitals and
1. Friedlander, S. K., Smoke, Dust and Haze: Fundamentals of Aerosol Behavior,
Wiley-Interscience, pp. 34-36, 1977.
2. Herdan, G., Small Particle Statistics, Academic Press, Inc., London, pp. 56-58
and 344, 1960.
3. Ibid, pp. 22-27 and 175-181.
4. Crutcher, E. R., PARTICLES AND HEALTH: ENVIRONMENTAL FORENSIC
ANALYSIS, American Industrial Hygiene Association Professional Development
Course 124, pp. 23-43, June 2, 2007.
5. Personal Experience in a Class 10 Cleanroom
6. Spengler, John D., Jonathan M. Samet, and John F. McCarthy (editors), INDOOR
AIR QUALITY HANDBOOK, McGraw-Hill, pp. 57.4-57.6, 2000.
9. Chosky, S. A., D. Modha, and G.J.S. Taylor, “Optimisation of ultraclean air”,
THE JOURNAL OF BONE AND JOINT SURGERY, vol. 78-B, no. 5, pp.835-
837, Sept. 1996.
10. Deijkers, R.L.M., R.M. Bloem, P.L.C. Petit, R. Brand, S.B.W. Vehmeyer, and
M.R. Veen, “Contamination of Bone Allografts”, THE JOURNAL OF BONE
AND JOINT SURGERY, vol. 79-B, no. 1, pp.161-166, Jan. 1997.
February 10, 2009
11. Spengler, John D., Jonathan M. Samet, and John F. McCarthy (editors), INDOOR
AIR QUALITY HANDBOOK, McGraw-Hill, p. 9.22, 2000.
14. Edmiston, Charles E. Jr., Gary R. Seabrook, Robert A. Cambria, Kellie R. Brown,
Brian D. Lewis, Jay R. Sommers, Candace J. Krepel, Patti J. Wilson, Sharon
Sinski, and Jonathan B. Towne, “Molecular epidemiology of microbial
contamination in the operating room environment: Is there a risk for infection?”,
SURGERY, vol. 138, no. 4, pp.573-582, Oct. 2005.
15. Blom, A.W., A.H. Taylor, G. Pattison, S. Whitehouse, and G.C. Bannister,
“Infection after total hip arthroplasty”, THE JOURNAL OF BONE AND JOINT
SURGERY, vol. 85-B, no. 7, pp.956-959, Sept. 2003.
17. LeDoux, R.H. and E. A. Gustan, HOSPITAL ENVIRONMENTAL
MONITORING, final report prepared for Swedish Hospital Medical Center under
Contract No. 080573, Boeing Aerospace Company, Seattle Washington, 1974.
18. Charnley, J., “Postoperative infection after total hip replacement with special
reference to air contamination in the operating room”, Clinical Orthopedic
Surgery, vol. 87, pp.167-187, 1972.
19. Nelson, C. L. and A. S. Greenwald, “Clean air and total hip arthroplasty”, CLEVE
CLINICAL QUARTERLY, vol. 39, no. 3, pp. 101-107
20. Lutz, Brock D., Jiankang Jin, Michael G. Rinaldi, Brian L. Wickes, and Mark M.
Huycke, “Outbreak of invasive Aspergillus infection in surgical patients,
associated with a contaminated air-handling system”, CLINICAL INFECTIOUS
DISEASES, vol. 37, pp. 786-793, 2003.
21. Personal Experience, Owner of Microlab Northwest.
22. Partington, Marshall T., INITIAL EXPERIENCE WITH THE MA2SI UNTRA
CLEAN AIR FILTRATION SYSTEM IN AN OFFICE BASED
AMBULATORY SURGICAL UNIT, Report for MA2SI, Feb. 7, 2009.
23. Personal Experience, Contamination Control Engineer, Boeing Aerospace