|
Determining Negative Air, Air Changes, Containment During Mold
Remediation
By Gary Kruse
Advances in technology and science continue to occur at ever increasing
speeds. As more information is being produced, companies must stay abreast
of new developments or find themselves falling behind their competitors.
The disaster restoration industry is certainly no exception.
Contractors and suppliers are regularly developing new techniques and
products that enhance the performance of restoration contractors.
Complicating matters is the fact that the medical and scientific
communities are continuing to discover the effects on human health of some
of the many contaminants that professional restoration contractors face on
a daily basis.
There is probably no subject that has received more attention in the
restoration industry in the past year than that of mold and the health
effects of mold exposure. This topic has not just been isolated to the
restoration arena. There have been some very high profile cases that have
received national media attention.
Mold-related problems have also caught the attention of many in the
legal community who are beginning to view mold as "the next
asbestos." It is of utmost importance that every contractor coming in
contact with structures contaminated with mold not only understand the
proper procedures and techniques used in remediation, but also the
techniques used to avoid contaminating the structure further by releasing
additional mold spores. This is critical in protecting their employees
from the potential health effects of mold and their businesses from the
possible cost effects of litigation.
Three terms that continually arise in discussions and in protocols for
mold remediation are: "containment," "negative air,"
and "air changes per hour." These different terms are all
interrelated and all involve the use of air scrubbers or negative air
machines. This article will attempt to describe and define each of these
terms and how they relate to a typical mold remediation job.
Negative Air Containment Principles
Containment is the process of isolating a contaminated area from the
rest of the facility. This is nothing new. For years, restoration
contractors have used common sense and hung a piece of poly or a tarp over
a doorway to prevent the spread of contaminants from one area to another.
Today the process is a little more sophisticated, but the end goal is the
same - to prevent the contaminants from spreading.
Negative pressure can be described as a partial vacuum and can be
demonstrated with the use of a drinking straw. Sucking on one end of a
straw creates a negative pressure zone inside the straw. Air flows into
the opposite end of the straw because of the negative pressure inside the
straw. This pressure differential is maintained as long as more air is
removed from the straw than can be replaced. Containment areas under
negative pressure behave in the same way, as lower pressure inside the
containment pulls outside air in and prevents the contaminated air from
escaping.
There are basically two types of containment: source containment and
full area containment. Severity of the mold growth and the size of the
contaminated area will generally dictate which type of containment to
use..
Local Containment Procedures
When the contaminated area is small and easy to dispose of without
causing further contamination, simple source containment may be an option.
Negative pressure is generally not used during this type of operation.
Removing a moldy ceiling tile and sealing it in a plastic bag is a good
example of a source containment application. This method should only be
used when there is no chance that any appreciable number of mold spores
will be released while the contaminated object is being removed and
bagged.
Source containment can also be used with localized negative pressure if
a very small area with mold must be removed. A one-square foot patch of
mold on a wall would be an example. A small piece of poly can be taped
around the area to be removed. A small HEPA vacuum can be used for this
purpose by inserting the inlet nozzle inside this small containment to
create a negative pressure and to vacuum up released particles.
Full Containment Procedures
Full containment is used when the contaminated area is larger or
requires some invasive activities to remove the mold source. In full
containment the entire containment area must be kept under negative
pressure at all times during the mold abatement process to prevent the
escape of any air that could potentially contaminate other parts of the
facility. An example of this type of containment would be the removal of
mold-contaminated wallboard.
If the contaminated area is small, an enclosure of polyethylene
sheeting is typically constructed that is large enough to completely cover
the work area and to accommodate a worker. The poly can be taped, glued or
tacked to the wall area around the portion that is being removed to form
one side of the containment. Adjustable poles or similar devices can be
used to form the other three sides.
When the structure is complete it should look like a large shower or a
tent made of poly, with a double flap door to allow entry without losing
negative pressure. If the structure is built properly, it will be almost
airtight around the perimeter and will allow make up air to enter through
the door opening.
If the contaminated area is larger a full-scale containment area must
be erected. Poly is applied to cover all the walls, floor, and ceiling of
the area under containment. It may be necessary to also construct
airlocks, pass throughs, and equipment rooms, depending on the work that
must be done and the equipment required.
Full containment always requires that negative pressure be maintained
inside of the containment area. The purpose of the negative pressure is to
ensure that any directional air movement is from the clean air environment
into the contained area, and not the other way around. This ensures that
contaminated air from inside the containment will not flow out through
doors, openings, cracks, etc. into the clean area.
Creating A Negative Pressure Containment
Negative pressure in containment is typically achieved using some type
of air moving equipment. For many years restoration contractors have used
fans for this purpose. If a fan is placed in a window and other doors and
windows are closed negative pressure will be created inside the room. Air
will stream into the room from the space under the door and other small
openings. Since contaminants will not flow against this air stream the fan
is acting as a simple negative air machine.
Today's contractor uses negative air machines or even more
sophisticated portable air scrubbers to accomplish what the fan
accomplishes in the above example and more. These machines contain high
output blowers designed to move large quantities of air. They also contain
multi-stage filtration systems to trap the contaminated particles that are
being pulled into the unit by the blower.
PAS Placement At The Jobsite
There are two ways that a negative air machine or air scrubber can be
used to create negative air pressure. If the containment is large enough,
the machine can be placed inside the containment and a flexible exhaust
air hose (flex duct) can be run from the outlet of the machine to an area
outside of the containment. In some instances, space restrictions or other
reasons may necessitate that the machine be left outside of the
containment. In these installations the flex duct is attached to the inlet
of the machine and used to draw air out of the containment.
What's The Difference?
The terms negative air machine and portable air scrubber are sometimes
used interchangeably since they can both be used to achieve a negative
pressure inside of a containment area. However, there are some significant
differences between the design and capabilities of the two types of
equipment. In essence, all portable air scrubbers are negative air
machines but all negative air machines are not portable air scrubbers.
Negative air machines were developed in the early 1980s for the
asbestos abatement industry. They are designed to HEPA filter large
volumes of air and generally used in larger commercial and industrial
facilities that are typically unoccupied. These machines are left on a
jobsite for a lengthy period of time, so not much concern is given to
their size, weight, or ease of transport. Consequently, they are basically
large, heavy boxes on four hard casters that move a lot of air and require
at least two people to carry.
Portable air scrubbers (PAS) were introduced in the early to mid 1990s
for a wide variety of disaster restoration applications in addition to
creating negative pressure containments. These devices were specifically
developed to provide a much more lightweight, portable and maneuverable
alternative to negative air machines.
A true portable air scrubber can be transported to the job site by one
person and can fit through much tighter spaces than a negative air machine
without damaging the customer's facility or property. For those reasons,
portable air scrubbers usually have an upright design. Features like
built-in stair climbers with non-marking belts and transportation dollies
with non-marking wheels are standard on many PAS.
Properly designed PAS also utilize special fans that have the capacity
to pull high volumes of air through dirty filters to maintain sufficient
airflow and negative pressure. This makes them especially useful in mold
abatement applications where a great deal of airborne debris can be
created during the remediation process.
Variable speed controls are especially useful on smaller projects where
exhausting too much airflow from the containment could cause the poly
walls to be pulled away from the containment frame. This feature is
particularly important since a large number of mold remediation projects
require small containments. The single speed negative air machines simply
cannot be used on many of the smaller containments without taking the risk
of collapsing the containment. Operating hour meters on PAS's are also
useful for tracking operating hours for filter changes and for billing
purposes.
How Much Airflow Will You Need?
Most specifications for containment areas require that a pressure
differential of 0.02" of water column be maintained between the
containment area and the surrounding area to achieve the necessary
negative pressure. It is a good practice to use a pressure monitor to
constantly monitor the pressure differential. This gauge should be viewed
regularly during the containment and the results recorded in the event
questions about the integrity of the containment are broached at a future
date.
There are too many variables to establish a fixed formula to determine
how many negative air machines or PAS would be needed to maintain the
0.02" on a given job. The integrity of the seals around the
containment and the size and type of door openings are two important
issues that affect the pressure differential. Obviously, huge openings for
the air to enter the containment will require a significant amount of air
to be exhausted to maintain a negative pressure. Conversely, if there is
little air entering the containment, less air is required to be exhausted.
A room with very poor poly installation and large holes in the poly may
require multiple negative air machines to maintain a 0.02"
differential whereas with better seals and well-built doors one machine
may suffice.
Some contractors use a visual test to estimate if their containment has
good negative pressure. When the negative air machines are operating, the
walls of the containment should be sucked in slightly. If they are not,
chances are there is not an adequate pressure. If they are bowed in
significantly and look like they are going to be pulled off the wall,
there is most likely too much pressure and either more air should be let
in or less air should be exhausted.
This visual test is not a substitute for a pressure monitor, but it can
provide a way to evaluate the integrity of the containment at all times
and from any location. Other contractors use smoke tubes to periodically
test for directional airflow.
One important aspect that must be considered during the planning of any
containment is the effect of filter loading on the airflow of the negative
air machine or air scrubber. During many mold remediation projects
significant amounts of debris can become airborne. As filters become
loaded with debris airflow can quickly drop by 50 percent or more from the
"clean filter" airflow, especially in machines equipped with
forward-curved "squirrel cage" blowers. If this loss of airflow
is not accounted for in the planning process, it is very possible that
negative pressure could be lost.
Air Changes Per Hour
The third term that is used in protocol for mold remediation projects
is "air changes per hour," or ACH. ACH measure the rate at which
fresh air is replacing contaminated air within containment. ACH refer to
how many times per hour an amount of air equal to the total volume of air
in the contained space is exhausted and replaced with clean air from
outside containment. Most mold remediation projects call for four air
changes per hour. Some professionals recommend designing the system for
six ACH to provide an extra cushion for filter loading and other
variables.
To determine air changes per hour it is necessary to know the size of
the containment area and the actual output of the negative air machine or
scrubber.
The 4.8 changes per hour shown above would exceed the 4 changes per
hour specification. However, if the filters were allowed to get too dirty
and the airflow was reduced to 1,200 cfm, one machine would not provide
the necessary air changes. (1,200 cfm x 60 = 72,000 cf per hour; 72,000/
25,000 = 2.9 air changes per hour.) This is where air scrubbers have a
distinct performance advantage over negative air machines since they are
designed to withstand much more filter loading while maintaining high
airflows. It is very possible on some jobs that one air scrubber can be
used where it would require multiple negative air machines.
Gary Kruse is general manager for the IAQ Products Division at
Abatement Technologies, Inc. in Duluth, Ga. He has been actively involved
in the asbestos abatement, duct cleaning, mold abatement and air
filtration industries for more than 12 years. He has written numerous
articles relating to the design and use of air filtration equipment for
these industries. You can reach him by calling (800) 634-9091 or by e-mail
at gkruse@abatement.com.
|