UE News, March 1997 (Part I of II)
Your buildings heating, ventilation and
air-conditioning (HVAC) systems are designed to provide warm air in the winter, cool air
in the summer and general comfort all year round. If operating properly, they can also
provide sufficient ventilation to prevent the slow build-up of relatively low levels of
toxic gases and vapors in the indoor air.
(But for concentrated sources of pollution, such as welding, or for
large volumes of toxic gases and vapors, as in degreasing and other large industrial
operations, a good local exhaust ventilation system is needed. Well discuss
this in the next column.)
If your HVAC is not operating properly, you can have lots of different
kinds of problems you can become drowsy, especially after lunch, due to the
build-up of gases such as carbon dioxide. You can develop headaches and become nauseous
from the build-up of solvent vapors and/or carbon monoxide in the room air. You can
develop allergic skin problems from various chemicals in the room air, and these can come
either from use in the building or from outdoor air which get drawn into the building.
In general, if the HVAC systems are not operating well, your office can
become a pretty unpleasant place to work in and I havent even begun to talk
about your boss yet!
Basically, your HVAC system draws outside air into the building through
ductwork (essentially, a system of large pipes) which connects to air supply fans. The air
is filtered to remove particles, heated, cooled and/or humidified if necessary, and sent
into your work area through the supplied air vents in your office. Then, the air is drawn
back through the return air vents by the buildings exhaust fans. Some of this air is
exhausted back out of doors, but much of it is filtered and returned to the heating or
cooling unit for recirculation with the incoming fresh air.
TIGHT BUILDING SYNDROME
This recirculation saves the building owners much money, since the
circulating air is already heated or cooled as is appropriate. But this recirculation
contributes greatly to the unpleasant indoor environment often described as "Tight
Building Syndrome" (TBS). Indeed, TBS was a rare phenomenon before 1973; then the oil
embargo drove up the cost of gasoline and heating oil and caused some building owners to
recirculate up to 90 percent of the indoor air. The resulting TBS crisis made
owners back off, but they often still try to recirculate as much of the indoor air as they
feel they can get away with.
No HVAC system, not matter how well designed in the first place, can
continue to operate properly without an active preventive maintenance program. Here are
some basic elements of a good preventive maintenance program. (Thanks go to a fine
pamphlet on the subject by the New York State Civil Service Employees Association, Local
Check for obstructions in the outdoor air intake openings. First,
are they open during the buildings operating hours? Do they have large wiremesh
screens to keep out birds and other "small living creatures?" And are the
screens blocked or obstructed? Are they regularly cleaned?
Are the air filters for the intake air checked regularly? Are
they generally clean when you inspect them? Are the filters the right size to fill the
duct, or are they too small so that unfiltered air can get around them and into the
building? How do the filters smell? If they smell bad, this smell can circulate throughout
every workroom in the building.
HOMES FOR BACTERIA
Is water accumulating in drip pans? Standing water in
pans, ductwork or near intake vents can provide a good home for homeless bacteria and
fungi, and easy entry into the buildings circulating air. These can cause
Legionnaires disease, among others, as well as allergic reactions. Like the drip
pans underneath your home refrigerators, these pans need to be inspected and cleaned
Inspect the ductwork. This is usually done by use of access doors
or by removing elbow joints and checking the ductwork near them. Are the insides of the
ducts clean? Dry? Are there any visible holes in the ductwork, or tears in flexible ducts?
Are fire dampers open? Are all access doors closed between inspections?
Are heating and cooling coils clean and free of dust or lint?
Do the humidification controls and equipment keep the relative
humidity between 20 and 60 percent all year round? Do you notice any standing pools of
water near this equipment? This is an especially important and potential source of
dangerous bacteria in a building.
Workplace Exhaust Ventilation
UE News, April 1997 - Part II of II
Your buildings heating, ventilation and air-conditioning (HVAC)
system is supposed to provide a comfortable indoor climate and protect you from small amounts
of toxic materials. (In the last column, we discussed the HVAC system and how to check if
its working properly.)
But when relatively large amounts of toxic vapors and dusts are released
in a shop, or when the toxic materials are highly concentrated in a small working area, a local
exhaust ventilation system is needed to protect your health and life.
Local exhaust ventilation systems basically consist of ventilation hoods,
air ducts, exhaust fans and the motors which power them. It is very important that the
hoods be placed as close to the source of the toxic emissions as possible, consistent with
letting people do their jobs. This is because the velocity of air being drawn into the
ventilation hood falls off very rapidly with distance from the face of the hood.
(For example, if the velocity of air at the center of a simple round pipe
six inches in diameter is 85 ft./second at the face of the pipe, it is less than 3
ft./second just 9 inches out along the centerline of the pipe.)
CHECK THE EXHAUST SYSTEM
How can you quickly check out how well a local exhaust system is
functioning? One way is to get hold of a velometer, a meter which measures air velocity,
and put it near the hood and see if the wind velocity is steady and toward the hood.
Another quick and dirty check, which is remarkably effective, is to light a cigarette (but
dont smoke it!) and see if its smoke gets drawn into the nearby hood. If the smoke
misses the hood or wanders out into the room, you know the local exhaust ventilation
system is not doing its job.
Dont be surprised if you find this with your own system the first
time you check it. Like HVACs, if these systems are not regularly checked and cleaned,
they quickly become ineffective.
What are some of the common problems which can interfere with a local
exhaust system and what can you do about them?
Cross-drafts caused by open doors and windows, operating machines
and even lots of people walking by can interfere with airflow in a room, and direct air
away from the ventilation hood. Sometimes a ventilation system which worked well in winter
will suddenly go haywire in spring and summer when people open windows and/or install
air-moving fans. Check out your own shop and see if cross-drafts are interfering with the
exhaust ventilation system.
Check that enough make-up air is supplied to the building to
replace the air drawn out by the local exhaust system. If not, the workplace will become
starved for air, and negative air pressure will result. Then air will seep into the room
through door cracks and other openings, creating cross-drafts and poor ventilation. You
might notice this by the way the door seems to stick when you open it up to get into the
Check whether too many new exhaust openings have been added to
the system over time, beyond what the system was designed to handle. If this happens, all
the exhaust hoods will operate at lower air intake velocities, and none will be effective.
Often this happens slowly; one vent is added here, later, another is added somewhere else
far away. No one added vent seems like it will cause any problem, but together they can be
Check to make sure that the air ducts dont have holes in
them caused by corrosive or acid gases flowing through the pipes. Make sure that abrasive
and gritty dusts dont wear away the ducts at elbow joints. And of course, check that
cleaning ports and caps are not left open while the system is operating.
Check that air filters are replaced regularly. If they get
clogged, the airflow in the system is slowed down.
A few more basic reminders:
Make sure that the vent system doesnt draw dust and gases into
peoples breathing zones. For example, an overhead canopy may do a fine job of
drawing vapors or welding fumes up above the source and out of the plant. But if a worker
has to put his or her head over the job regularly, the system is then drawing toxins into
his or her breathing zone. In this case, a slotted side hood across the worker might give
Make sure that exhaust vents to the outdoors from the local exhaust
system are not located near the intake vents of the HVAC system. This common
occurrence causes the toxic emissions to be recirculated into the plant, slowly poisoning
all of the workers instead of rapidly poisoning a few. This kind of a vent system (and
toxic democracy) none of us need. And you wont believe how often this happens in
To avoid it, have the health and safety committee walk around the plant
and on the roof, and locate every air exhaust vent and every air intake vent. If they are
within a few feet of each other at any of these locations, you are having trouble or will
have trouble on some days when the wind is blowing in the right (that is, the wrong)