Control of pollutants
at the source is the most effective strategy for maintaining
clean indoor air. Control or mitigation of all sources, however,
is not always possible or practical. Ventilation, either natural
or mechanical, is the second most effective approach to
providing acceptable indoor air.
In the past, most buildings had windows that
opened; airing out a stuffy room was common practice. In
addition, indoor-outdoor air pressure differences provided
ventilation by movement of air through leaks in the building
shell.
Today however, most new office buildings are
constructed without operable windows, and mechanical ventilation
systems are used to exchange indoor air with a supply of
relatively cleaner outdoor air.
The rate at which outdoor air is supplied to a
building is specified by the building code. Supply rates are
based primarily on the need to control odors and carbon dioxide
levels; carbon dioxide is a component of outdoor air, but its
excessive accumulation indoors can indicate inadequate
ventilation.
Most indoor air pollution comes from sources
inside the building. For example, adhesives, carpeting,
upholstery, manufactured
wood products, copy machines, pesticides, and cleaning agents
may emit volatile organic compounds (VOCs), including
formaldehyde.
Environmental tobacco smoke
contributes high levels of VOCs, other toxic compounds, and
respirable particulate matter. Research shows that some VOCs can
cause chronic and acute health effects at high concentrations,
and some are known carcinogens.
Low to moderate levels of
multiple VOCs may also produce acute reactions. Combustion
products such as carbon monoxide, nitrogen dioxide, as well as
respirable particles, can come from unvented kerosene and gas
space heaters, woodstoves, fireplaces and gas stoves.
The outdoor air that enters a
building can be a source of indoor air pollution. For example,
pollutants from motor vehicle exhausts; plumbing vents, and
building exhausts (e.g., bathrooms and kitchens) can enter the
building through poorly located air intake vents, windows, and
other openings. In addition, combustion products can enter a
building from a nearby garage.
Bacteria, molds, pollen, and
viruses are types of biological contaminants that may breed in
stagnant water that has accumulated in ducts, humidifiers and
drain pans, or where water has collected on ceiling tiles,
carpeting, or insulation. Sometimes insects or bird droppings
can be a source of biological contaminants.
Physical symptoms related to
biological contamination include cough, chest tightness, fever,
chills, muscle aches, and allergic responses such as mucous
membrane irritation and upper respiratory congestion.
One indoor bacterium,
Legionella, has caused both Legionnaire’s Disease and Pontiac
Fever.
These elements may act in
combination, and may supplement other complaints such as
inadequate temperature, humidity or lighting. Even after a
building investigation, however, the specific causes of the
complaints may remain unknown.
Pollutant source removal or
modification is an effective approach to resolving an IAQ
problem when sources are known and control is feasible. Examples
include routine maintenance of HVAC systems, e.g., periodic
cleaning or replacement of filters; replacement of water-stained
ceiling tile and carpeting; institution of smoking restrictions;
venting
contaminant source emissions to the outdoors; storage and use of
paints, adhesives, solvents and pesticides in well ventilated
areas, and use of these pollutant sources during periods of
non-occupancy; and allowing time for building materials in new
or remodeled areas to off-gas pollutants before occupancy.
Several of these options may be exercised at one time.
Increasing ventilation rates and air
distribution often can be a cost effective means of reducing
indoor pollutant levels. HVAC
systems
should be designed, at a minimum, to meet ventilation standards
in local building codes; however, many systems are not operated
or maintained to ensure that these design ventilation rates are
provided.
In many buildings, IAQ can be improved by
operating the HVAC system to at least its design standard, and
to ASHRAE Standard 62-1989 if possible. When there are strong
pollutant sources, local exhaust ventilation may be appropriate
to exhaust contaminated air directly from the building. Local
exhaust ventilation is particularly recommended to remove
pollutants that accumulate in specific areas such as rest rooms,
copy rooms, and printing facilities.
Air cleaning can be a useful adjunct to source
control and ventilation but has certain limitations. Particle
control devices such as the typical furnace filter are
inexpensive but do not effectively capture small particles; high
performance air filters capture the smaller, respirable
particles but are relatively expensive to install and operate.
Mechanical filters do not remove gaseous
pollutants. Some specific gaseous pollutants may be removed by
adsorbent beds, but these devices can be expensive and require
frequent replacement of the adsorbent material. In sum, air
cleaners can be useful, but have limited application.
Education and communication are important
elements in both remedial and preventive indoor air quality
management programs. When building occupants, management, and
maintenance personnel fully communicate and understand the
causes and consequences of IAQ problems, they can work more
effectively together to prevent problems from occurring, or to
solve them if they do.
IAQ Summary
