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Active pathways for air contamination include HVAC
systems, cooling towers designed into the building, and local
spot humidifiers, dehumidifiers, fans and heaters added by the
occupants after occupancy. If the contaminants are not trapped
or eliminated at the source, the HVAC systems can move them
throughout the building. Since the HVAC system is the primary
active pathway for contamination it plays a significant role in
the solution to the IAQ problem. Filtration is a significant aspect
of the HVAC system where air is recirculated.
Cooling towers that are not properly treated can promote the
growth of legionella which may then be introduced into the HVAC
system and spread throughout the building. Local devices such
as humidifiers and dehumidifiers can promote fungus growth and
spread it into the room space where it is picked up by the HVAC
system and distributed throughout the building. While designers
cannot control what the occupants place in the building after
turnover, they can provide documentation and education as to the
limits of the systems installed.
HVAC systems operating at 100 percent outdoor air produce
indoor air concentrations of contaminants that approach outdoor
concentrations. If there is a 50 percent probability that the
outdoor air will not meet NAAQ standards there is a 50 percent
probability the IAQ will be substandard unless filtration systems
are designed for these contaminants. This includes HVAC
systems with economizers during moderate weather conditions.
REMEDIATING CONTAMINANT LEVELS
Particulate Contaminant Remediation
Methods of particulate remediation used to maintain
acceptable IAQ levels include filters and electronic wire
elements. Particulate filters are used to remove contaminants
from the air stream. Standard particulate filters with a 75 percent
dust spot efficiency will remove all pollen and larger particles
as well as oil smoke. High efficiency HEPA filters having a
DOP efficiency rating of 98 percent have been in use in the
health care industry for many years. These remove viruses and
bacteria. Portable and fixed installation HEPA filters are also
available in the residential and light commercial market.
Gas Contaminant Remediation
Gas phase filtration is used to reduce and control gas and vapor
contaminants. Gas phase filtration systems utilizing virgin coconut
shell carbon activated to 60 percent carbon tetrachloride activity
should be capable of removing parts per billion of Volatile Organic
Compounds, ozone, nitrogen dioxide, and sulfur dioxide.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
INDOOR AIR QUALITY FUNDAMENTALS
According to the EPA one half the population of the
United States lives in areas that do not meet the National
Ambient Air Quality Standards with ozone being the most
prevalent contaminant. Electronic air cleaners are high
efficiency filter devices that produce ozone as a by-product.
Ozone is listed as a harmful contaminant, yet it has been
used to improve IAQ by removing VOCs. If ozone is used
as a filtration or cleaning device it is necessary to follow
these devices with activated charcoal or other gas phase
filters to removing any residual ozone.
Radioactive particles and gases can be removed from air by
devices such as filters and absorption traps, but the gamma
radiation from such material is capable of penetrating solid walls.
EFFECTS OF TEMPERATURE
Lack of thermal comfort control is an often ignored cause of
presumed IAQ problems.
Discomfort is often confused with sick building syndrome.
Thermal comfort is a function of six parameters: air temperature,
mean radiant temperature, relative air velocity, humidity, activity
level, and clothing thermal resistance. The International
Organization for Standardization has adopted the Predicted
Mean Vote (PMV) thermal comfort index to provide a method
to index comfort and discomfort levels taking into account the
six PMV parameters. PMV sensors are available on a limited
basis. These sensors may become commonplace in the future,
but for today most people who recognize that discomfort can
create the perception of IAQ problems are focused on the
temperature and relative humidity aspects of the problem.
Air is perceived to be fresher and less stuffy with decreasing
temperature and relative humidity, and there is a direct
correlation between increases in sick building syndrome and
rises in room temperatures. The effect of temperature is linear
and stronger than humidity. Temperature also effects relative
humidity. For example, if space conditions are 26 C and 16
percent relative humidity and the temperature is decreased 4
degrees to 22 C, then the relative humidity will rise to 20 percent
or a 4 percent increase. These conditions are not uncommon in
the afternoon in the winter in temperate climates. Discomfort
attributable to dry air can usually be remedied by lowering the
air temperature a few degrees
Failing to maintain the temperature will affect the relative
humidity which can have an adverse effect on the growth of
viruses, mold, mildew, and bacteria. A temperature increase of
2.5 to 4 kelvins can double the levels of formaldehyde.
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