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14.2 Threshold Limit Values 431

Society of Heating, Ventilating, and Air-conditioning Engineering (ASHRAE) or
even individual researchers.
The recommended TLVs are not the fine lines between safe and dangerous
concentrations, nor are they a relative index of toxicity. Serious adverse health
effects do not necessarily take place as a result of exposure to the indoor air
pollutants above their TLVs. It is the best practice to maintain concentrations of all
indoor air pollutants as low as practical.
According to the exposure period, TLVs can also be classified into the following
three categories,

(1) Time-Weighted Average Threshold Limit Value (TLV-TWA),
(2) Short-Term Exposure Limit Threshold Limit Value (TLV-STEL), and
(3) Threshold Limit Value—Ceiling (TLV-C).
TLV–TWA is the time-weighted average concentration based on 8 h workday
and a 40 h workweek, when all workers may be repeatedly exposed, day after day.
The second one, STEL is defined as a 15 min TWA exposure that should not be
exceeded at any time during a workday even if the 8 h TWA is within the TLV-
TWA. TLV-STEL is the concentration to which it is believed that workers can be
exposed continuously for a short period of time without suffering from (1) irritation,
(2) chronic or irreversible tissue damage, or (3) narcosis of sufficient degree to
increase the likelihood of accidental injury, impair self-rescue, or materially reduce
work efficiency. STELs are recommended only where toxic effects have been
reported from high short-term exposures in either humans or animals. There should
be at least 60 min between successive exposures in this range. An averaging period
other than 15 min may be recommended when this is warranted by observed
biological effects.
Example TLVs of typical indoor air pollutants are listed in Table 14.2. More
data can be found in literature such as [7] guidelines for indoor air quality.
American Conference of Governmental Industrial Hygienists (ACGIH) also pub-
lishes annual threshold limit values for chemical substances and physical agents and
biological exposure indices. Note that many air pollutants do not have all the three
TLVs. It is important to observe whether any one of these types of TLVs is
exceeded, a potential hazard from that contaminant is presumed to exist.

14.2.1 Normalized Air Contaminant Concentration

In most indoor environments, more than one air pollutants exist simultaneously. For
example, carbon dioxide and airborne particles are always present in a typical
working environment. When multiple pollutants exist, the effects of some on human
health may be independent of others. In this case, TLVs for individual contaminants
can be used to determine the indoor air quality. In general, however, the combined
effects of multiple air pollutants on the health and comfort are considered greater