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substances that do not have TLV-TWA or TLV-C established, the maximum admissible peak
concentrations must not exceed:

• Three-times the value of the TLV-TWA for no more than 15 minutes, no more than four times per
workday. Exposures must be at least 1 hour apart during the workday.
• Five times the TLV-TWA under any circumstances.

Limitations of Exposure Limits

It is important to remember that WELs have their limitations:

• WELs are designed only to control the absorption of harmful substances into the body following
inhalation. They are not concerned with absorption following ingestion or through contact with
the skin or eyes. So, for example, the concentration of organic solvent in a person’s body may be
at damagingly high levels, even though the WEL has not been exceeded, because most of the
solvent may have been entering through the skin by direct contact with the liquid solvent rather
than by inhalation of solvent vapour.

• They take no account of individual personal susceptibility. This is particularly important in the
case of substances that produce an allergic response; once a person has become sensitised, the
exposure limit designed to suit the average person has no further validity.

• They do not take into account the synergistic (or combined) effects of mixtures of substances
(e.g. the use of multiple substances). Some chemicals are harmful individually but much more
harmful in combination. One example is asbestos and cigarette smoke – both can cause lung
cancer, but the lung cancer risk of an asbestos worker who smokes is multiplied many times
(it isn’t simply doubly dangerous).

• They may become invalid if the normal environmental conditions are changed (e.g. changes in
temperature, humidity or pressure may increase the harmful potential of a substance).

• Some limits do not consider all the possible health effects of a substance (e.g. impact on
the skin, such as dermatitis, would not be considered with an airborne limit).

Comparison of Measurements to Recognized Standards

As we have seen, different measurements are applied in different regions of the world. In the
USA alone, several different groups recommend what occupational exposure limits should be:

• The ACGIH sets Threshold Limit Values (TLVs), as described above.
• The National Institute for Occupational Safety and Health (NIOSH) recommends RELs
(Recommended Exposure Limits).

ENSIGN| Unit IG2 – Element 7 – Chemical and Biological 28
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