From the ACGIH website: http://www.acgih.org/Products/tlvintro.htm (The little ® means a registered trademark.)
Introduction to the Chemical Substances TLVs®
Threshold Limit Values (TLVs®) refer to airborne concentrations of substances
and represent conditions under which it is believed that nearly all workers
may be repeatedly exposed day after day without adverse health effects. Because
of wide variation in individual susceptibility, however, a small percentage
of workers may experience discomfort from some substances at concentrations
at or below the threshold limit; a smaller percentage may be affected more seriously
by aggravation of a pre-existing condition or by development of an occupational
illness. Smoking of tobacco is harmful for several reasons. Smoking may act
to enhance the biological effects of chemicals encountered in the workplace
and may reduce the body's defense mechanisms against toxic substances.
Individuals may also be hypersusceptible or otherwise unusually responsive to some industrial chemicals because of genetic factors, age, personal habits (e.g., smoking, alcohol, or other drugs), medication, or previous exposures. Such workers may not be adequately protected from adverse health effects from certain chemicals at concentrations at or below the threshold limits. An occupational physician should evaluate the extent to which such workers require additional protection.
TLVs® are based on available information from industrial experience; from experimental human and animal studies; and, when possible, from a combination of the three. The basis on which the values are established may differ from substance to substance; protection against impairment of health may be a guiding factor for some, whereas reasonable freedom from irritation, narcosis, nuisance, or other forms of stress may form the basis for others. Health impairments considered include those that shorten life expectancy, compromise physiological function, impair the capability for resisting other toxic substances or disease processes, or adversely affect reproductive function or developmental processes.
The amount and nature of the information available for establishing a TLV® varies from substance to substance; consequently, the precision of the estimated TLV® is also subject to variation and the latest TLV® Documentation should be consulted in order to assess the extent of the data available for a given substance.
These limits are intended for use in the practice of industrial hygiene as guidelines or recommendations in the control of potential workplace health hazards and for no other use, e.g., in the evaluation or control of community air pollution nuisances; in estimating the toxic potential of continuous, uninterrupted exposures or other extended work periods; as proof or disproof of existing disease or physical condition; or adoption or use by countries whose working conditions or cultures differ from those in the United States of America and where substances and processes differ. These limits are not fine lines between safe and dangerous concentrations nor are they a relative index of toxicity. They should not be used by anyone untrained in the discipline of industrial hygiene.
The TLVs®, as issued by ACGIH, are recommendations and should be used as
guidelines for good practices. In spite of the fact that serious adverse health
effects are not believed likely as a result of exposure to the threshold limit
concentrations, the best practice is to maintain concentrations of all atmospheric
contaminants as low as is practical.
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