The dose response effect from exposure to radioactive materials was not well known, as evidenced by the fact that so many of the early researchers died of radiation poisoning. Even as knowledge concerning the health effects of ionizing radiation grew, specific dose response correlations remained uncertain. Consequently, the approach to keep all exposures to radiation as low as reasonably achievable seemed to be prudent.
Thanks to the American Conference of Industrial Hygienists (ACGIH), we have the threshold limit values for chemical substances or TLVs. The 1968 edition was adopted and made enforceable by OSHA.
My experience suggests that many people, including some industrial hygienists, are misguided in their use of TLVs and PELs (permissible exposure limits), often treating them as if they were an accurate indicator of dose response for chemical substances. As a practical matter, they are not! In fact, the introductory notes in the ACGIH TLV booklet are filled with important caveats with respect to the use of the TLVs.
Unfortunately, many people do not read the introductory notes and instead immediately consult the tabular data for the TlV that interests them.
What’s the TLV for cyclohexanol? Answer: 50 ppm (parts per million). Well, that’s all I need to know; if I keep exposure levels below 50 ppm, employees will be protected, right? Not necessarily!
The prefatory note in the TLV booklet titled, “Introduction To the Chemical Substance,” which I have discovered many don’t read, offers many cautions with respect to the use of the TLVs. For example, it states, “These TLVs 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 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 any 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 or where substances and processes differ.”
It continues by stating that, “These TLVs are not fine lines between safe and dangerous concentrations nor are they a relative index of toxicity. In other words, a substance with a TLV of 1 ppm is not 10 times more toxic than a substance with a TLV of 10 ppm because the TLVs may have been set for different reasons: in one case to prevent respiratory irritation, in the other to prevent long-term kidney damage. These two health effects cannot be compared nor can the two TLVs.”
In addition to this, the TLV committee takes a rather simple approach to exposures that are a mixture of chemicals. It assumes that the effects are additive. In reality, we don’t know how mixtures of chemicals react in the body. A metabolite of one substance may chemically react with the metabolite of another substance to produce an adverse effect not predicted by exposure to either of the initial contaminants.
This, coupled with fact that the TLVs themselves are imprecise and are arrived at by consensus of a panel of toxicological experts who have reviewed the scientific literature, suggests that perhaps the concept of ALARA should be applied to chemical exposures. As a matter of fact, one of the preface notes says as much, stating, “The best practice is to maintain concentrations of all atmospheric contaminants as low as practical.”
Contributing Editor John F. Rekus, PE, CIH, CSP, has over 30 years' experience in occupational safety and health, spanning industry, government and education, and more than 10 yearss experience in technical program planning, development and administration. He has extensive knowledge of technical standards, inspection techniques and methods of sampling and analysis applicable to the investigation and control of workplace safety and health hazards. He can be reached at [email protected]