During the Middle Ages a superstitious society believed that black cats portended bad luck. But a program to rid society of black cats brought little improvement in hunger and misery. So, it was concluded that not just black cats, but all cats must be evil. A massive campaign soon rid Europe of all its evil cats. Rats and mice proliferated then, and a great plague descended on Europe.

I'd like to suggest that airborne exposures are industrial hygiene's black cats. The further they are reduced, the more workers become endangered by dermal exposures. The industrial hygiene profession argues endlessly over what represents a safe airborne exposure limit. If workers are sick, the universal response is to lower the airborne exposure limit. But the profession is misled by a myth that says lowering airborne exposures will better protect workers.

The TLV myth

Lowering threshold limit values can actually increase exposures. The TLV myth is killing the industrial hygiene profession and making workers ill. I cite several recent examples to support my professional heresy:

In 1992, OSHA issued a standard to regulate the safe use of methylene-dianaline (MDA). The agency cited biological monitoring data showing that 95 percent of all documented MDA exposures were via skin contact. Then it set an airborne PEL at 10 ppb to protect the five percent of inhalation exposure cases and rejected as *unscientific* a biological limit to protect the 95 percent of cases that were dermal exposures.

Evidence that glycol ethers, used as solvents in the electronics industry, caused reproductive hazards and miscarriages led recently to a new regulation. Dermal absorption, in part due to the use of ineffective gloves, was concluded to be the primary exposure. The new OSHA standard lowered the airborne exposure limit 100-fold and merely warned users to prevent skin contact. Enforcing biological monitoring to measure skin absorption was again rejected.

Occupational asthma reduction is part of NIOSH's top priority "national occupational research agenda" (NORA). Latex gloves and isocyanates, the key component in polyurethane formulations, are linked to respiratory sensitization and asthma. But recent research indicates that initial sensitization occurs through skin exposure. Later, inhalation elicits the respiratory symptoms.

Substituting chemicals is a commonly recommended method for reducing exposures to toxic chemicals. Typically, low vapor pressure chemicals get substituted for volatile chemicals. Airborne exposures are reduced and compliance with TLVs documents a safe workplace.

But what we have really accomplished is to substitute an inhalation hazard (volatile) for a dermal hazard (non-volatile). Then we continue to ignore dermal exposures which we have just effectively increased.

For example, substituting low volatility isocyanates (MDI) for volatile TDI has resulted in substantial reductions in airborne exposures. Yet some industries are sensitizing five times more workers to MDI than were ever sensitized to TDI.

The industrial hygiene paradox

When using volatile TDI, workers were taught to avoid spills and surface contamination because such accidents could cause airborne levels to exceed the PEL. Ironically, workers thereby avoided skin contact by avoiding air exposures.

That was dumb luck. But the practice of industrial hygiene should be foremost the exercise of common sense. Questioning our assumptions, then using good science to test and verify those assumptions is good practice. Until we dispel the scientific black cat myth that compliance with TLVs protects workers, we do a disservice to the workers, the industries that employ our services, and our profession.