Hazardous Chemicals/Materials

Understanding Exposure Levels

All chemicals or chemical compounds can be hazardous to your health if the exposure is of sufficient length and concentration. In order to prevent exposure, businesses and manufacturers are expected to comply with specific guidelines, however combing through the maze of regulations to ensure compliance can be difficult for even the most thorough and well trained companies. That’s why OEHS, Inc. commonly assists businesses and manufacturers in assessing their compliance with requirements and guidelines pertaining to hazardous chemicals and materials and provides step-by-step plans to enable companies to achieve compliance cost effectively and within reasonable time parameters.

To begin, it is important for employers to understand the basic tenets of the exposure guidelines as well as the laboratory safety standard. The American Conference of Governmental Industrial Hygienists (ACGIH) and the Occupational Health and Safety and Health Administration (OSHA) have developed exposure guidelines known as Threshold Limit Values (TLV’s) published by ACGIH and Permissible Exposure Levels (PEL’s) published by OSHA which are utilized to determine safe exposure levels to chemicals.

Both of these values refer to chemical and physical boundaries under which it is believed nearly all workers may be repeatedly exposed eight hours a day forty hours per week without adverse effect. Neither of these values represent a fine line between safe and dangerous nor are they a relative index of toxicity. Because of the wide variation in individual susceptibility, a small percentage of workers may experience discomfort from some substances at concentrations at or below the published levels.

The only difference between the two exposure levels is that TLV’s are a consensus standard and PEL’s have the force of law. Because of this, it is difficult to update the PEL’s. However, the Threshold Limit Values are updated annually. The OSHA Permissible Exposure Levels are actually based on ACGIH TLV’s which were published in 1968. There have been several attempts to update the PEL’s but the courts have overthrown each effort.

Three categories of TLV’s and PEL’s

The specifics are as follows:

Threshold Limit Value (TLV) or Time Weighted Average (TWA) and Permissible Exposure Level (PEL). This is the time weighted average concentration for a normal 8 hour workday and 40 hour workweek to which nearly all workers may be repeatedly exposed, day after day, without adverse effect The Short-term Exposure Limit (STEL) is the concentration to which workers can be exposed continuously for a short period of time without suffering from any of the following:

  1. irritation
  2. chronic or irreversible tissue damage
  3. narcosis of sufficient degree to the likelihood of accidental injury, impair self-rescue or materially reduce work efficiency.

The STEL is not a separate independent exposure limit; rather it supplements the TWA limit where there are recognized acute effects from a substance whose toxic effects are primarily of a chronic nature.

The Ceiling is a concentration that should not be exceeded during any part of the working exposure. For some substances, i.e. irritant gases, only one category, the Ceiling, may be relevant. For other substances, one or two categories may be relevant, depending upon their physiologic action.