PELs, STELs, and Ceiling Limits

The air we breathe is never contaminant free in the sense that the atmospheric gases comprising air (nitrogen, oxygen, argon, carbon dioxide, and small amounts of other gases) are “littered” with dust particles, pollen, dander, microbes, and chemicals released from processing operations.  This is quite obvious when shafts of sunlight shine through windows revealing minute particles suspended in air. 

So in realizing this, what is considered to be a safe level of exposure to air contaminants, particularly in an occupational setting?

OSHA has developed occupational exposure limits, commonly referred to as permissible exposure limits (PELs), by adopting exposure limits established by the American Conference of Governmental Industrial Hygienists (ACGIH).  These limits are mostly derived from worker experience and research on laboratory animals and are based on an 8-hour time weighted average (TWA). 

OSHA defines a TWA as “the employee's average airborne exposure in any 8-hour work shift of a 40-hour work week which shall not be exceeded."  Therefore, the 8-hour TWA PEL is considered to be “the highest level of exposure an employee may be exposed to without incurring the risk of adverse health effects.”  There are approximately 500 PELs listed in the Z tables of OSHA’s air contaminants standard, 29 CFR 1910.1000.

Additionally, OSHA has established two other legal limits of air contaminant exposure, the short term exposure limit (STEL) and the ceiling limit defined as follows:

  1. Short Term Exposure Limit – Maximum concentration to which workers can be exposed for a short period of time (15 minutes) for only four times throughout the day with at least one hour between exposures.
  2. Ceiling Limit – An airborne concentration of a toxic substance in the work environment, which should never be exceeded.

PELs, STELs, and ceiling limits refer to employee exposure without regard to use of a respirator, in other words with no protection.  STELs and ceiling limits are intended to “cap” excessive exposure where an 8-hour TWA allows a worker to have exposure to airborne concentrations above the PEL, albeit for limited periods provided the average concentration remains lower over 8 hours.

Chemical manufacturers are required to provide information on exposure limits for their products in Section 8, Exposure Controls and Personal Protection of the corresponding Safety Data Sheet (SDS).  Employers, in turn, are required to evaluate the level of employee exposure to chemicals and other hazardous substances and must take appropriate action when limits are exceeded.  First and foremost, engineering controls (product substitution or ventilation) need to be implemented and if not feasible, personal protective equipment with training must be afforded to employees. 

For more information on OSHA exposure limits click here.