Air pollution

 Definitions

MAC

Maximal Admissible Concentrations. This is a drinking water standard concerning the health of Ontario that is fixed for polluting substances that have unfavorable impact for health above a certain concentration. The quantity time belonging at MAC-value can be exceeded without impact to health depending of the nature and concentration of the parameter.

PEL (TWA)

Permissible Exposure Limit. PEL is an occupational exposure limit established by OSHA's regulatory authority. It may be a time-weighted average (TWA) limit or a maximum concentration exposure limit.

PEL’s can be defined in two different manners become which are discussed in the OSHA regelement concerning air polluted substances, 1910.1000:  

§         De ceiling value –may never exceed the exposure. Sometimes indicated with the character C.

§         8-hour Time Weighted Averages (TWA) - are an average value of exposure over the course of an 8 hour work shift.

TWA levels are usually lower than ceiling values. Thus, a worker may be exposed to a level higher than the TWA for part of the day (but still lower than the ceiling value) as long as he is exposed to levels below the TWA for the rest of the day.

STEL

A Short Term Exposure Limit (STEL) is defined by ACGIH as the concentration to which workers can be exposed continuously for a short period of time without suffering from:  

§         Irritation

§         Chronic or irreversible tissue damage 

§         Narcosis of insufficient degree to increase the likelihood of accidental injury, impair self-rescue or materially reduce work efficiency 

STEL's are generally used only when toxic effects have been reported from high acute (short-term) exposures in either humans or animals. An STEL is not a separate independent exposure limit, but supplements time-weighted average limits where there are recognized acute effects from a substance whose toxic effects generally chronic (long-term) in nature.

For example, one can not be exposed to an STEL concentration if the TLV-TWA (time weighted average for an 8 hour shift; see Permissible Exposure Limit (PEL)) would be exceeded. Workers can be exposed to a maximum of four STEL periods per 8 hour shift, with at least 60 minutes between exposure periods.

TLV

Threshold Limit Values (TLV's) are guidelines (not standards) prepared by the American Conference of Governmental industrial Hygienists, Inc (ACGIH) to assist industrial hygienists in making decisions regarding safe levels of exposure to various hazards found in the workplace.

A TLV® reflects the level of exposure that the typical worker can experience without an unreasonable risk of disease or injury. TLVs® are not quantitative estimates of risk at different exposure levels or by different routes of exposure

Some definitions were taken from MSDS Hyperglossary  (HTTP://www.ilpi.com/msds/ref/).