ABBREVIATIONS: used with gas exposure limits

ABBREVIATIONS: used with gas exposure limits

Definition of PEL = permissible exposure limit. PEL's are a regulatory limitation to exposure used to specify the allowable exposure to a substance in the workplace and assume that the exposure takes place over an 8-hour shift in a 40-hour work week. Note that there are more stringent exposure limits for higher levels of exposure that may occur over a shorter time interval

Definition of PPM = parts per million of concentration of the gas of interest in air. 1 ppm means one part of gas to 1 million parts of air.

Definition of MAX = maximum exposure in ppm for any individual in the work area over an 8-hour period

Definition of MSDS = Material Safety Data Sheet, published for every chemical or substance that may be hazardous; if an exposure limit has been published for a substance being discussed in an MSDS, that limit, such as TWA or PEL, is required to be included in the MSDS publication.

TWA's and PEL's are not available for most chemicals. There are simply too many chemicals, many of which have not gone through the rigorous scientific study and peer review required. The absence of an exposure limit for a substance should not be used to assume that a substance is not hazardous.


Definition of TIME = as used in environmental exposure literature: the point in time when the maximum exposure will occur from the beginning of any 8-hour period

Definition of TE = total exposure in ppm per hour

Definition of TLV = threshold limit value: the level of exposure that a worker can experience in the workplace without an unreasonable risk of disease or injury. These are not estimates of "level of risk" for different exposure levels nor do they address the different means by which a person may be exposed to a substance.
TLV's are specified by ACGIH, the American Conference of Government Industrial Hygienists. TLV's are guidelines prepared by ACGIH and are solely concerned with health risk. They do not address economic considerations.
TLV's are not regulatory but rather are advisory. (See PEL and TWA which are specified by OSHA).

Definition of TWA = time weighted average exposure level. TWA's are a regulatory exposure limit. The TWA calculation takes into account that exposure level may vary over a time period.