Time-weighted average concentration definition

TLV The TLV or Threshold Limit Value refers to a safe level of exposure by inhalation. The definition was established by the American Conference of Governmental Hygienists. There are several variations or criteria levels for the TLV. As an example, hydrogen sulfide has a TLV for short-term exposure limits (STEL) of 15 minutes of only 5 ppm. Comparing this to the TLV-STEL of 400 ppm for carbon monoxide provides an indication of the need to be extremely careful when H2S is suspected. Under OSHA Standards, and particularly on MSDS (Material Safety Data Sheets) compounds are associated with a time weighted average (TWA) TLV, which is the allowable concentration for an 8-hour continuous exposure period. For firefighting purposes, the short-term exposure is likely more realistic. 

Threshold Limit Value. (TLV). A set of standards established by the American Conference of Governmental Industrial Hygienists for concentrations of airborne substances in workroom air. They are time-weighted averages based on conditions that it is believed that workers may be repeatedly exposed to day after day without adverse effects. The TLV values are revised annually and provide the basis for the safety regulations of OSHA. They are intended to serve as guides in control of health hazards rather than definitive marks between safe and dangerous concentration. In this book, these are indicated by TLV. 

In the European community, the occupational exposure level is defined as the limit of the time-weighted average of a chemical agent in the air within the breathing zone of a worker in relation to a specified reference period, usually 8 h or a work shift. In addition, to avoid adverse health effects, there is a need to set limits for short-term exposure peaks. In the occupational safety legislation of most countries within the European Community the lists of occupational exposure levels include limits for short-term exposure for many substances, either in the form of definite concentration values or so-called short-term exposure factors. In case of factors, the allowed maximum short-term concentration is calculated by multipHca-tion of the OEL by the short-term exposure factor. Typically, many hazardous substances have factors within the range of 1-4. 

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