Exposure continued long-term average

Aromatic Hydrocarbons. These are the most toxic of the hydrocarbons and inhalation of the vapor can cause acute intoxication. Benzene is particularly toxic and long-term exposure can cause anemia and leukopenia, even with concentrations too low for detection by odor or simple instmments. The currendy acceptable average vapor concentration for benzene is no more than 1 ppm. PolycycHc aromatics are not sufftcientiy volatile to present a threat by inhalation (except from pyrolysis of tobacco), but it is known that certain industrial products, such as coal tar, are rich in polycycHc aromatics and continued exposure of human skin to these products results in cancer. 

The workplace exposure limits are subject to time-weighted averaging. There are two such time-weighted averages (TWA) - the long-term exposure limit (LTEL) or 8 hour reference period and the short-term exposure limit (STEL) or 15 minute reference period. The 8 hour TWA is the maximum exposure allowed over an 8 hour period so that if the exposure period was less than 8 hours the workplace exposure limit is increased accordingly with the proviso that exposure above the LTEL value continues for no longer than 1 hour. 

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