Reported releases are annual estimates. The amounts reported could have been released evenly over the course of the year or, possibly, in a single large burst. Release estimates alone are not sufficient to determine exposure or to calculate potential adverse effects on human health and the environment. TRI does not cover toxic chemicals that reach the environment from non-industrial sources, such as dry cleaners or auto service stations. Air releases are reported either as stack emissions or fugitive emissions.
Stack emissions occur through confined air streams, such as stacks, vents, ducts or pipes.
Fugitive emissions are all releases to the air that are not released through a confined air stream. Fugitive emissions include equipment leaks, evaporative losses from surface impoundments and spills, and releases from building ventilation systems. The following charts can be used to analyze the TRI data for Pennsylvania air releases from 1990 through 1996. The EPA's Cumulative Exposure Project uses a computer model to estimate 1990 average outdoor concentrations of selected chemicals from the TRI data. Selected chemicals in TRI 1996 .