A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

Threshold: The dose or exposure level below which a significant adverse effect is not expected.

Threshold Limit Value (TLV): The concentration of an airborne substance to which an average person can be repeatedly exposed without adverse effects. TLVs may be expressed in three ways: (1) TLV-TWA--Time weighted average, based on an allowable exposure averaged over a normal 8-hour workday or 40-hour work- week; (2) TLV-STEL--Short-term exposure limit or maximum concentration for a brief specified period of time, depending on a specific chemical (TWA must still be met); and (3) TLV-C--Ceiling Exposure Limit or maximum exposure concentration not to be exceeded under any circumstances. (TWA must still be met.)

Time-weighted Average (TWA): In air sampling, the average air concentration of contaminants during a given period.

Topography: The physical features of a surface area including relative elevations and the position of natural and man-made (anthropogenic) features.

Total Petroleum Hydrocarbons (TPH): Measure of the concentration or mass of petroleum hydrocarbon constituents present in a given amount of soil or water. The word "total" is a misnomer--few, if any, of the procedures for quantifying hydrocarbons can measure all of them in a given sample. Volatile ones are usually lost in the process and not quantified and non-petroleum hydrocarbons sometimes appear in the analysis.

Toxicity: The degree to which a substance or mixture of substances can harm humans or animals.

Transmissivity: The ability of an aquifer to transmit water.

Transpiration: The process by which water vapor is lost to the atmosphere from living plants. The term can also be applied to the quantity of water thus dissipated.

Treatment: Any method, technique, or process designed to remove contaminants from a given media (e.g. water, air).

Turbidity: A cloudy condition in water due to suspended silt or organic matter.

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z