Home > Electronic Tutorials > Science Glossary / Terms Dictionary > Science Definitions - T

Science Glossary / Terms / Dictionary

Science Terms Definitions - T

TCE - Trichloroethene (trichloroethylene) is an organic solvent and degreaser.

Technology-Based Limitations - Industry-specific effluent limitations applied to a discharge when it will not cause a violation of water quality standards at low stream flows. Usually applied to discharges into large rivers.

Technology-Based Standards - Effluent limitations applicable to direct and indirect sources which are developed on a category-by-category basis using statutory factors, not including water-quality effects.

Teratogenic - Substances that are suspected of causing malformations or serious deviations from the normal type, which can not be inherited in or on animal embryos or fetuses.

Thermonuclear Weapon - A nuclear weapon that gets a large part of its explosive power from fusion reactions.

Thorium (Th) - A radioactive element with the atomic number 90 and, as found in natural ores, an atomic weight of approximately 232. There are 11 other known isotopes. Thorium is used in the process of making nuclear bombs. Thorium is also used in ceramics, glass and gas mantles.

Threshold - A pollutant concentration [or dose] below which no deleterious effect occurs.

Threshold Dose - The minimum application of a given substance required to produce an observable effect.

Threshold Level - Time-weighted average pollutant concentration values, exposure beyond which is likely to adversely affect human health.

Threshold Limit Value (TLV) - Refers to airborne concentrations of substances and represents conditions under which it is believed that nearly all workers are protected while repeatedly exposed for an 8-hr day, 5 days a week (expressed as parts per million (ppm) for gases and vapors and as milligrams per cubic meter (mg/m3) for fumes, mists, and dusts).

TNT Equivalent - The weight of TNT which would release the same amount of energy as a particular nuclear explosion. One ton of TNT releases approximately 1.2 billion calories (that is, 5.1 kiloJoules per gram). Nuclear explosions are usually measured in kilotons (KT) or megatons (MT).

Tolerances - Permissible residue levels for pesticides in raw agricultural produce and processed foods. Whenever a pesticide is registered for use on a food or a feed crop, a tolerance (or exemption from the tolerance requirement) must be established. EPA establishes the tolerance levels, which are enforced by the Food and Drug Administration and the Department of Agriculture.

Ton - A metric ton is 1,000 kilograms. This is approximately 2,200 pounds, and very nearly equal to a British ton (2,240 pounds). The U.S. ton is 2,000 pounds.

Toxic Substance - A chemical or mixture that may present an unreasonable risk of injury to health or the environment.

Toxic Wastes - Wastes that contain substances in sufficient quantity to impinge harmfully on biological systems.

Toxicant - A substance that kills or injures an organism through chemical or physical action or by altering the organism's environment; for example, cyanides, phenols, pesticides, or heavy metals; especially used for insect control.

Toxicity - The degree of danger posed by a substance to animal or plant life.

Toxicity Assessment - Characterization of the toxicological properties and effects of a chemical, with special emphasis on establishment of dose response characteristics.

Toxicity Profile - An examination, summary, and interpretation of a hazardous substance to determine levels of exposure and associated health effects. See our toxicity profiles.

Toxicology - The study of the adverse effects of chemicals and radionuclides on living organisms.

Trace - A very small amount of a material. Usually used in reference to concentrations which are on the order of or less than 1-10 parts per million.

Trace Metals - Metals normally found in trace amounts due to their insolubility or to their relative lack of abundance in the crust of the earth.


(1) Any method, technique, or process designed to remove solids and/or pollutants from solid waste, waste streams, effluents, and air emissions.

(2) methods used to change the biological character or composition of any regulated medical waste so as to substantially reduce or eliminate its potential for causing disease.

Tritium - A radioactive isotope of hydrogen whose nucleus contains one proton and two neutrons.

TRU (transuranic waste) - Waste that contains more than 100 nCi/g of alpha emitting isotopes with atomic numbers greater than 92 and half-lives greater than 20 years. Such wastes result primarily from fuel reprocessing and from the fabrication of plutonium weapons and plutonium-bearing reactor fuel. Generally, little or no shielding is required ("contact-handled" TRU waste), but energetic gamma and neutron emissions from certain TRU nuclides and fission-product contaminants may require shielding or remote handling ("remote-handled" TRU waste).

Trust Fund (CERCLA) - A fund set up under the Comprehensive Environmental Response, Compensation and Liability Act (CERCLA) to help pay for cleanup of hazardous waste sites and for legal action to force those responsible for the sites to clean them up.

Tumor - Any abnormal mass of cells resulting from excessive cellular multiplication.

Note: To report broken links or to submit your projects please send email to Webmaster