Glossary

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acute toxicity	The toxicity of a substance caused by a single exposure to a large dose. The usual method of expressing it, for ingested substances, is LD₅₀ meaning the lethal dosage, expressed in mg/kg of body weight, that will cause death in 50% of victims within 24 hours. For airborne poisons, the usual method is LC₅₀, meaning the lethal concentration expressed in mg/m³ of air, that will cause death in 50% of victims within 24 hours.

Atmospheric Residency Time	The concentration of a "foreign" gas in the atmosphere decays exponentially as it is destroyed, usually hydrolytically by reaction with OH radicals or photolytically by destruction by radiation from the sun. The measure used is the average time required to reduce the concentration to 1/e or 0.3679 of the original concentration. It is sometimes referred to as the folded-e lifetime. Note that it can vary considerably, especially with short-lived gases, as the concentration of OH radicals is variable in place and season. The Atmospheric Residency Time is often referred to as the half-life, but this is incorrect and misleading.



carcinogenicity	The quality of a substance to cause cancer when an animal is exposed to it. Note that different species, including humans, metabolise chemicals in different ways and what is proven to be carcinogenic in one non-human species does not mean that it is carcinogenic to humans or vice versa.

chronic toxicity	The toxicity of a substance caused by continual or repeated exposures to small doses, having a cumulative effect on the body. There are no recognised methods of expressing this. See Operator Exposure Limit




















Operator Exposure Limit (OEL)	The limit of concentration of a substance in air that an operator should be exposed to, as a time-weighted average of 8 hours per day, five days per week, with no risk of damage to his/her health over long periods of time. It is usually expressed in parts per million (ppm) or in mg/m³of air.

Ozone Depletion Potential (ODP)	An index arbitrarily related to the amount of stratospheric ozone that is destroyed, compared with CFC-11 = 1. For example, carbon tetrachloride has an ODP of 1.3, meaning that 1 kg of it destroys 1.3 times as much ozone as would 1 kg of CFC-11.

Permitted Exposure Level (PEL)	The legally binding OEL in the USA as set by OSHA.




toxicity	The quality of a substance absorbed into a living body by any means to cause organic harm to that body or death. See acute toxicity and chronic toxicity.

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