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H

Half-life

The half-life of a substance is the time required for 50% of its amount to be reduced by natural processes of transformation, degradation, or elimination.

Environmental half-life: the time it takes for half of a chemical to break down or disappear in the environment (air, water, soil, sediments). This depends on ambient factors like temperature, light/UV radiation, presence of oxygen, and microbial activity.
Biological half-life: the time it takes for half of a chemical in a living organism to be metabolized, excreted, or otherwise removed from the body.

The concept of half-life is important because it indicates how persistent a pollutant is in nature and in organisms. Substances with long half-lives can accumulate and cause long-term effects, while those with short half-lives tend to be removed more quickly.

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Halogenated compounds

Halogenated compounds are chemical substances that contain one or more halogen atoms (fluorine, chlorine, bromine, iodine, or astatine) bonded to carbon atoms. These compounds are widely used in industrial applications such as solvents, refrigerants, pesticides, and pharmaceuticals.

The term halogenated comes from the word “halogen,” derived from Greek:

“hal-” (ἅλς) meaning salt, and
“-gen” (γεννάω) meaning to produce -referring to the salt-producing nature of halogens when they react with metals (e.g., forming sodium chloride).

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Homologue

In organic chemistry, a homologous series is a sequence of compounds with the same functional group and similar chemical properties in which the members of the series differ by the number of repeating units they contain- Wikipedia