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.