From Coastal Wiki
Definition of Half-life (of a pollutant):
1)The time required for a pollutant to lose one-half of its original concentration. For example, the biochemical half-life of DDT in the environment is 15 years.
2)The time required for the elimination of half a total dose from the body.
This is the common definition for Half-life (of a pollutant), other definitions can be discussed in the article