The length of time a chemical species can survive without reacting, photolyzing, dissociating, or otherwise changing into another chemical species. Highly reactive chemicals have short lifetimes. For instance, if the reaction rate of a target species with an attacking species is very fast then the lifetime of the target species will be short. For example, the reaction of smalls radicals like hydroxyl radical in the troposphere is very fast with many common tropospheric species, and therefore the tropospheric lifetime of hydroxyl radical is measured in seconds.