The time that is required for a substance to reach fifty percent or one-half its initial mass. This can be radioactive decay or through chemical interaction. For instance, the half-life of carbon-14 is 5730 years; it decays to nitrogen-14. The atmospheric half life of hydroxyl radical (seconds) is not based on radioactive decay (it's radioactively stable) but on the chemical processes by which it is removed from the atmosphere. The half-life of atmospheric chemical species is often most strongly affected by reactions that have the fastest rate of reaction (see Reaction Rates).