After being spewed into the atmosphere for the past 30 years, mainly through the use of fire extinguishers, ozone-depleting halon concentrations are now decreasing (Nature, Vol 359 No 6394). Although halons H-1301 and H-1211 are each present in the troposphere at a concentration of two parts per trillion, the gases have long lifetimes and carry significant amounts of bromine to the stratosphere, where it destroys ozone.
Under the Montreal Protocol, the manufacture of the gases is to be discontinued globally soon. Halons are stored in extinguishers or stockpiled and their production began declining after 1988. But it is uncertain when halon releases started to decrease.
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