Nature produces massive amounts of chloromethane and bromomethane.  Most concern focuses on anthropogenic sources, which are potential toxins, even carcinogens. Similarly, great interest has been shown in remediation of man made halocarbons such as those produced on large scale, such as dry cleaning fluids.  Volatile halocarbons degrade photochemically because the carbon-halogen bond can be labile.  Some microorganisms dehalogenate halocarbons.  While this behavior is intriguing, the rates of remediation are generally very slow.