Bromine compounds are still used in well drilling fluids, in photographic film, and as an intermediate in the manufacture of organic chemicals. Bromine has long been considered to be possibly essential in humans, but with the support of only limited circumstantial evidence, and no clear biological role. However, one recent study has purportedly identified an essential role in a key protein. Bromine is used preferentially to chlorine by one antiparasitic enzyme in the human immune system.