Ozone

Ozone Ghosts, spirits, and other supernatural beings often leave behind evidence of their presence. One type of evidence is ozone, which is a poisonous unstable gaseous form of oxygen, formed from diatomic oxygen by electric discharge or exposure to ultraviolet radiation. Ozone can be detected by its smell. The stronger the smell, the stronger the spirit's presence. Ability to smell ozone differs from person to person, but most people can smell it at about 0.015 ppmv. Ozone is also the primary cause in the scent that precedes a rainstorm. – Pad of Definitions (1.07 Hookman), Official Website