From the Latin effluere
, to flow out.
Effluvia is the run-off, and the gas which comes from rotting organic matter. As such, it can refer to the odor of feces, or to the last givings-off of a freshly dead creature, be they brachial or intestinal.
It is generally agreed that effluvia smell bad to humans. We are able to capitalise on this fact, however, by adding the essence of rotting flesh to natural gas, causing an aversion to the smell, and saving us from unintentional poisoning and asphyxiation.
Effluvia can refer to any resin, waste, or byproduct in production, and is often used poetically to refer to thoughts put in writing. In another sense, it is often considered an aura.