Hydrogen Sulfide is a highly toxic gas that is heavier than air. It can cause dizziness, unconsciousness and death. At low concentrations it may smell like rotten eggs, but at higher concentrations, no odor can be detected because it deadens the sense of smell. Carbon dioxide is an orderless, tasteless gas that is heavier than air. It displaces the oxygen supply in the bloodstream, which can cause unconsciousness and death.
Ammonia is a gas that is lighter than air.
It has a pungent smell and can irritate the eyes and respiratory tract. Ammonia displaces oxygen in the bloodstream. Methane is also a gas that is lighter than air. The primary hazard of methane gas is that can create an explosive atmosphere. This gas also displaces oxygen.