Foul-smelling air can be described as mephitic. If you accidentally combine ammonia and bleach when you're cleaning, you will make a mephitic cloud of gas that is not only smelly but is also dangerous to breathe.
Mephitic comes from the Latin word mephitis, which means noxious vapor or exhalation. Mephitis was also the name of the Roman goddess of foul-smelling gases from volcanoes and swampy areas. She loans her name to the very common North American striped skunk, Mephitis mephitis. If you can remember the Latin name for skunk, you'll have no trouble remembering that mephitic describes air that smells terrible.