Sulfur dioxide, a common air pollutant, is a good example. Unfortunately, when we buy a bottle of selenium toner concentrate, we are buying a soluble form of selenium. So the bottles of selenium toner we have on hand contain a toxic substance, although not a hugely toxic nor highly concentrated one. It does not travel through the air to poison us, although the powerful odor of ammonia could make us believe it does. That odor comes from the ammonium thiosulfate which comprises a large part of the selenium toner concentrate.
Ammonia is a dangerous substance in itself, although it is also a very common household chemical.
Ventilation is advised to get rid of ammonia if we are going to be closeted with it in the darkroom. To avoid getting it in the body, avoid drinking it and avoid contact with the bare skin. If a splash occurs and it gets on bare skin, there is no need to fear. Washing it off immediately with soap and large amounts of water suffices. Ansel Adams is reputed to have exposed his hands to selenium toner, as well as every other darkroom substance.