But I Don't Smell Anything

For a variety of reasons, people tend to associate toxic chemicals with "smells."  Many chemicals do have strong, identifiable odors. The association is also made because when people begin to recognize that chemicals cause them problems, they often first notice symptoms caused by perfumes and other synthetic fragrances. Unfortunately, the flip side of this association is that people tend to believe that if they can't smell something, it isn't there or can't be problematic.

The truth is that many dangerous chemicals can be neither seen nor smelled. Carbon monoxide is a well-known example. As with carbon monoxide, people can be greatly affected by other toxins without being aware that they are being exposed to them.

Some chemicals do have a noticeable odor, but in any given situation the odor may not be detected. This is generally due to a phenomenon known as olfactory fatigue (also known as odor fatigue or olfactory adaptation). Olfactory fatigue occurs when prolonged exposure to a compound renders a person unable to smell it any longer.

When people stop smelling an odor, they naturally assume that the odor-causing chemicals have dissipated. This may or may not be true. In fact, in the case of olfactory fatigue, the inability to smell a compound is not due to the fact that there is less of it, but to the fact that there is more of it than the nose can process.

One online source explains the phenomenon this way:

"Have you ever noticed a particular scent upon entering a room, and then not noticed it ten minutes later? This is due to olfactory fatigue. The olfactory sense is unique because it relies on mass, not energy to trigger action potentials. Your ears do not "stop" hearing a sound after a certain period of time, nor do your eyes stop seeing something you may be staring at. This is because both the ears and the eyes rely on energy to trigger them, not mass. In the nose, once a molecule has triggered a response, it must be disposed of and this takes time. If a molecule comes along too quickly, there is no place for it on the olfactory hairs, so it cannot be perceived."

All of our senses, including our sense of smell, can be helpful in warning us of danger. Relying on our sense of smell too heavily, though, isn't wise. Our noses can help us, but the organ we most need to rely on is the brain.