When we encounter foul odors, we instinctively recoil — we may pull our noses up, pinch them with our fingers, or even gag. Is this instictive response of disgust merely a reaction to the fact that something smells bad, or do nasty odors have such a profound effect on us because they warn us of hazard?
How does your sense of smell actually work?
The human sense of smell is activated when olfactory sensory neurons inside the nasal cavity, each able to perceive a different kind of odor, are activated. These neurons — sensors, if you like — then transport that information to the brain, which interprets the odor. A smell isn't just a sense, feeling, or perception, but a physical response to contact with tiny particles emitted by their originating substances.
The particles that trigger your sense of smell can reach the olfacory sensory neurons both directly through the nostrils, and through another channel within the throat. This second channel helps you taste the foods you eat, but can also help you perceive smells, to a lesser extent, when your nose is blocked and you are breathing through your mouth.
Another system, the so-called common chemical sense, is distrubuted all over your face by way of nerve endings, especially around the eyes, the mouth, and the throat. This helps you sense potentially irrititating and hazardous chemicals.
Where do outdoor or environmental odors originate from?
Odors are all around us. Some are welcoming; the smell of fresh coffee or a nice perfume, for example. Others are foul. They can originate from a wide variety of sources, from rotten foods to feces or body odor. The foul odors in the air outside can come from sources such as vehicles, animals and manure (agriculture), sewage, and industry.
Different industries emit different types of odors. The pungent smell of ammonia, which will be well-known to many who use it to clean their homes, may indicate the presence of fertilizer, textile and paper, or pharmaceutical companies in the vicinity, for example. Hydrogen sulphite is often said to smell of rotten eggs, and is associated with paper mills, food (including animal feed) processing plants, and sewage. Formaldehyde, with its musty and pickle-like smell, is linked to vehicle emissions and coal-fueled plants. Sulfur dioxide, which smells a bit like burned matches (some may not perceive the odor as bad), is toxic and associated with the same sources.
Does the (sudden) presence of foul environmental odors mean that your health is in peril?
In some cases, you will only experience transient symptoms — nasty things like headaches, nausea or even vomiting, a cough, a sore throat, and irritated eyes. Some people will suffer heart palpitations, difficulty breathing, or light-headedness in response to an odor. The symptoms, combined with the odor, will usually be enough to cause people to move out of the immediate area as a result of exposure. If you were not exposed to the point of irritation, the symptoms will subside as soon as the smell no longer affects you. With higher exposure, however, you may suffer for longer.
It is also interesting to note that prolonged exposure, even to generally non-toxic or non-hazardous, foul odors, can have a significant impact on mental health. It has been associated with stress and even clinical depression, in part because continuous poor odors may cause you to have trouble sleeping well. This may happen if you live in the vicinity of a factory or landfill site, for instance.
What can you do to combat bad outdoor smells?
If you're at home when you suddenly catch a whiff of a pervasive and terrible odor that you're not used to, it may signify danger. It is best to close your doors and windows and to look for announcements about possible industrial or other disasters in this case. Should you be out when this happens, seeking shelter is advised. Don't forget to set any air conditioning system you may have in place to recycle indoor air, rather than allowing it to bring air from outside in, where possible. Otherwise, turn your air conditioning off. If you have an N95 mask (as many of us do, since COVID-19 entered the picture), wearing it would be prudent.
For more information about what odor you may be smelling, you can take a look at the CDC's odor database.
Long-term odor problems can, in the United States, be reported to your local government as well as the Agency for Toxic Substances and Disease Registry (ATSDR). Actual investigations will be conducted by the Environmental Protection Agency, and it may be most effective if your whole neighborhood, including leading community figures, take action. PurpleAir offers general information about the outdoor air quality in your locality, while the Smell MyCity app allows any US resident to report a bad odor anywhere in the country, then sharing the information you collect with other users.
If you experience serious symptoms, like severe shortness of breath and wheezing, after exposure to a foul smell, calling emergency response services is in order.
Foul smells don't always pose a serious health hazard, but because they can, it is always good to take any precaution you can, at least until you can find out where the odor comes from.