Your ability to sense a particular odor depends on its "odor threshold," the level at which your nose can detect it. Most people have about the same sense of smell, but some people are more or less sensitive than others. Some people like odors that other people don't like, such as different perfumes. Other people, such as allergy sufferers, don't like the smell of a freshly mown lawn not because it smells bad but because they associate the odor with breathing difficulty. Another complication is that people who are exposed to a specific odor for a long time can become so accustomed to it that they don't sense it at all.
Chlorine is a common household chemical that most people can relate to. Found in bleach and swimming pool tablets, it can be detected at 0.08 parts per million (ppm)--that's its "odor threshold." Usually we detect chlorine right away. The Occupational Safety and Health Administration (OSHA) permissible exposure level in the workplace is 1 ppm. This means we can smell it long before it reaches a concentration (amount) that government regulators feel is harmful--greater than 1 ppm. You wouldn't want to be near a derailed tank car of chlorine, however, because a leak could produce a chlorine cloud with a deadly concentration.
Ammonia is another common household chemical that is easily detected. Its pungent odor threshold is 17 ppm, much higher than chlorine, and sometimes it's so strong that you want to avoid smelling it. Ammonia's exposure limit is 50 ppm, however, so even though you don't much like the smell, it isn't going to hurt you in the kitchen.
One household odor that can be harmful is undetectable until it is too late: Teflon®. If a Teflon® pot or pan is left cooking without food in it, it gives off a gas strong enough to kill your pet canary or parakeet.
One of the greatest concerns of the Environmental Protection Agency (EPA) is industrial emissions of the chemical benzene, a long-term carcinogen or cancer-causing chemical. It's odor threshold is 61 ppm, which means you need a very high concentration to detect it. Yet the OSHA permissible exposure limit is only 1 ppm. In other words, you've been affected long before you smell benzene, which is why its presence is regulated so carefully in the workplace. The inability to smell it is also the reason people ignore the danger. They figure "out of sight, out of mind." By the way, benzene is found in gasoline, so every time you fill your car or truck and sniff in those fuel odors, your catching a whiff of benzene even though you can't detect it amidst the other gasoline odors. Better stand upwind.
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