Most of us in the hazardous materials are familiar with the concept of "olfactory fatigue." In general terms what it means is that by prolonged or concentrated exposure to certain odorous chemicals, we lose the ability to detect them by our sense of smell.
Hydrogen sulfide, for example, can be detected at roughly 1 ppm. However, for concentrations above 100 ppm, most people lose the ability to smell the characteristic "rotten egg" odor. This can also be precipitated by long term exposure to lower concentrations. This phenomena is well documented.
However, there is another form of olfactory fatigue that is not so well known among responders and safety workers.
I can tell you from personal experience that stressful environments can bring on a form of olfactory fatigue relatively quickly. For example, when I have been around situations where an explosion is possible, my ability to smell hydrogen sulfide dropped dramatically.
This is because there are two components to smell- the chemicals interaction with olfactory receptors and the brain's process of this information. When the brain is on overload because of an intensely stressful situation in progress, the olfactory data can be reduced.
Hazardous gas detectors are therefore our first line of defense, but these are not always available to the first person on the scene and, like all equipment, they too can fail.
So before you count too much on your sense of smell, just remember that stress, too, can bring on olfactory fatigue.