Why do smells make some people sick?

Normally your smell perceptions diminish rapidly, as when you enter a friend's apartment. Even though you clearly notice smells just inside the door, you don't think about them for long. For people with chemical intolerance, on the other hand, smells seem always to be present. Psychology researcher Linus Andersson has exposed both intolerant and non-intolerant individuals to smells and compared their reactions.

"The hypersensitive individuals felt that the smell was getting stronger even though its concentration had not changed. Their brain activity images also differed from those in the other group," he says.

The results were observed using methods based on both electroencephalography (EEG) and functional brain imaging technology (fMRI). The EEG method involved placing electrodes on the heads of trial subjects and registering the minute changes in tension in the brain that arise following exposure to smells. Unlike the people in the normal group, Linus Andersson explains, the intolerant people did not evince a lessening of brain activity during the period of more than an hour they were exposed to a smell. The inability to grow accustomed to smells is thus matched by unchanging brain activity over time.

"These individuals also have a different pattern in the blood flow in their brains, compared with those who perceive that a smell diminishes. A similar change can be found in patients with pain disorders, for example."

Sensitivity to smell impacts the entire body A further finding in the dissertation is that chemical intolerant people also react strongly to substances that irritate the mucous linings of their nose and mouth. People who cough more when they inhale capsaicin, the hot compound in chili peppers, also have heightened reactions in the brain to other smells. Besides the fact that intolerant individuals perceive that smells grow stronger, effects are also seen in mucous linings and in the brain.

"In other words we can see indications that this intolerance affects both the body and the mind, and that it's important not to blindly focus on just one of these aspects," says Linus Andersson.

Chemical intolerance is surprisingly common -- up to ten percent of the Swedish population report they are bothered by everyday smells, whereas roughly two percent experience severe symptoms. Yet, in contrast to the situation regarding allergies and asthma, there is very little research about what causes this condition. Linus Andersson maintains that if it were possible to identify what characterizes this hypersensitivity then it would be possible to develop methods for diagnosis and treatment. But research can also provide new knowledge about how we should think about our work and everyday environments.

"Some co-workers are bothered more than others by the smell of the printer -- what should we do to make our working conditions acceptable to as many people as possible?"

Dissertation.