ScienceDaily (Nov. 9, 2009) — People who become ill from harmless smells are not being silly, says Dutch researcher Patricia Bulsing. Rather, they perceive these smells differently than other people. The smell is detected more rapidly by the brain and processed more deeply. If you expect to become ill from a smell, then the smell in question might really make you ill.

Would your favourite perfume smell just as attractive if the bottle displayed a large label saying 'Warning: perfume can be toxic'? Probably not. But some people react even more violently, actually becoming ill. Analyses of odour molecules and receptors in the nose have not yet been able to show why people become sick from what are actually harmless odours. According to Patricia Bulsing, our unconscious perception may well have a part to play in this. She has discovered, for instance, that people subconsciously associate the notion of odours with illness. Also, our own experiences exert a significant influence on the way our brains process incoming odours.

If you've ever eaten anything that actually made you ill, you know that afterwards you cannot tolerate the smell of the food concerned for a while. You then associate the odour with a feeling of queasiness. This is the sort of association that Bulsing taught her trial volunteers. She combined a smell with a painful stimulus in the nose. This led to the volunteers expecting a specific odour to be associated with pain.

Not in your nose and not between your ears, but in your brain.

The psychologist looked at what was happening in the brain as soon as people inhaled an odour they associated with the expectation of pain. Using electrodes attached to the head, Bulsing was able to see that the volunteers processed the incoming information more quickly when they expected pain. An unpleasant smell, associated with pain, also meant that the information was processed more intensely, with the brain devoting more energy towards processing the information. So, when people expect that an odour can cause pain, their brains process the odour quite differently.

Government departments and industrialists who have to deal with people who become ill from odour nuisance often assume that they are imagining their illness. Bulsing has now demonstrated that this is not the case. The subconscious association between odour and illness indicates that people regard odours as warning signals.

In such cases, being able to detect and analyse odours rapidly and subconsciously is advantageous. However, a clear disadvantage of this response mechanism is the increased risk of a false alarm. According to Bulsing, this mechanism may also be the basis for exaggerated physical reactions to odours.

Patricia Bulsing undertook her research within the project 'Chemosensory irritation from disagreeable and agreeable odours: subjective versus objective effects' headed by Vidi prizewinner Monique Smeets.

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Adapted from materials provided by NWO (Netherlands Organization for Scientific Research).

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