On the trail

“Not having the scientific knowledge of what’s going on [in the body of someone with MCS] is what causes some doctors to tell patients that it’s not a real problem; that it’s all in their head,” says MCS researcher and psychiatrist Dr Thomas Uhde. But experts do have some idea of what’s happening inside your body:

IMMUNITY GLITCH

Immune cells, known as mast cells, are created in the bone marrow before spreading through the body to repel invaders. Dr Miller believes mast cells that are erroneously called up, even when chemicals aren’t dangerous, cause the wide-ranging reactions.

THE NOCEBO EFFECT

Some experts suggest that people become intolerant to chemicals when symptoms are linked in the brain with certain smells (the opposite of the placebo effect). Then, when the person later experiences those odours, their body sets off a reaction. “Nocebo symptoms are real; it’s people’s attributions of the symptoms to environmental factors that’s wrong,” notes Dr Omer Van den Bergh, a psychology professor who published this theory in the journal Clinical Psychological Science.

NEGATIVE ASSOCIATION

“If you have a traumatic experience that involves an odour in the environment, an association is going to be laid down,” which is helpful from an evolutionary perspective to keep you from danger, says psychiatrist Dr Bernadette Cortese, who works with Dr Uhde. The pair hypothesise that, especially in people with certain kinds of anxiety, a traumatic or chronic stress involving a smell might then increase sensitivity to other odours that are strong, acidic or burning (the kinds most likely to be hazardous). In a study they published in 2018, people with anxious personalities were more bothered by a smoky smell (one of these so-called ‘trigeminal odours’) than other people were. Meanwhile, a roselike scent didn’t faze them.