How bad smells affect your health - BBC

For Elaine Corner, stepping into her garden on a summer's day can be unbearable. She likens it to "walking behind an open bin lorry". Even with all the windows shut in her home in the English market town of Westbury, the retired teacher says she still often can't escape the nausea-inducing stench from a nearby waste treatment plant. "We can't use our garden or go for a walk, you feel as though you're going to vomit," says Corner.

At some point, we've all caught the rank waft of decomposing waste as we take out the refuse, drive past a rubbish dump or encounter the sickly pong from a factory. Just imagine living constantly with such a stench.

Yet we give little attention to the health and quality-of-life impacts of such odour pollution. Bad smells are often dismissed as subjective or trivial. Research shows people typically value their sense of smell less than vision, hearing, touch and taste. Some US college students have even admitted they would rather lose their sense of smell than their phone.

It's not just the discomfort that people experience from living with terrible smells: studies link unpleasant odours in urban areas to health complaints ranging from headaches and nausea to difficulty breathing or disturbed sleep. They can also have long-lasting mental and physiological effects. A growing body of research is helping us understand the surprisingly significant role scent plays in the health of our bodies and minds.

Smell has partly evolved into an early-warning signal to help us avoid a getting ill or infected. Something that smells rancid is likely to be packed with bacteria looking to do us harm. As such, it is part of our so-called behavioural immune system, says Johan Lundström, a professor in the science of smell at the Karolinska Institute in Stockholm, Sweden. "The olfactory system primarily functions as an avoiding system to learn to warn us about danger in the environment," says Lundström. His work has even shown that odour signals are processed in the brain within about 300 milliseconds of being inhaled through the nose. Study participants exposed to the bad smells showed a rapid physical response, instinctively withdrawing from the source of the smell.

Smell's defensive default makes it surprisingly easy to convince someone that an odour is negative, even if it's normally considered pleasant. "If we detect an odour and we don't know what it is, it's almost always a negative experience," says Lundström.

When a smell is associated with a threat, our sensitivity to it can also increase dramatically. In one study, Lundström and his colleagues showed that pairing a smell with an electric shock can make people detect that odour at much lower concentrations – a response that likely evolved to help humans react quickly to potential dangers, even when the smells are only faint. Similarly, the rotten eggs scent of hydrogen sulfide, a gas produced in sewage processing, can be perceived in concentrations as low as 0.5 parts per billion – an alarm bell for a gas that is lethal in higher concentrations.

Smell isn't just about threat detection: odours can have very real effects on people's health and well-being. Scientists have shown that good smells, like the fragrance of a woodland, can be beneficial for our mental health, partly because they stimulate brain regions linked to emotion and memory.

There is also evidence that the opposite can be true, with malodours damaging our health, although scientists are still trying to disentangle the precise link between odour pollution and any direct physiological impacts. A 2021 review of studies found some "biological plausibility" behind symptoms like headache or vomiting caused by bad smells. For instance, nasty odours can trigger the vagus nerve, a key part of the nervous system linking the brain and gut, making one feel sick or nauseous. But the scientists behind that review ultimately concluded that more research into the physiological impacts of smells is needed to draw clear-cut conclusions.

The extent of the affect on our health also depends on how worried we are by the odour. "The health impact is mediated through an individual dislike or fear of an odour," says Pamela Dalton, a cognitive psychologist at the Monell Chemical Senses Center in Philadelphia, in the US, who has spent 32 years investigating the health impacts of odours. The more anxious you are about an odour, the more it can impact your health and wellbeing.