The Science of Smell: How Odors Affect Health and Well-being

That bad smell not only makes you nauseous, it can also affect your body and mind.

For Ellen Corner, going out into her garden on a summer day can be unbearable. She describes it as 'walking behind an open garbage truck'.

The retired teacher says that even when she closes all the windows of her house in Westbury, England, she can rarely escape the nauseating stench coming from a nearby waste treatment plant. 'We can't use our garden or go for a walk, we feel like vomiting,' says Corner.

We have all smelled the stench of garbage when throwing it away, driving past a landfill, or the smell of rot from a factory. Imagine living with such a bad smell constantly.

Yet, we pay little attention to the impact of such odor pollution on health and quality of life. Bad smells are often dismissed as personal or trivial matters. Research shows that people generally give less importance to the sense of smell than to sight, hearing, touch, and taste. Some college students in America have even admitted that they would rather lose their sense of smell than their phone.

It's not just the discomfort people experience living with terrible smells: studies have linked unpleasant odors in urban areas to health problems ranging from headaches and nausea to difficulty breathing or disturbed sleep. They can also have long-term mental and physical effects. Growing research is helping us understand the surprisingly important role smell plays in the health of our body and mind.

• The Smell of Danger

The sense of smell has evolved as a warning sign to help prevent diseases or infections. Something that smells rotten may be filled with bacteria that can harm us.

Johan Lundström, a professor of olfactory science at the Karolinska Institute in Stockholm, Sweden, considers it part of our 'behavioral immune system'. 'The olfactory system primarily functions as a defense system to learn to warn us about dangers in the environment,' says Lundström.

His research shows that smell signals are processed in the brain within about 300 milliseconds of breathing through the nose. Participants in the study who were exposed to bad smells showed rapid physical reactions, easily moving away from the source of the smell.

Because of this protective nature of smell, it is surprisingly easy to convince someone that a smell is bad, even if it is generally considered pleasant. 'If we detect a smell and don't know what it is, it's almost always a negative experience,' says Lundström.

When a smell is associated with danger, our sensitivity to it can also increase dramatically. In one study, Lundström and his colleagues proved that when smells were associated with electric shocks, people could detect the smell even in very small quantities.

Bad smells can activate the vagus nerve, a key part of the nervous system connecting the brain and the stomach, causing a feeling of nausea.

This is likely a response that evolved to help people react quickly to potential dangers even when the smell fades. Similarly, the rotten egg-like smell of hydrogen sulfide, a gas produced in sewage treatment, can be detected in quantities as low as 0.5 parts per billion, which is a warning for a gas that can be fatal in high concentrations.

• Real Health Effects

Smell's job is not just to detect danger; it can have many real effects on people's health and well-being. Scientists have shown that pleasant smells, such as the scent of forests, can be beneficial for our mental health. Part of the reason for this is that such scents stimulate areas of the brain connected to emotions and memories.

Conversely, there is also evidence that bad smells can harm our health, although scientists are still trying to understand the exact relationship between odor pollution and direct physical effects. A review of studies conducted in 2021 found some 'biological plausibility' behind symptoms like headaches or vomiting caused by bad smells.

For example, bad smells can activate the vagus nerve, a key part of the nervous system connecting the brain and the stomach, causing a feeling of nausea. However, the researchers concluded that further research is needed on the physical effects of smell to draw clear conclusions.

The extent of the impact on our health also depends on how worried we are by the smell. Pamela Dalton, a cognitive psychologist at the Monell Chemical Senses Center in Philadelphia, USA, says that the health effects are determined by an individual's aversion or fear of smell. She has spent 32 years researching the health effects of smell and says, 'The more worried you are about a smell, the more it can affect your health and well-being.'

• Lifestyle Changes

Constant bad smells can dominate many parts of life. It can force someone to make lifestyle changes that can be detrimental to their health. These changes are called 'maladaptive behaviors'.

For example, on a summer day, one might feel compelled to keep windows closed or avoid going out to exercise or spend time with friends. Living amidst the stench in Westbury 'affects social life,' says Corner. 'If you're planning a barbecue in the summer, you just hope the smell doesn't come,' she says.

Life can be difficult for those living in communities plagued by unpleasant odors, but there is at least one small consolation: a good sense of smell is an important part of good health.

However, even a slight whiff of a smell can be unbearable for some people, while for others it may be barely noticeable. Dalton, who regularly investigates odor complaints, says, 'There is a wide range of reactions; some say they only smell it occasionally, while for others it has no effect.' Age, gender, allergies, and lifestyle choices like smoking affect how people perceive smells.

You might think that people can get used to unpleasant smells over time, but repeated exposure to unpleasant smells, such as those from landfills, does not make them easier to tolerate.

Conversely, it is common to get used to neutral or pleasant smells. 'Once you smell something and recognize that it won't kill you, then you stop smelling it,' says Lundström. This is also why it is difficult for people to name smells that come from things that are generally not dangerous, even though the human nose can detect one trillion smells. Sniff tests have shown that less than half of us can correctly name everyday smells like coffee or vanilla.

• Fighting Odors

Sometimes odors can come and go with the wind or be felt only in certain parts of a neighborhood. Amanda Jiang, an associate professor of environmental modeling and policy at the University of British Columbia in Canada, calls this 'hyper-local'. She, who has researched the impact of odor pollution on Vancouver residents, says, 'I could live a block away and never know that there's a smell of rotten fish on another block.'

But not all odors are experienced equally: poorer communities with affordable housing are often located near smelly landfills or heavy industries. Studies in Europe and the UK have shown that people in lower-income communities are more likely to live within 2 km of incinerators, landfills, and hazardous waste sites than those in higher-income groups in some countries.

Complaints about terrible smells can bring about change: residents' campaigns have forced sewage treatment plants and fish processing factories to reduce or cease their operations. Efforts to better control odor pollution are increasing worldwide, albeit unevenly. Examples include new regulations in Chile regarding odors from fish feed industries, and strict rules in Lithuania on how much odor companies can emit in residential areas.

• Benefits of Smell

Life can be difficult for those living in communities plagued by unpleasant odors, but there is at least one small consolation: a good sense of smell is an important part of good health.

Research has shown that people with a good sense of smell also derive more pleasure from eating and sexual intercourse. A 2018 study of 70 adults found that those with high olfactory sensitivity reported greater pleasure from sexual activities. Among them, women reported experiencing a higher frequency of orgasm during intercourse.

Conversely, an estimated 5% of people who lack the sense of smell, known as anosmia, often face widespread health consequences. Interviews with people with anosmia show that they have a higher risk of poor appetite and a lower quality diet due to difficulty enjoying food. 'If you lose your sense of smell, you will feel like your appetite is gone,' says Lundström.

Worse still, research has linked impaired sense of smell in older adults to a 46% higher risk of death within 10 years. Scientists are still trying to find the cause, although they have found it to be linked to deaths from heart disease and neurodegenerative diseases such as Alzheimer's and Parkinson's.

Dalton, who has researched smell for decades, says, 'I don't mind smelling bad smells because it means my sense of smell is really working!'

BBC