Air pollution has affected humankind since the beginning of civilisation. Paleopathological studies have shown the presence of carbon deposits and other pollutants in the lung of Egyptian mummies.
Following the industrial revolution, air pollution became a visible presence in urban areas, where the combination of domestic and industrial coal burning caused thick and vast smog which provided inspiration for writers and painters of the time. London’s toxic air was a particular fascination to Charles Dickens, who often referred to it as a metaphor of the city’s moral slide into decadence caused by greed and corruption.
Unlike the visible smog of Victorian London, modern pollution that can be found in today’s cities is made of fine particular matter and gases such as sulfur oxides, nitrogen oxides, carbon monoxide – all of which are indiscernible at ground level.
The detrimental effects of air pollution on physical health have long been recognised. In 1952 air pollution in London reached breaking point due to cold stagnant weather conditions that trapped coal burning emissions at ground level for several days. This caused an enormous increase in respiratory and cardiovascular complications, and an estimated 4,000-12,000 deaths.
This public health disaster – which came to be known as London’s Great Smog – led to a set of policies aimed at reducing air pollution. For example, the Clear Air Act (1956) introduced smoke-free areas in which only smokeless fuels could be burned, leading to considerable reductions in respiratory and cardiovascular disease across the city.
In contrast the detrimental impact of air pollution on mental health is a more recent discovery. Since the turn of the century, several studies have reported associations between air pollution and psychiatric disorders. For example, a recent investigation found that the risk of developing depression – the most prevalent mental disorder in the world characterised by with low mood and feeling helpless – is 50 per cent higher in people exposed to greater levels of air pollution. The risk of developing bipolar disorder – where people swing between feeling low and lethargic and feeling very high and hyperactive – is 29 per cent higher.
The risk of developing schizophrenia – a severe psychiatric disorder associated with hallucinations, delusions, paranoia and disorganised thought – is 147 per cent higher. And the risk of developing personality disorder – characterised by difficulties relating to other people and controlling impulses and emotions – is 162 per cent higher.
Similar findings have been reported for other mental health problems such as anxiety, autism spectrum disorder, cognitive decline and dementia. In addition, in people with a diagnosis of mental illness, high levels of air pollution are associated to worsening of symptoms, including more hospitalisations and emergency department visits. There is even evidence that air pollution leads to higher risk of self-harm and suicide in the general population.
Most of these effects have been reported across all stages of life, including children, adolescents, adults and the elderly. In addition it appears that prenatal exposure to air pollution, which occurs when the mother lives or works in close proximity of busy roadways, power plants, industrial facilities or other pollutant sources, can be as harmful as postnatal exposure.
Is there a safe threshold? A Swedish investigation of 5,000 children and adolescents found that living in areas with higher than average nitrogen dioxide concentration led to higher rates of prescribed medication for psychiatric disorders. Remarkably, this was the case even when the concentration of this pollutant was half of the recommended threshold by standard guidelines by the European Union and the World Health Organization. This is consistent with studies on the impact of air pollution on respiratory and cardiovascular disease, which have found no evidence for a “safe” level of exposure.
Explaining the link
So how does air pollution increase the risk of developing mental health issues? Although there is little disputing that exposure to air pollution is associated with higher risk of mental health issues, at present we have little understanding of the pathways that underlie this association. One possibility is that air pollutants enter the blood stream via inhalation into the lungs and reach the brain, causing harmful effects such as neuroinflammation, hormonal dysregulation and neurotoxicity.
Another possibility is that air pollution leads to changes in our day-to-day physical and/or social activities, which in turn have detrimental effects on our mental health. For example, high levels of air pollution can lead to a reduction in the amount of time people spend exercising or socialising outdoors, which in turn affects the release of endorphins and other mood-boosting hormones.
Another possible pathway is sleep: when pollutants such as nitrogen dioxide and particular matter increase, people are more likely to experience sleep disturbances, including disordered breathing and nocturnal hypoxemia (i.e. low oxygen in the blood). These disturbances, when occurring over a protracted period of time, increase the risk of developing mental health issues.
Who is most vulnerable?
Children tend to be more vulnerable to the harmful effects of air pollution for at least two reasons. Firstly, they breathe more air per unit body weight and therefore, all other aspects equal, inhale more pollutants than adults. Secondly, their bodies are still developing and as such are more susceptible to damage, for example, exposure to traffic-related pollution leads to slower lung growth which, in turn, is linked with higher risk of developing chronic illness in adulthood.
People with low socioeconomic background also tend to be more vulnerable to the harmful effects of air pollution. This is because they are more likely to live in close proximity of busy roadways, power plants, industrial facilities and other sources of pollution; and less likely to live in proximity of green spaces or have access to high-quality health care.
Finally, vulnerability to the harmful effects of air pollution is greater in people who are experiencing psychosocial stress. For example, children are more likely to develop asthma from traffic-related air pollution if they are also exposed to intense parental stress. Similarly, adults are more likely to develop cancer from air pollution if they are also experiencing multiple adverse social conditions.
The overall vulnerability of an individual to the harmful effects of air pollution will therefore be especially high in people with low socioeconomic background who are under intense psychosocial stress. This means that reducing exposure to air pollution is only part of the solution; the other part must involve reducing the existing social inequalities which magnify the harmful effects of air pollution in disadvantaged communities.
How long do we have?
Most of these results are based on long-term exposure to air pollution over a period of several years. Yet recent evidence demonstrates that short-term exposure can also have a measurable adverse impact on our minds. For example, in a pioneering investigation examining the relationship between daily variation in air pollution and expressed happiness on social media, using 210 million geotagged tweets across 144 Chinese cities, increases in fine particular matter concentration were associated with lower levels of happiness.
This is consistent with earlier reports of greater risk of suicide and compulsive behaviours on days when air pollution is higher. The impact of short-term exposure is also evident in the brain. In particular, animal studies have shown that brief exposure to air pollutants is sufficient to cause changes in the brain which resemble the type of neurodegeneration typically observed in dementia.
The good news is that air pollution is controllable, and therefore its adverse effects on mental health could be prevented. Several studies have shown that improving air quality can have almost immediate health benefits.
For example, following improvements to air quality in Beijing at the time of the 2008 Olympic Games, beneficial effects were observed within a matter of months. These included higher infant birth weight and improvement in markers of cardiovascular health, such as blood pressure and heart rate, as well as markers of inflammation and thrombosis in young people. In addition, studies have shown a linear relationship between pollutant concentration and adverse health effects, suggesting that any reduction in air pollution would generate public health benefits. While these studies focused on physical health, one would expect similar immediate benefits in the domain of mental health.
How reliable are the research findings?
It is important to remind ourselves that these results are based on observational rather than experimental studies, and therefore it was not possible to demonstrate a causal relationship. This means that the observed associations between air pollution and mental illness could be due to other factors that happen to differ between people who live in areas with low and high air pollution, such as smoking or drinking.
Nevertheless, most studies were able to account for these factors by including them in the statistical modelling of the data. Furthermore, several studies have reported a so-called dose-response effect, meaning that the higher the exposure to air pollution the greater the adverse effects. This provides indirect evidence that air pollution does cause an increase in the risk of developing mental health issues.
If this interpretation of the data is correct, one would expect to be able to detect a biological impact of air pollution in the brain. Consistent with this expectation, long-term exposure to air pollution is associated with neuroinflammation, neuronal damage, hormonal dysregulation and cerebrovascular changes in the human brain. Furthermore, animal studies, in which the amount of air pollution was experimentally manipulated, have confirmed that air pollutant exposure has neurotoxic effects in the brain and leads to impaired cognition as well as depression-like symptoms.
What can we do?
Despite a number of recent policies aimed at improving air quality, such as the introduction of the ultra low emission zone in London, levels of air pollution in our cities remain high.
It might be tempting to think that the solution is to abandon our cities and move to the countryside. In fact, it turns out that rural areas can be more polluted than urban areas – a surprising statistic that can be observed all over the world, and is particularly evident in developing countries.
Air pollution in rural areas tends to have both natural and human sources. Natural sources include forest fires, dust storms and underground coal fires, whereas human sources include indiscriminate use of insecticides and pesticides, burning of straw and other crop residue and the production of grain dust when cereal crops and maize are harvested. A further source of air pollution is the wind-propelled transport of emissions that originated far away – in some cases even continents away.
An alternative solution is to reduce our carbon footprint, for example by avoiding highly polluting diesel vehicles and boycotting companies which are responsible for high amounts of air pollution, to help create a cleaner and healthier environment. While the idea that we could all make a difference by adjusting our lifestyle might seem idealistic, the impact of our collective action on the quality of the air we breathe could be enormous.
But perhaps the most effective solution is to increase pressure on our politicians who have the power and responsibility to improve the quality of our air, for example through investment in clean and affordable public transport and housing. This is where our increasing knowledge and understanding of the pervasive impact of air pollution on our health become essential. We can use this knowledge and understanding to advocate for policies and practices that will improve the quality of the air we breathe.
It is time for the emerging evidence on air pollution and mental health to become part of this conversation, strengthening the case for cleaner air as a fundamental human right.
Andrea Mechelli is Professor of Early Intervention in Mental Health at King’s College London.