Here’s why air pollution affects mental health

Los Angeles? We think? Not sure. Image: Getty.

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.

 
 
 
 

To build its emerging “megaregions”, the USA should turn to trains

Under construction: high speed rail in California. Image: Getty.

An extract from “Designing the Megaregion: Meeting Urban Challenges at a New Scale”, out now from Island Press.

A regional transportation system does not become balanced until all its parts are operating effectively. Highways, arterial streets, and local streets are essential, and every megaregion has them, although there is often a big backlog of needed repairs, especially for bridges. Airports for long-distance travel are also recognized as essential, and there are major airports in all the evolving megaregions. Both highways and airports are overloaded at peak periods in the megaregions because of gaps in the rest of the transportation system. Predictions for 2040, when the megaregions will be far more developed than they are today, show that there will be much worse traffic congestion and more airport delays.

What is needed to create a better balance? Passenger rail service that is fast enough to be competitive with driving and with some short airplane trips, commuter rail to major employment centers to take some travelers off highways, and improved local transit systems, especially those that make use of exclusive transit rights-of-way, again to reduce the number of cars on highways and arterial roads. Bicycle paths, sidewalks, and pedestrian paths are also important for reducing car trips in neighborhoods and business centers.

Implementing “fast enough” passenger rail

Long-distance Amtrak trains and commuter rail on conventional, unelectrified tracks are powered by diesel locomotives that can attain a maximum permitted speed of 79 miles per hour, which works out to average operating speeds of 30 to 50 miles per hour. At these speeds, trains are not competitive with driving or even short airline flights.

Trains that can attain 110 miles per hour and can operate at average speeds of 70 miles per hour are fast enough to help balance transportation in megaregions. A trip that takes two to three hours by rail can be competitive with a one-hour flight because of the need to allow an hour and a half or more to get to the boarding area through security, plus the time needed to pick up checked baggage. A two-to-three-hour train trip can be competitive with driving when the distance between destinations is more than two hundred miles – particularly for business travelers who want to sit and work on the train. Of course, the trains also have to be frequent enough, and the traveler’s destination needs to be easily reachable from a train station.

An important factor in reaching higher railway speeds is the recent federal law requiring all trains to have a positive train control safety system, where automated devices manage train separation to avoid collisions, as well as to prevent excessive speeds and deal with track repairs and other temporary situations. What are called high-speed trains in the United States, averaging 70 miles per hour, need gate controls at grade crossings, upgraded tracks, and trains with tilt technology – as on the Acela trains – to permit faster speeds around curves. The Virgin Trains in Florida have diesel-electric locomotives with an electrical generator on board that drives the train but is powered by a diesel engine. 

The faster the train needs to operate, the larger, and heavier, these diesel-electric locomotives have to be, setting an effective speed limit on this technology. The faster speeds possible on the portion of Amtrak’s Acela service north of New Haven, Connecticut, came after the entire line was electrified, as engines that get their power from lines along the track can be smaller and much lighter, and thus go faster. Catenary or third-rail electric trains, like Amtrak’s Acela, can attain speeds of 150 miles per hour, but only a few portions of the tracks now permit this, and average operating speeds are much lower.

Possible alternatives to fast enough trains

True electric high-speed rail can attain maximum operating speeds of 150 to 220 miles per hour, with average operating speeds from 120 to 200 miles per hour. These trains need their own grade-separated track structure, which means new alignments, which are expensive to build. In some places the property-acquisition problem may make a new alignment impossible, unless tunnels are used. True high speeds may be attained by the proposed Texas Central train from Dallas to Houston, and on some portions of the California High-Speed Rail line, should it ever be completed. All of the California line is to be electrified, but some sections will be conventional tracks so that average operating speeds will be lower.


Maglev technology is sometimes mentioned as the ultimate solution to attaining high-speed rail travel. A maglev train travels just above a guideway using magnetic levitation and is propelled by electromagnetic energy. There is an operating maglev train connecting the center of Shanghai to its Pudong International Airport. It can reach a top speed of 267 miles per hour, although its average speed is much lower, as the distance is short and most of the trip is spent getting up to speed or decelerating. The Chinese government has not, so far, used this technology in any other application while building a national system of long-distance, high-speed electric trains. However, there has been a recent announcement of a proposed Chinese maglev train that can attain speeds of 375 miles per hour.

The Hyperloop is a proposed technology that would, in theory, permit passenger trains to travel through large tubes from which all air has been evacuated, and would be even faster than today’s highest-speed trains. Elon Musk has formed a company to develop this virtually frictionless mode of travel, which would have speeds to make it competitive with medium- and even long-distance airplane travel. However, the Hyperloop technology is not yet ready to be applied to real travel situations, and the infrastructure to support it, whether an elevated system or a tunnel, will have all the problems of building conventional high-speed rail on separate guideways, and will also be even more expensive, as a tube has to be constructed as well as the train.

Megaregions need fast enough trains now

Even if new technology someday creates long-distance passenger trains with travel times competitive with airplanes, passenger traffic will still benefit from upgrading rail service to fast-enough trains for many of the trips within a megaregion, now and in the future. States already have the responsibility of financing passenger trains in megaregion rail corridors. Section 209 of the federal Passenger Rail Investment and Improvement Act of 2008 requires states to pay 85 percent of operating costs for all Amtrak routes of less than 750 miles (the legislation exempts the Northeast Corridor) as well as capital maintenance costs of the Amtrak equipment they use, plus support costs for such programs as safety and marketing. 

California’s Caltrans and Capitol Corridor Joint Powers Authority, Connecticut, Indiana, Illinois, Maine’s Northern New England Passenger Rail Authority, Massachusetts, Michigan, Missouri, New York, North Carolina, Oklahoma, Oregon, Pennsylvania, Texas, Vermont, Virginia, Washington, and Wisconsin all have agreements with Amtrak to operate their state corridor services. Amtrak has agreements with the freight railroads that own the tracks, and by law, its operations have priority over freight trains.

At present it appears that upgrading these corridor services to fast-enough trains will also be primarily the responsibility of the states, although they may be able to receive federal grants and loans. The track improvements being financed by the State of Michigan are an example of the way a state can take control over rail service. These tracks will eventually be part of 110-mile-per-hour service between Chicago and Detroit, with commitments from not just Michigan but also Illinois and Indiana. Fast-enough service between Chicago and Detroit could become a major organizer in an evolving megaregion, with stops at key cities along the way, including Kalamazoo, Battle Creek, and Ann Arbor. 

Cooperation among states for faster train service requires formal agreements, in this case, the Midwest Interstate Passenger Rail Compact. The participants are Illinois, Indiana, Kansas, Michigan, Minnesota, Missouri, Nebraska, North Dakota, Ohio, and Wisconsin. There is also an advocacy organization to support the objectives of the compact, the Midwest Interstate Passenger Rail Commission.

States could, in future, reach operating agreements with a private company such as Virgin Trains USA, but the private company would have to negotiate its own agreement with the freight railroads, and also negotiate its own dispatching priorities. Virgin Trains says in its prospectus that it can finance track improvements itself. If the Virgin Trains service in Florida proves to be profitable, it could lead to other private investments in fast-enough trains.

Jonathan Barnett is an emeritus Professor of Practice in City and Regional Planning, and former director of the Urban Design Program, at the University of Pennsylvania. 

This is an extract from “Designing the Megaregion: Meeting Urban Challenges at a New Scale”, published now by Island Press. You can find out more here.