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.


How the rise of anti-crime politics caused lasting harm to Black Americans

"I see an awareness that has developed in the Black community in the last 10 years or so about how deeply racist the criminal justice system has become," James Forman Jr. says. (David McNew/Getty Images)

The police killing of George Floyd, and the protest movement that emerged from it, has reinvigorated a national conversation around reinventing criminal justice policy in the United States.

At the same time, reports that violent crime is rising in many US cities have resurrected talk of the much-disputed “Ferguson effect,” a theory put forward by law enforcement professionals, and some researchers, who argued that police slowdowns in the wake of the first wave of Black Lives Matter protests resulted in elevated rates of violent crime. President Donald Trump is trying to weaponise this narrative, paired with images of federal officers clashing with protesters in the streets of Portland, to wage a 1968-style backlash election campaign.

“People who want to mobilise a lock-them-up style of either policing or prosecution have tried to weaponise those short-term increases,” says James Forman Jr., professor of law at Yale Law School. “Criminologists will say you have to be very, very cautious about short-term movement [in crime statistics]. We don't know whether or not what we're seeing right now [with violent crime increasing] is going to sustain itself. But the fact is, it's here and people are talking about it.”

In 2018, Forman won the Pulitzer Prize in nonfiction for his book Locking Up Our Own: Crime and Punishment in Black America. Drawing on his experience as a public defender in Washington, DC, he traced the emergence of anti-crime politics in late 20th century Black communities. Forman showed how newly empowered Black politicians fought for policies they believed would protect and uplift Black Americans, but inadvertently contributed to mass incarceration. 

CityMetric recently caught up with Forman to discuss crime trends, where he sees reason for hope in this moment and how the Black political class’s attitude toward crime and punishment has shifted since the latter part of the 20th century. 

This interview has been edited and condensed. 

There is talk right now about a resurgence of crime and violence in American cities. We saw similar, more localised concerns after the initial 2015 Black Lives Matter protests in Ferguson and Baltimore. Do you fear this could reinvigorate the kind of politics you describe in your book among segments of the Black community and political class?

I fear that it could be reinvigorated nationally and also in the Black political class. Look at the political conversations that are happening in Atlanta right now, for example, a city that also has seen a short-term uptick in crime as it is a site of a lot of protests about George Floyd and Breonna Taylor on the national level, as well as Rayshard Brooks and Ahmaud Arbery more locally in Georgia.

I think that you can already see in some of the language of the local elected officials this idea that we have to be very careful about pulling back. [They are saying] “while the protesters may make some valid points, we can't risk returning to the ‘80s and ‘90s.” Those decades really traumatised the United States, and particularly traumatised Black communities. There's a deep fear about returning to the levels of the violence that we saw in the crack years.

You write a lot about class divides among Black Americans, where middle income and elite Black people don't suffer as much from extremely punitive policies. They also have closer ties to the politicians who are creating these policies. There are very specific groups of people, even in marginalised communities, whose voices are heard.  As a result of these dynamics, you write about Black politicians fighting for things like mandatory minimum prison sentences or against decriminalising marijuana. Is there still that disconnect between those who suffer the most from criminal justice policies and those who are actually heard in political discourse?  

Let me just say a caveat, that when we talk about class divisions in the Black community it's important to hold two truths in our head at the same time. Bruce Western and others have shown the way in which class, educational status, income can dramatically reduce the likelihood of being hardest hit by the criminal system – namely incarcerated. Middle class and upper middle class Black people get some measure of protection. It's also true at the same time that Black people of all classes are worse off relative to their class counterparts in the white community. 

One area where class is least protective is policing and police stops. The police do not know how many degrees you have. They don't know how much money you have in your bank account. I want to be very clear that in making this point about class, I'm not making the argument that race or racism don't matter in this context. 

In terms of how it plays out now, I see an awareness that has developed in the Black community in the last 10 years or so about how deeply racist the criminal justice system has become. Twenty or 30 years ago they had a consciousness, but there's levels of understanding. Many of the people I write about in the book wanted to promote the interests of the Black community. They weren't motivated by indifference or callousness. When presented with mounting evidence of how awful this system has been in Black lives, they're reconsidering and recalibrating. 

Lots of former elected officials have said to me some version of “I didn't know at the time and I appreciate that you showed us in our full complexity. I appreciate that you showed the pressures we were under. If I had known then what I know now, maybe I would have been less quick to go along with some of these harsh measures.” 

The second thing that has affected the Black political class has been the emerging movements, led by Black people in particular and led by young people. They not only educated leaders, but pressured them and made them understand that there is a political cost. If you're not moved by the moral argument, then you'll be moved by the political argument. You'll be moved by the people protesting outside the office of District Attorney Jackie Lacey in Los Angeles, for example, where Black Lives Matter LA has held, I believe, a year of consecutive protests against a Black district attorney who has had really some of the worst practices.

From what I can tell, she's been pressured by the movement to change some of her positions on important issues like prosecution of low-level drug offenders, for example, and the aggressiveness with which she prosecutes police officers for acts of violence.

What do you make of the calls to defend or even abolish the police?

What I find so compelling about abolition, initially in the prison context and extended to the police as well, is that it shifts the conversation and forces us to go through experiments in which we imagine what it would take to build that world. I think that exercise is very important, because it pushes us further than we are naturally inclined to go. Cultivating a broader imagination is an incredibly important part of this work, because as you know from my book, often it was lack of imagination that caused people to fall back on [punitive policies]. 

That's what caused D.C. Councilmember David Clarke to call the police rather than public health experts when he was overwhelmed with letters about heroin addicts in public space. He was anti-drug war, but he couldn't imagine responding to a call for help with heroin addicts with anything other than police. That's a very common move from even really good and progressive people. 

People who are for defunding, for abolition, are absolutely right about reinvesting that money into alternative structures that support communities. But the reinvestment part doesn't follow naturally from the terms. We might want to come up with a term that captures the new stuff we want to do. I think that's particularly important because one of the reasons Black communities have ended up supporting more police is that Black communities have always wanted their fair share of the resources.

Then, the evidence suggests the United States has too many police officers doing prophylactic, preventative, or stop-and-frisk style policing. The style of policing that leads to district level harassment, pulling people over for no reason. But we have too little investment in the parts of police departments that investigate unsolved crimes. I'm talking about the investigator or the detective who comes to your house after there's been a robbery, an assault, a rape, or homicide. 

As compared to European countries, in the United States we actually underinvest in those parts of our police departments. Jill Leovy’s book Ghettoside shows this in dramatic detail. She describes an LAPD that's stopping and frisking Black drivers wantonly and yet the homicide detectives are still relying on a fax machine and the fax machine is broken. They have to go with their own money to Staples to buy a printer. Meanwhile, other aspects of the department are kitted out in this ridiculous riot gear that makes them look like they're in Fallujah. 

That under investment is particularly damaging to Black communities because we're disproportionately victimised by crime. Because of racism and this allocation of resources, the police are less likely to respond in Black communities. The kids I used to work with in the charter schools in DC, we talk about no snitching, but one of the reasons they would never call the police after they'd been victimised by crime is they would say, “They're not even going to come. You're wasting time.” 

I did a Q&A with Jill Leovy too and her argument is one I've struggled to articulate in our present moment. She argues the state doesn't have a monopoly on violence in low-income Black neighbourhoods, because investigations of violence are deemphasised and crime victims or their loved ones often take retribution into their own hands.  But right now, establishing or preserving the state's monopoly on violence isn't an appealing talking point. 

Yes, this is another thing nobody's talking about. Whatever we're going to do instead of the police has to be accountable to the public. The best, most direct way to have accountability is to have the individuals be public employees. As long as we have 300 million guns in this country at least some of those state employees are going to themselves be armed. It's unreasonable to ask them to do the job without it. Not as many need to be armed as are armed now, but some of them need to be. But they can't be hiding behind union contracts or civil service protections which make it impossible to remove even the worst performing, most abusive officers. 

We can not call them police if we want to. That's semantic, but maybe symbolism matters. But those people have to be state employees. They can work with community-based nonprofits, but there are also communities that don't have as robust of a nonprofit network, and they deserve protection too. These [community] groups have to be accountable to the state and, when they don't exist, the state has to be there. 

Progressives get all the points I just made when it's applied to education. The notion that things be public and accountable to the state is understood when it comes to schools. It's exactly why so many people on the left are opposed to charter schools, because they say they don't have public accountability. They want these things to be a state function. But this point about the difficulty in removing this entirely from the hands of the state is, I think, one that liberals and progressives understand from other contexts.

Jake Blumgart is a staff writer at CityMetric.