Where industrialized humans live, air pollution follows. That’s bad news for your lungs, which, in exchanging the gasses necessary to keep us alive, encounter particles that can make the body function less well.
Though scientists know that chronic exposure to air pollution is toxic to several organ systems, the exact mechanism of how our bodies accrue damage over time is still unclear. Over the decades, scientists have pieced together epidemiological data to see how different levels of pollutants affect long-term health. But now, they want to fix it by finding ways to prevent and undo the damage. If they can more concretely define the relationship between air pollution and various health outcomes, they could drastically improve public health.
Early evidence suggests that at a population level, it is possible to reverse some of the damage caused by polluted air. “Clean air can help,” says Aruni Bhatnagar, the director of the Envirome Institute at the University of Louisville in Kentucky, who studies how the environment impacts public health. It’s just a matter of finding the most efficient ways for cities to do it.
Moving to the country
Scientists know that air pollution impacts health, but quantifying that effect isn’t so easy. In 2018, the World Health Organization (WHO) estimated that 4.2 million people die annually from poor outdoor air quality. But death certificates generally won’t show that. Instead, causes of death are attributed to the conditions air pollution exacerbates. Acute air pollution exposure has been tied to health events like asthma attacks and heart attacks, while long-term exposure can exacerbate conditions like chronic obstructive pulmonary disease, emphysema, heart disease, cancers, preterm births in pregnant people, and even dementias like Alzheimer’s disease.
Generally, there are two kinds of air pollution public health officials like to keep track of: polluting particles that are larger than are up to 10 microns (one millionth of a meter), and those that are 2.5 microns or smaller. Larger and coarser air pollution is less worrisome from a public health perspective, because it gets “removed by the nose” as Bhatanagar puts it, and so has less impact on our health.
PM2.5, or fine particulate matter, is small enough to work its way deep into the lungs and blood vessels. There, particles can cause damage: Particles of PM2.5 immediately kick the lungs’ inflammatory response into gear, as the lungs’ immune system responds to what it sees as a potential invader. In the long-run, these particles cause tiny tears that harden the blood vessels that line the heart, lungs, and eventually the rest of the body.
To avoid this damage, the WHO has advised that people shouldn’t be exposed to air that contains (pdf) more than 10 micrograms of PM2.5 per cubic meter of air on a yearly average, and no more than 25 micrograms on average for a single day. But most people aren’t exposed to air so free of pollutants; WHO also estimates that roughly 95% of the population breathes air that contains elevated levels of PM2.5. Studies have shown that the more urbanized an area is, the more pollution there usually is, too.
Recent work suggests that for things like heart disease, people incur most of the risks at the very lowest levels of PM2.5 contamination. As air quality deteriorates, the incurred risks plateau off. In other words, the potential health gains from cleaning up more polluted air to slightly less polluted air are less than the gains from cleaning up relatively clean air to make it pristine. “You can think of [improving air quality] like quitting smoking,” says Bhatnagar. People who minimally smoke—up to five cigarettes per day—have roughly the same health consequences as those who smoke 30 per day.
This means that small adjustments won’t cut it. In order for people to enjoy the health benefits of improved air quality, governments need to make their air as clean as possible. And for areas that already have decent air quality, like countries in North America and Europe, that may be harder to do without concrete evidence to support that the efforts are worth it. The good news is, scientists are working to find low-cost interventions that can passively improve a city’s ambient air quality over time.
Making the city more like the country
Scientists are still debating just how much damage can be averted by relocating from an area with high air pollution to lower air pollution.
There’s some reason to believe that relocating for a short time period—say, a vacation—can reduce some of the more acute risks from heavily polluted air. For example, people who are predisposed to asthma or heart attacks are less likely to have one while breathing cleaner air.
The effects of a permanent relocation, however, are harder to quantify. Because we know air pollution is linked to acute and chronic health conditions, researchers can’t ethically expose people to it. Instead, they conduct observational studies in which they measure the ambient air quality of an area and record various aspects of residents’ health.
Though there are dozens of confounding variables—factors like diet, socioeconomic class, genetics, lifestyle, to name a few—these kinds of studies can give epidemiologists rough ideas about how cleaner air impacts health. Rural areas—which have less commuting traffic, less industrial activity, and more greenery—tend to have better air quality, and public health experts are exploring how they can make urban areas more like rural areas in these ways. (Even if they’re wrong, we know that better air quality can’t hurt people’s health.)
One of the best ways cities have historically reduced their air pollution is by setting strict emission limits from cars and industrial activity. In the US, changes instituted by the Environmental Protection Agency’s Clean Air Act are estimated to have decreased air pollutants by 73% from 1990 to 2015, and saved roughly 230,000 American lives per year, due to fewer heart attacks or chronic lung conditions. Additionally, smaller studies from Europe, Ireland, and Hong Kong have shown that respiratory deaths and deaths of any cause decreased after banning coal as a fuel source or limiting fossil fuel contaminants like sulfur.
Researchers are exploring what else they can do to continue to make small gains in cities that already have comparatively clean air. One such intervention could be adding greenery bringing nature in. There’s ample evidence that green space in urban areas improves residents’ moods and physical activity. Now, researchers are working to see if adding more trees to urban areas can improve air quality, too.
“The central hypothesis is that greenness and vegetation could decrease air pollution levels by filtering it out,” says Jay Turner, an environmental engineer at the Washington University in St. Louis. “Therefore, you get a health benefit.”
Turner is working with Bhatnagar and others on an ongoing study taking place in Louisville, Kentucky, a city of roughly 600,000 people. He and his colleagues plan to plant pine trees and other conifers in front of some of the noise barricades between highways and residential areas. (Pine trees have needles year-round, which means their benefit, if any, should be continuous.) Then, the research team will look at incidences of emergency room visits due to heart attacks, general physical exams from residents, and biomarkers in wastewater to see if residents’ health improves over time. They’ll compare these data to those from people who live near noise barriers that don’t have any greenery nearby.
The work is just in its early stages because the Covid-19 pandemic set the team back about a year. They were able to plant some greenery at the end of last year, but not the entire area they’d planned. During the last week of March 2021, the researchers were furiously taking measurements of air pollution near the highway before other plants in the area go into bloom. They should be able to plant their additional trees at the close of this year, and take new measurements in the March and April of 2022. Throughout the study, they also plan to take passive air quality measurements in 60 sites around the control and intervention areas using special sponges that can soak up pollutants, as well as mobile measures taken from cars as they drive over every street.
“We’d hope that the intervention [area air] would become cleaner,” says Turner. If it does, it could prompt the team to plant additional trees in the control area too, provided they get additional funding. And, it could open the door for future studies investigating vegetation’s role in cleaning the air.
Without much work done in this space, it’s impossible to say which kind of plants will have the biggest impact on air pollution. More work could help them narrow down which plants may also help improve other environmental measures, like biodiversity. “It’s not as simple as bringing greenness back to urban areas,” he says. “It’s doing it in a way that makes sense.”