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Expert Q&A

Is Air Pollution Getting Worse or Better?

by Jeanine Barone  

Michael Jerrett, PhD, is professor and chair of the Department of Environmental Health Sciences at the University of California, Los Angeles. He is internationally recognized as an expert on the health effects of air pollution. Here he discusses what we know about the state of air pollution and its effect on health in the U.S. and other parts of the world.

Is air pollution in the U.S. getting better or worse?

Overall, air quality is improving nationally. We’ve seen an improvement in carbon monoxide. Particulate matter has been reduced in most locations by significant amounts, oftentimes by about 50 percent over the past 30 years. But there is a problem with pollutants that are very difficult to control because of various chemical processes that occur in the atmosphere. A good example is ozone. More than 150 million Americans live in areas that are above the ozone standard. While we’ve seen declines in heavily polluted areas like Southern California in terms of most of the other pollutants that are regulated under the Clean Air Act, we have not seen the levels of decline in ozone that we would expect.

Air pollution is a public health concern in areas that are close to roadways. Roadways are like the smokestacks of the 21st century. There are higher levels of nitrogen dioxides that come either directly from the tailpipe or from the atmosphere as a result of emissions. There are also ultrafine particles that can penetrate deep into the lungs and even bypass the blood barrier, traveling to major organs in a matter of minutes to hours. Diesel exhaust, another pollutant, is a carcinogen. And, though diesel exhaust has been heavy regulated and its levels have come down in many places, areas near major roadways and near airports and seaports have elevated levels. People who spend a lot of time in their cars or people who are active (biking or walking) near roadways are likely to be exposed to elevated doses.

When there’s traffic congestion, pollution emissions tend to go up. Those are areas where we still have a lot of work to do. The other problem is that these major roadway areas are often inhabited by people of lower socioeconomic status, who are more likely to be overweight or have diabetes or other health problems. So they face additional burdens on their health because these illnesses make them more susceptible to a given level of air pollution.

Is global air pollution getting better or worse?

We’re facing one of the largest public health crises of the 21st century when we look at air pollution in places like New Delhi and Beijing and many similar cities in Southeast Asia. You have a confluence of very large populations and dramatically increasing levels of air pollution because of a number of things, including governments’ not knowing how to formulate public policies to combat the air pollution that comes with the increasing growth, and dramatic increases in economic wealth leading to people purchasing private automobiles instead of using public transit or bicycles. In areas like New Delhi and Beijing, we’re seeing levels of pollution that are almost historically unprecedented—not seen since the 19th century in industrial England and other parts of Europe, when people were burning coal in the houses and the industries were right in the center of the city. And there are many places in South America and Africa that are also experiencing dramatically increasing levels of pollution.

What we see in a lot of these places is that people still rely on burning wood, cow dung, sugar cane, and other solid fuels to cook in their houses. Huge emissions come from these sources. This affects indoor air quality, and it’s also a big factor in terms of the outdoor air. In China and India, you have a double threat: You have the air pollution that comes from burning solid fuels combined with pollution from the rapidly expanding industrial capacity and a shift to driving automobiles. The levels of air pollution might be five to 10 times higher than what you find in American cities. During one recent extreme pollution event in New Delhi in late 2016, the levels were more than25 times the U.S. Environmental Protection Agency’s (EPA) standard. Even in highly polluted areas in the U.S. like Houston or L.A., it would be rare to have a day where you have much more than 1.5 to 2 times that standard.

We don't know how this affects health, but we suspect there’s a very large burden of illness associated with these exposures. The World Health Organization estimates that more than 6 million people are prematurely dying every year from the combined effect of household air pollution and ambient air pollution. Most are living in Southeast Asia.

How much of the increases in asthma that we're seeing in developed or high-income countries are related to air pollution?

In many of the wealthier countries of the world there have been increases in asthma over 20 or 30 years. The rapid pace and increase in prevalence has raised the concern that there must be an environmental contributor. It’s not resolved that all of this increase is due to environmental factors; it may be due to increases in diagnosis. But it’s rapid enough and large enough that many people think there are environmental causes at work.

The EPA considers that both ozone and nitrogen dioxide exacerbate asthma. That’s what’s been found in all the epidemiological literature as well as animal and lab studies. There’s a suggestion that asthma incidence in children is related to nitrogen dioxide, which is a traffic-related pollutant. But it's bundled in a package with ultrafine particles, diesel, and black carbon. So it’s not clear if it’s nitrogen dioxide itself or some of these other pollutants. We also have evidence that when air pollution goes up in the span of one to two days, we observe many more admissions to hospitals for respiratory systems and an exacerbation of asthma symptoms.

How does air pollution affect society?

There are several impacts of air pollution on society. One of them is productivity. We have strong evidence from well-controlled studies from Southern California that when there are elevations of air pollution, children's absences from school go up. We also know that once people have asthma, they may be more likely to miss work. And, if you look at how much the person can contribute to society over the life span, essentially when life is cut short or they live years with a debilitating disease, people are less likely to contribute productively. Parents may not be willing to let their asthmatic children go out and play on days when there’s high air pollution. This may make these children more prone to obesity. Adults with asthma may be less inclined to do physical activity outdoors

In addition, fixing air pollution at the tailpipe and at the factory are usually very expensive solutions. As a result, consumers will have to pay quite a bit more for products than they would if those industries or automobiles were designed with pollution controls in the first place. When we have to pay more to regulate cars, that money could've been spent on something else that’s more productive and makes people better off. Instead, it’s being spent to try to restore air quality to the healthier levels we had before. It diverts a lot of our resources away from more productive uses that could benefit people’s welfare as we compensate for the damage that’s being done to the environment.

Does wearing a surgical mask outdoors, as many people do in China, help keep out pollutants?

Unless the masks are specifically approved to remove fine particles or gases, such as ozone, by the National Institutes of Occupational Safety and Health or similar agencies, they’re probably not doing very much. Most of the particles will pass through easily.

Aside from respiratory problems, are there any surprising negative health effects of air pollution?

Air pollution affects health throughout the life course, starting even before we’re born. When we look at the effects, in utero exposures are well documented. People who are exposed to secondhand tobacco smoke or high levels of ambient air pollution are more likely to have babies with low birth weight and congenital abnormalities. There’s convincing evidence that air pollution contributes to atherosclerosis in adults.

Some of the more recent evidence points to air pollution contributing to diabetes. There are at least five studies that have carefully tracked people over time and looked at the incidence of diabetes in the population in relation to air pollution exposure. For both nitrogen dioxide and fine particulate matter, we see that the effects of air pollution are significant, increasing diabetes risk by about eight to 10 percent. Almost half the U.S. adult population is affected by diabetes or prediabetes. So if you have a pervasive environmental exposure that’s contributing to this devastating disease, this is a major shift in our understanding.

There is also some evidence that air pollution can affect obesity, something we didn’t expect. In two studies—I was the lead author on one, and co-author on the other—we tracked children over eight years and we controlled for 50 variables. And we found that children living in areas of higher air pollution have a higher body mass index (BMI) than those living in less polluted areas. I wouldn’t say it’s causal at this point, but it follows the same metabolic pathways that we see with diabetes.

Finally, there's an emerging literature that air pollution over time is leading to earlier cognitive decline in adults as they get older. The influence of air pollution on the blood vessels in the brain and the autonomic signals from the heart and the nervous system could be responsible for this. But this is still a new, active area of research.

This opinion does not necessarily reflect the views of the UC Berkeley School of Public Health or of the Editorial Board at

Also see Pollution Inside Your Car.