One of the great environmental success stories of the past half century has been significant reductions in smog across the United States and other parts of the world, thanks largely to laws and technological advances that reduced emissions that create ground-level ozone, a key component of smog. But some places have recently experienced spikes in ground-level ozone during particularly hot periods, such as during heat waves in 2021 and 2022 that made the air particularly unhealthy in parts of Texas, the Pacific Northwest, and Europe.
More days like that might be on the way, in more places. Recent studies project that rising global temperatures will increase ground-level ozone — the core component of smog — to higher levels in the coming years, causing more respiratory illnesses and deaths.
“Climate change and changes in emissions of ozone precursors will elevate future ground-level ozone in many parts of the world — the short-term exposure to which is linked to increased mortality,” concluded a study led by researchers at the Yale School of Public Health in Connecticut, published this year by the journal One Earth.
Meanwhile, efforts to curb the rise in global temperatures continue moving forward, albeit slowly and sometimes haltingly. The United Nations reported last fall that “the world is not on track to meet the long-term goals of the Paris Agreement,” the 2015 treaty through which nearly 200 nations agreed to adopt stricter controls on emissions that cause pollution. The United States, one of the world's biggest polluters, signed on to the Paris Agreement in 2021, and its participation in the future might be in jeopardy, given the politicization of climate change in the United States.
In June, the U.S. Supreme Court blocked implementation of the Environmental Protection Agency’s (EPA’s) Good Neighbor Plan, which aims to reduce ozone-producing emissions from industrial sources, and overturned the so-called Chevron Supreme Court ruling, which gave the EPA broad leeway to enact rules for carrying out environmental laws passed by Congress.
The health impacts of those policy changes could be significant. Breathing in ozone, the EPA explains, is linked to coughing, sore throats, increased susceptibility to lung infections, aggravation of existing lung conditions (including asthma, emphysema, and bronchitis), and respiratory and cardiac mortality.
What is ozone?
Say “smog” and people typically envision air that looks dirty; a clear sky feels clean. But the main component of what scientists call smog — ozone — is a colorless gas, says Kai Chen, PhD, co-faculty director of the Yale Center on Climate Change and Health. “Ozone pollution is invisible.”
He contrasts that with the visibility of high levels of fine particulate matter, such as particles carried by wildfire and industrial smoke. Ground-level ozone and fine particulate pollution sometimes occur at the same time — thus, the dirty air that people see — and are identified by the EPA as the two most common forms of air pollution in the United States.
Ozone can be healthy or dangerous to humans, depending on where it sits. The gas exists both in the stratosphere (at about 10 miles above the Earth’s surface) and at ground level. In the stratosphere, ozone is formed by a reaction between oxygen and ultraviolet radiation from the sun. That high-level ozone acts as a natural sunscreen, blocking much of the sun’s radiation from reaching the Earth, where it can cause skin cancer, cataracts, and other ills.
“It forms naturally, and it’s actually a good thing,” says Dmitri Kalashnikov, PhD, a graduate research assistant at the Washington State University School of the Environment in Vancouver, British Columbia.
On the ground, ozone is dangerous because “it’s what we breathe” and harms the lungs, Kalashnikov notes. Ground-level ozone forms through reactions between two types of compounds — nitrogen oxides and volatile organic compounds (VOCs) — that occur naturally to some degree but are produced in abundance by man-made processes.
Sources of nitrogen oxides include fuel production and combustion (vehicles, power plants, and gas stoves), burning (forest fires), and some microbial processes in soil. VOCs — these are “volatile, which means they’re ready to react with other compounds,” Kalashnikov notes — come from the use and production of many man-made chemicals, such as paints, solvents, and dry cleaning agents, explained by the EPA.
“Generally, when we talk about ozone pollution, it’s [primarily] caused by human sources,” Kalashnikov says.
The good news: Federal and state regulations have driven down ground-level ozone and other forms of pollution since Congress passed the Clean Air Act in 1970. The act and its subsequent amendments authorized federal and state regulations to limit emissions of ozone precursors from industrial facilities, transportation sources, and household products (such as cleaners). One example of the impact: The EPA reports that average ozone levels across 132 consistently measured sites in the United States went down 29% from 1980 to 2022.
(Ozone is measured regionally, by parts per million [ppm]. In 2015 the EPA lowered the allowable ppm from 0.075 to 0.070. In 2022, the EPA reports, the mean level in the areas measured in the United States was 0.066 ppm. Regions that exceed the limits for certain periods of time are required to enact more restrictions on sources of nitrogen oxides and VOCs.)
So why worry about ozone now? Because of what climate and environmental scientists call the climate penalty, whereby rising temperatures cause increases in ground-level ozone, thereby threatening to counteract efforts by governments to reduce air pollution.
Simply put, heat accelerates the process by which nitrogen oxides and VOCs create ozone.
“One of the key factors in the reaction is temperature,” explains James East, PhD, a postdoctoral fellow in the Harvard John A. Paulson School of Engineering and Applied Sciences in Boston, who studies environment and climate. “The higher the temperature, the faster the reaction rate and the more ozone that gets formed.”
That phenomenon has prompted studies of how rising temperatures have increased ground-level ozone and will continue to do so, Chen explains. A study focusing on the western United States, led by Kalashnikov and published in Science Advances, looked at the dangerous combination of ozone and fine particulate matter. Fine particulate matter — scientifically identified as PM 2.5, because it’s made up of inhalable particles with diameters of 2.5 micrometers or less — comes from things like smokestacks and fires. Many sources that emit PM 2.5 also emit nitrogen oxides and VOCs.
The study found increasing occurrences of ozone and PM 2.5 exposure from 2001 through 2020. Two of the most significant factors were the increasing occurrences and intensity of heat domes — which develop when intense hot air essentially sits over wide geographic swaths — and wildfires, which spread particulate matter hundreds of miles from their sources. Heat domes “persist for days, and that’s conducive to producing more ozone,” Kalashnikov says.
The study predicted “an increasing potential for co-occurring pollution episodes in the western U.S., with continued climate change.”
The One Earth study led by Yale researchers examined epidemiological data from more than 400 cities in North America, Europe, Asia, Australia, and Africa, and estimated air quality and ozone-related deaths under multiple scenarios. Those scenarios included rising temperatures and potential government regulations to compress ozone levels. The study compared the period of 2010-14 to projections for 2050-54, and calculated that total ozone-related deaths would rise by 45 a year under the best-case scenario (widespread adoption regulations to meet the Paris Agreement goals), and by 6,200 annually under the worst-case scenario (no advancement on climate policies and weak ozone restrictions).
“If we keep the current levels” of government air-pollution regulations, “that will make it harder to meet the air-quality goals” that countries established, says Chen, lead author of the study.
A 2019 study of ozone in the United States, mid-Atlantic region, led by the University of Delaware, concluded that progress on ozone reductions “will be partially negated as temperatures continue to increase.… Maintaining current ozone levels will require even stricter regulations.”
The future
Scientists who warn about the need for further ozone restrictions are not optimistic about the prospects of governments taking such action. Actions to mitigate climate change can be politically controversial in the United States. Governments, by and large, are not on target to meet their goals to stem climate change and reduce air pollution.
One challenge to raising concerns about ozone is its literal invisibility. Significant air-quality measures, including the Clean Air Act, were spurred in large part because of increasing images of cities (such as New York in the 1960s) being blanketed by fine particulate matter from industrial production and transportation. Wildfire smoke drifting across the United States over the past two years has again sparked discussions about the dangers of particulate pollution and the need for government action to curtail it.
At Harvard, East wonders about the ability to get people to worry about ozone when they don’t see it. He thinks about air-quality alerts that government agencies send to people on their phones (such as the EPA’s AirNow app, which tracks ground-level ozone and PM 2.5 levels for cities across the world).
“If you want to ride your bike and it looks clean outside, and you get a push notification on your phone” that warns of high ozone levels, “you think, ‘Well, it looks clean. I want to ride my bike; I don’t want to sit inside.’”
East and Chen say that warnings about ozone must be communicated, because there is evidence of harmful health impacts even under current government standards and because those impacts are likely to grow worse as ozone mixes with particulate matter, and as temperatures keep rising.
“When you have more hot days and hotter days in the summer,” Chen says, “that risk is going to increase.”
