Smoke from massive Canadian wildfires has been blowing thousands of miles across the U.S., blotting out the sun and shrouding many cities in a cough-inducing haze. As the smoke drifts with the wind, it also creates a new hazard along the way that is greater than the sum of its parts: a troubling ozone cocktail that results from a chemical reaction between wildfire smoke components and urban air pollution.

Scientists have long known that ozone is a by-product of the reaction between sunlight and chemicals in wildfire smoke called volatile organic compounds (VOCs) and nitrogen oxides (NOx). If ozone is high in the atmosphere, it is beneficial because it blocks harmful ultraviolet rays. But ozone at Earth’s surface can cause breathing problems and other health issues. A study published earlier this year in Environmental Science and Technology has shown that the smoke can create ozone even when all the NOx in it is gone. As plumes blow over cities, any VOCs that have not already chemically reacted have another chance to mix with the abundant levels of NOx that are produced by the burning of fossil fuels in urban areas.

Much of the attention on air quality issues caused by the wildfire smoke that has blanketed the eastern U.S. in recent days has focused on particulate matter: fine particles that can be breathed deeply into the lungs and pose considerable health risks. But prolonged exposure to elevated ozone levels is also dangerous. It can exacerbate asthma and cause coughing, a sore throat and difficulty breathing, particularly in children, older adults and those who spend a lot of time outside.

Ozone concerns aren’t just for cities near wildfires: VOCs can be long-lasting, meaning that even fires from more than 1,000 miles away can increase ozone levels in NOx-filled cities. And climate change heightens the risk because it is leading to more frequent and more intense wildfires. “This is a serious issue right now,” says Steven Brown, an atmospheric chemist at the National Oceanic and Atmospheric Administration and a co-author of the new study. “We face a very different challenge now that we have this response to climate change. These new emissions that are coming from wildfires present a challenge to people who want to see continued improvement in air quality.”

Ozone levels in the U.S. are regulated by the National Ambient Air Quality Standards, which set them at 70 parts per billion (ppb), averaged over eight hours. This standard imposes legal consequences for cities that have more than 70 ppb of ozone for too many days each year. The ozone borne out of wildfires is “easily sufficient to make the difference between whether a city is above or below the standard,” Brown says. On May 23, with wildfire smoke from western Canada enshrouding the city, one site in Denver surpassed 81 ppb of ozone pollution, compared with an average level of about 40 ppb for the year so far. Atmospheric chemist Suzanne Paulson of the University of California, Los Angeles, says the new study’s findings are important because increased ozone levels are “significant to particular groups, especially those with respiratory illnesses.” Paulson, who was not involved with the new research, says it shows “there will definitely be more ozone than there would have been otherwise.”

When unhealthy ozone levels are present, the Environmental Protection Agency recommends staying indoors, buying an air purifier, limiting outdoor recreation to the early morning and evening and wearing an N95 mask.

Mitigating wildfire smoke’s contribution to ozone is going to be a unique challenge for each city, Brown says. Ozone production rises in step with NOx concentrations—but only up to a certain point. Above a specific threshold of NOx, ozone production becomes less efficient.

A city is considered to be “NOx-sensitive” if it has not reached the maximum amount of ozone that can be created from the levels of NOx in the atmosphere. Conversely, if a city has passed the maximum level of ozone that can be created from the available NOx, it is considered to be “NOx-saturated.” NOx saturation tends to be prevalent near cities’ core, where the majority of emissions are released.

Counterintuitively, a NOx-saturated city that tries to reduce its NOx levels to accommodate for wildfire smoke would temporarily increase its ozone levels. NOx-sensitive cities, on the other hand, would have a much more straightforward approach to mitigation: stronger regulations on NOx production from cars, trucks, construction equipment and other industrial sources could improve ozone levels.

Despite this complication, Brown is optimistic that the problem can be tackled. “You can’t figure out how to solve a problem until you understand it,” he says. For many, the recent shocking images of smoke blotting out the Statue of Liberty drive home the importance of understanding how the intricacies of air quality are becoming essential to the health of millions in our changing climate. As Paulson puts it, our smoke-laden landscape is a “strong visual for the impacts of climate change.”

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