Heat Waves Are Unleashing a Deadly but Overlooked Pollutant

LUTYENS’ DELHI IS one of the most iconic neighborhoods of India’s capital. Home to the country’s parliament, numerous embassies, and a lush, 90-acre Mughal-era park, it’s an architectural paradise, connected by tree-lined streets and roundabouts with mini-gardens. Yet despite being one of the city’s most refined districts, this clean, green neighborhood is home to something sinister. It is a hot spot for a dangerous and overlooked air pollutant: ozone.

India is no stranger to pollution, with many of its cities reporting some of the worst air quality in the world. Every winter, New Delhi gets shrouded in smog for days. But discussions about air pollution and policies to mitigate it mostly focus on particulate matter: PM2.5 and PM10—small particles or droplets that are only a few microns in diameter. However, scientists are increasingly raising the alarm about surface ozone. It’s a secondary pollutant that isn’t released from any source, forming naturally when oxides of nitrogen and volatile organic compounds—such as benzene, which is found in gasoline, or methane—react under high heat and sunlight. This makes ozone a particularly ugly modern threat—a problem that arises where pollution and climate change coincide.

“Even an hour of exposure can give you very poor health outcomes,” says Avikal Somvanshi, a researcher at the Center for Science and Environment in New Delhi. While ozone is beneficial in the high atmosphere, where it absorbs ultraviolet radiation, down on Earth’s surface, concentrations of it can be deadly. Data on its impacts is patchy, but a 2022 study estimates that ozone killed more than 400,000 people worldwide in 2019, up 46 percent since 2000. And according to the State of Global Air Report 2020, it is in India where the number of ozone deaths has increased the most over the past decade.

Ozone wreaks havoc in the respiratory tract. The gas can “inflame and damage airways” and “aggravate lung diseases like asthma,” warns the US Environmental Protection Agency. It does this by affecting the cilia, the microscopic hair-like structures that line the airways to help protect them, explains Karthik Balajee, a clinician and community medicine specialist based in Karaikal, India. After exposure “we are more prone to respiratory infections,” he says, adding that inhaling ozone also affects lung capacity. Studies show that long-term exposure is associated with an increased risk of chronic obstructive pulmonary disease, a lung disease that makes it hard to breathe, and increases the risk of dying from other cardiovascular or respiratory conditions. Even short-term exposure can land you in the emergency room. “One or two days following a peak in ozone, there have been increases in hospital admissions due to respiratory problems,” says Balajee.

Delhi and other major Indian cities see spikes in ozone throughout the year, but particularly during summer heat waves, which have become increasingly common due to climate change. The World Health Organization says that exposure to ozone in the air, across an eight-hour period, shouldn’t exceed 50 parts per billion; India’s air quality standard says this WHO limit shouldn’t be broken on more than eight days a year, and not on two consecutive days. But analysis by Somvanshi and his colleagues has found that ozone has already exceeded limits in Delhi and its surrounding areas on 87 days between March and May this year. And they’ve seen similar results over the past three summers. And while the number of monitoring stations recording a breach of the ozone threshold was fewer this year than in previous years, the duration of the exceedance was higher. “We are not even close to compliance with the standard,” says Somvanshi.

Part of what’s driving this problem is ozone’s complicated relationship with other air pollutants. Ozone formation is a cyclic reaction, meaning that after it’s generated by reactions between air pollutants, ozone gets converted back to oxygen when it reacts again with pollutants in the air, such as oxides of nitrogen. But if those pollutants are not present after ozone has formed, it lingers. That’s why ozone levels shot up during India’s Covid-19 lockdown in the summer of 2020, when traffic screeched to a halt—the air pollutants needed to convert it back to oxygen weren’t being produced. It’s also why ozone is often found in green neighborhoods like Lutyens’ Delhi—because their air is cleaner, reactions that get rid of the ozone don’t take place.

Controlling particulate pollution is relatively simple—you just regulate its sources, such as vehicles and crop burning. Reducing ozone levels is harder. “Our prediction of how ozone might respond to pollutant changes or emission changes is complicated by environmental factors,” says Steve Arnold, professor of atmospheric composition at the University of Leeds. When trying to drive levels down, everything depends on the exact mixture of the different precursor pollutants in the air, he says.

Reducing precursor gasses is one way to reduce ozone formation. But bring them down too much and you won’t have any left to neutralize the ozone that’s already present. This is exactly what has happened in China, says Arnold, where stringent policies have reduced PM2.5 pollution in recent decades but the ozone problem has worsened. “There’s a delicate balance that needs to be made,” says Somvanshi.

At present, particulate matter is India’s focus, and rightly so—it’s a bigger problem. According to Arnold’s research, 900,000 deaths in India each year are linked to PM2.5, with 374,000 linked to ozone. But in the future, we should worry more about ozone, Arnold says. He has modeled how ozone pollution in India might look in the coming years. “If you go to 2050, then the health impacts from particulate matter and the health impacts from ozone pollution become much more similar in magnitude,” he says. One reason is that researchers believe pollution control policies in the future will be more successful in mitigating particulate matter than ozone.

Arnold’s study estimated that by 2050, India could see more than a million premature deaths a year linked to ozone exposure if there is no change in emissions. Even under a strict pollution control policy, premature deaths due to ozone were estimated to be 791,000—more than twice as many as in 2015.

While the gas is a problem in other countries, it is India’s poor air quality and increasingly common heat waves that makes ozone a particularly grave threat there. And for Arnold, how the country will evolve is concerning: India will have both a growing and aging population in the coming decades—meaning more people will be exposed to ozone, and in particular, more who are vulnerable.

And it’s not just human health that is damaged. Ozone also threatens food security by causing plant yields to “reduce drastically,” says Madhoolika Agrawal, a professor in the department of botany at Banaras Hindu University. Ozone enters plants through small pores on their leaves, explains Agrawal, and then kills cells by oxidizing them. Leaves then start yellowing and the plant is unable to photosynthesize. India’s wheat crop is particularly vulnerable. The country is a major producer and aspires to be a top exporter—yet studies show it loses millions of tons of wheat and rice annually to ozone. In fact, a 2021 paper shows that ozone affects the yield and seed quality of all major crops in India. Current policies are “insufficient in reducing crop loss to ozone,” its authors write.

At present, Somvanshi says India is “not doing anything” about ozone, and that the way the country’s Central Pollution Control Board monitors the toxic gas is flawed. The CPCB caps ozone pollution measurement at 200 micrograms per cubic meter—above that, the measurement software just records a blank cell in the data sheet. “We don’t really know exactly how severe the problem gets,” says Somvanshi, comparing the situation to checking someone’s fever using a thermometer that can only go up to 100 degrees Fahrenheit. The CPCB also reports ozone levels after averaging values at all the monitoring stations in a city, which Somvanshi calls a “disaster” because ozone is a hyperlocal pollutant. “Within the city, there is a massive variation and if you average it out, it will always be below the standard,” he says. WIRED reached out to the CPCB for comment but did not receive any response.

And while ozone monitoring infrastructure is robust in the capital region—Delhi and its five satellite cities have nearly 60 stations—more than 200 Indian cities don’t monitor ozone at all. The rest only have one or two ozone monitoring stations, which is basically “useless” says Somvanshi, because of the localized nature of the gas.

A quick way to mitigate the problem would be to issue targeted local ozone alerts, he says. In the US, for example, health advisories during periods of high ozone pollution warn residents to not go to refuel their cars when it is sunny outside, because petrol or diesel can evaporate to form volatile organic compounds that can further increase ozone formation. India could do something similar. Somvanshi suggests that factories could also be directed to operate during the evening or nighttime. That way, any nitrogen oxides they release wouldn’t be converted into ozone because there’s no sunlight. To reduce the precursor gasses, Arnold says the government should encourage “policies that can limit motor vehicle emissions in cities.”

As for addressing the threat to food security, Agrawal says farmers could choose crop varieties that are less susceptible to ozone. Some plants show resilience, she says, and by cross-breeding we could get crops that are less affected by it. Another solution could be to plant crops during seasons when ozone levels are low, like during the monsoon, but that’s not possible for all plants.

Above all, Arnold says, the Indian government should “take the time to understand the potential consequences of emission reductions” and learn from what happened in China. Use “modeling to try and predict the response you might get to emission control policies,” he advises. Ozone pollution, he says, cannot be ignored anymore.

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