Delhi of Lutyens is one of the most iconic neighborhoods of India’s capital. It is home to the country’s parliament, numerous embassies and a lush 90-acre Mughal-era park. It is an architectural paradise, connected by tree-lined streets and roundabouts with mini gardens. Despite being one of the city’s most sophisticated areas, this clean, leafy neighborhood harbors something sinister. It’s 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 is shrouded in smog for days on end. But discussions about air pollution and policies to reduce it usually focus on particulate matter: PM2.5 and PM10 – tiny particles or droplets just a few microns in diameter. However, scientists are increasingly raising the alarm about surface ozone. It is a secondary pollutant that is not released from any source and forms naturally when nitrogen oxides and volatile organic compounds, such as benzene, found in gasoline, or methane, react under high temperatures and sunlight. This makes ozone a particularly ugly modern threat – a problem that arises where pollution and climate change converge.
“Even an hour of exposure can have very bad health outcomes,” said 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, concentrations of it on the Earth’s surface can be deadly. Data on its impact is fragmentary, but a Study from 2022 estimates that ozone killed more than 400,000 people worldwide in 2019, a 46 percent increase since 2000. And according to the State of Global Air Report 2020it is in India where the number of deaths from ozone has increased the most in the past decade.
Ozone wreaks havoc in the respiratory tract. The gas can “inflame and damage airways” and “exacerbate lung diseases such as 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 physician and community medicine specialist in Karaikal, India. After exposure, “we are more susceptible 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 difficult 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 after an ozone spike, there are more hospitalizations due to respiratory problems,” says Balajee.
Delhi and other major Indian cities see ozone spikes throughout the year, but especially during summer heat waves, which are becoming more common due to climate change. The World Health Organization says exposure to airborne ozone should not exceed 50 parts per billion over an eight-hour period; India’s air quality standard say this WHO limit should not be exceeded more than eight days per 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 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 measuring stations that registered an exceedance of the ozone threshold was less this year than in previous years, the duration of the exceedance was longer. “We’re not even close to meeting the standard,” says Somvanshi.
Part of the cause of this problem is ozone’s complicated relationship with other air pollutants. Ozone formation is a cyclic reaction, meaning that after being generated by reactions between air pollutants, ozone converts back to oxygen when it reacts again with airborne pollutants, such as nitrogen oxides. But if those pollutants aren’t present after ozone is formed, it sticks. That’s why ozone levels skyrocketed during India’s Covid-19 lockdown in the summer of 2020, when traffic came to a screeching halt — the air pollutants needed to convert it back into oxygen weren’t being produced. It’s also why ozone is often found in green neighborhoods like Lutyens’ Delhi – because their air is cleaner, there are no reactions that remove the ozone.