Rising ozone pollution in China‘s cities has emerged as a major health risk, causing a rise in deaths from strokes and heart disease among vulnerable residents, according to a new study by a team of Chinese researchers.

Data from 272 Chinese cities between 2013 and 2015 showed “robust evidence” linking rising short-term ozone exposure with increased mortality from cardiovascular and heart diseases as well as strokes, according to a paper published in the Environmental Health Perspectives journal.

Ground-level ozone, also known as photochemical smog, is caused by the interaction of sunlight with nitrogen oxides (NOx) and the vast amounts of uncontrolled volatile organic compounds (VOCs) produced by burning fossil fuels or producing chemicals.

China is waging a “war on pollution” to reverse the environmental damage done by nearly four decades of untrammelled economic growth. But much of the focus has been on reducing concentrations of small airborne particles known as PM2.5, especially in winter.

Kan Haidong, director of the department of public health at Shanghai’s Fudan University, said while PM2.5 is currently a bigger contributor toChina‘s overall disease burden, ozone is already equally significant in regions like the Pearl River delta.

“Ozone has been increasing in the past several years in China,” said Kan, who was involved in the study. “In contrast, PM2.5 has decreased by about 30 percent in the past five years.”

Lauri Myllyvirta, a Beijing-based campaigner with environmental group Greenpeace, said soaring ozone is partly a result of China‘s success in reducing PM2.5, which has increased the amount of sunlight, but China has lagged when it comes to tackling NOx and VOCs.

China‘s average ozone exposure increased 17 percent over 2014-2017, implying an additional 12,000 premature deaths per year, Greenpeace estimated, using data from China‘s environment ministry and the Global Burden of Disease (GBD) database (http://www.healthdata.org/gbd).

China launched a 2013-2017 action plan aimed at reducing average PM2.5 levels by 25 percent in targeted regions, but there was no target for ozone.

Liu Bingjiang, head of the air quality department at China‘s environment ministry, said in September that 59 out of 338 monitored cities exceeded the national ozone standard of 160 micrograms per cubic metre last year.

But he said while China was paying close attention to the problem, recent increases were “still a normal fluctuation”.