Ozone—made up of three oxygen atoms—is a powerful chemical. In the upper atmosphere, it protects against the sun’s harmful ultraviolet radiation. But down below, breathing it harms the lungs and can lead to respiratory diseases.

China is finding this out the hard way. An analysis of government data by Unearthed, a Greenpeace-funded news publication, found that Beijing and its neighboring city Tianjin are suffering from record levels of ozone (O3).

Unearthed/Greenpeace

Though China has made impressive strides in cutting sulfur oxides (SOx) and particulate-matter pollution (PM 2.5), other pollutants remain. Ozone is created when nitrogen oxides (NOx) react with volatile organic compounds (VOCs) in the presence of sunlight. Hotter temperatures increase the rate of ozone production. Both NOx and VOCs are mainly the result of burning fossil fuels in industry, transport, and power plants.

Each year ozone pollution is estimated to cause 70,000 premature deaths in China, according the Global Burden of Disease. The country doesn’t yet have regulatory limits on ozone pollution, but one may be in the works, according to Unearthed.