Here’s another way China’s notorious air pollution is making citizens’ lives uncomfortable—it’s making the country’s cities hotter.

Researchers have found evidence that the pollution engulfing China’s cities enhances the warming effect of cityscapes, raising the temperature by one degree Celsius. Writing in Nature Communications, they say it’s not the bigger cities that suffer the most, but those with the worst of a certain type of air pollution.

Cities tend to be hotter than countryside areas because of the Urban Heat Island effect—the density of buildings and the materials they are built out of absorb heat and radiation from the sun extremely well, but don’t readily release it at night, keeping the area warmer for longer.

Meanwhile, China’s cities are often covered in a haze, as the researchers call it, that comes from the vehicles, factories, and coal-fired power plants that have driven China’s industrial development, which in turns has triggered a mass migration of citizens from rural areas into cities in search of work. The population regularly manifests as a suffocating smog that can engulf major cities for days.

Scientists have long suspected pollution exacerbates the Urban Heat Island, said Xuhui Lee, a professor at the Yale University School of Forestry & Environmental Studies, in a press release. The new finding, based on satellite data, is the first direct evidence that China’s infamous pollution problems are compounding the effect.

But not all pollution is created equal. Fine pollution particles, such as those in the 2.5 PM (particulate matter) range that are normally blamed for damaging health can actually be a protector when it comes to city heat. On the one hand, they cause asthma and penetrate the blood stream and internal organs, raising the risk of cancer and heart disease. On the other hand, those same sized particles actually block sunlight, which can help cool city surfaces.

Larger aerosol particles, such as those generated from road dust, coal burning, cooking stoves and sand—particularly a problem in cities in China’s northwest that are near to the Gobi and Taklimakan deserts—absorb and radiate heat while also being bad for health. The dust carried over from the deserts, combined with industrial pollution, mean these cities suffer the most from a thicker haze and a larger heating effect, compared to the bigger coastal cities. Hami City, population 450,000, has an Urban Heat Island effect three times worse than Shanghai, which has 14 million residents.

Lee says it shows how tackling China’s air pollution problems can benefit health in two very different ways. “Cleaning up has a co-benefit,” said Lee, “It helps improve human health, but it also helps to cool the local climate.”