OVER the past month, successive waves of thick smog have blanketed northern and central China. With visibility severely reduced, authorities have cancelled flights, shut highways and imposed emergency factory closures. Air quality usually deteriorates during the winter when demand for heating soars and coal-fired power plants rev up. But after the government declared a “war on pollution” in 2014, China had in fact made some headway in improving its environment. With stricter emission standards on power plants, it worked to curb its reliance on coal. Smog, though still too frequent, was at least a bit lighter.

What explains the reversion to “airpocalypse”? The answer lies in a rebound in heavy industry, according to Greenpeace, an environmental group. China’s steelmaking heartland, just a few hours’ drive south of Beijing, has been the centre of the recent pollution bouts. Output of crude steel, pig iron and cement began to pick up around the spring of 2016, tracking a property-market rally. The concentration of small, dangerous PM2.5 (particulate matter of less than 2.5 microns in diameter) has also increased since then, more or less in tandem with the industrial products.

Although it was easy to overlook this trend in the summer and autumn, when the worst of the air was blown away from Beijing, shifting wind patterns in the winter have let it settle over the capital. With the government now trying to cool the property market, there is the possibility of a renewed industrial downturn in the coming months, welcome news for urbanites gasping for clean air. But officials are also worried about jobs in China’s steel and coal sectors, and so will try to avert a more serious slowdown. The war against pollution will only go so far.