As countries around the world imposed tough restrictions on daily life this Spring to prevent the spread of COVID-19, a new study in Nature Sustainability looks at how air pollution levels were impacted by the subsequent declines in economic activity. The study uses timely and comprehensive air quality data from China. It finds that in the weeks after the Chinese government locked down one third of its cities, particulate pollution (PM2.5) levels dropped by 24 percent countrywide.

“With empty roads and industrial activities largely at a halt, people were consuming less energy, and as a result air pollution levels were significantly lower than before the crisis,” says study co-author Guojun He, the research director for the Energy Policy Institute at the University of Chicago in China (EPIC China) and a professor at the Hong Kong University of Science and Technology. “These plummeting pollution levels can be an early indicator of the deep economic impact of COVID-19.”

He and his co-authors, Yuhang Pan and Takanao Tanaka, also from the Hong Kong University of Science and Technology, compared air quality data in cities where a lockdown was imposed to cities without formal lockdown policies. Those with the lockdown saw their particulate pollution drop 17 percent. To investigate the impact of precautionary measures the government took in other areas not locked down—including extending the Spring Festival holiday, requiring social distancing, and urging people to stay at home—the researchers compared pollution levels in the months and years before COVID-19 hit. They discovered that even the precautionary measures caused pollution to decline by 7 percent. Across the board, lockdowns and precautionary measures led to a 24 percent drop in particulate pollution. The study found the biggest drop in pollution to be in cities with a larger economy, greater industrial activities and traffic, and higher demand for coal heating.

“Even with significant restrictions on daily life and commerce, pollution levels in China were still four times greater than what the World Health Organization considers safe,” He says. “This could be because, while the data confirmed that traffic, industrial, and business activities are important sources of air pollution, so are coal-fired winter heating systems. Those residential heating systems were still, necessarily, powered on during the coronavirus crisis. Moving forward, it will be important for China to develop policy tools that can effectively target these sources without compromising economic activity.”