As part of China’s “war on pollution,” local officials are permitted to require emissions reductions from polluting factories, and often do so by closing down factories that are high polluters. The researchers are taking advantage of a natural experiment where “yellow,” “orange,” and “red” alerts are triggered at different pollution levels, with higher alert levels requiring higher reductions in industrial emissions. For each alert type, the local government has a pre-set “action plan” describing which firms will be required to shut down or curtail production. The researchers are comparing days that had forecasted pollution barely below an alert threshold to those with forecasted pollution barely above an alert threshold, as well as comparing between locations just inside and just outside prefecture city boundaries. This allows them to estimate the effects of pollution forecasts on emissions, government actions, firm production, and health outcomes.