The unobserved responses of individual polluters are often used to rationalize the aggregate effects of international trade on the environment. In this paper, I provide the first evidence of these responses. I estimate the effects of NAFTA on the emissions of particulate matter (PM10) and sulfur dioxide (SO2) from manufacturing plants in the United States. My findings suggest that trade liberalization led to significant reductions of these pollutants at affected plants. On average, nearly two-thirds of the reductions in PM10 and SO2 emissions from the U.S. manufacturing sector between 1994 and 1998 can be attributed to trade liberalization following NAFTA. •I study the effects of NAFTA on the pollution emitted by U.S. manufacturing plants.•NAFTA significantly reduced emissions of PM10 and SO2.•Reductions are driven by within-plant responses to NAFTA.•My estimates suggest that trade played a key role in the clean-up of the U.S. manufacturing sector.