In the western United States, regulations on emissions, primarily from industry and automobiles, have helped reduce ozone-producing pollution. Nitrogen oxides and other pollutants react in the atmosphere to produce ozone, which, near Earth’s surface, is a health hazard. Between 2005 and 2010, nitrogen oxide pollution in the western United States dropped by 21%, and should have been accompanied by a similar reduction in ozone, according to a new study. Yet ozone levels barely budged. Why not? Using a combination of satellite observations and chemical transport modeling, researchers found that two rising sources of ozone offset what should have been a reduction. About half of the offset came from naturally produced ozone that has fallen down from the stratosphere. The other half came from ozone that has drifted across the Pacific from China, where ozone levels rose by 7% over the 5 years, according to a study published today in Nature Geoscience. The evidence suggests that ozone pollution is becoming a global rather than a local problem. Its transport halfway around the globe will make it harder for western U.S. states to satisfy air quality requirements set by the Environmental Protection Agency, which in November 2014 ratcheted down its ozone standard from 75 parts per billion to between 65 and 70 parts per billion. To avoid punishment, some air quality districts have already begun pointing the finger at China . Yet there are no agreements to limit the intercontinental transport of ozone and its precursors.