Wildfire smoke and scorching heat are reversing decades of progress in cleaning the world’s air, according to the World Meteorological Organization (WMO).

The UN agency warns that extreme fire-smoke events have tripled globally since the 1990s. At the same time, rising temperatures are making ground-level ozone—a harmful pollutant that forms when heat reacts with vehicle and industrial emissions—more severe. “This is a major setback for public health,” the WMO said in its latest air quality bulletin. The report links the surge in smoke to longer, more intense wildfire seasons driven by drought and heatwaves. Ground-level ozone, unlike the protective ozone layer high in the atmosphere, causes breathing problems and damages crops. Hotter weather speeds up its formation, meaning even regions with strict pollution limits are seeing worse air during summer spikes. The WMO stresses that cutting fossil fuel emissions remains the core fix. But it adds that even with current climate pledges, more frequent heatwaves will keep pushing ozone levels up unless nations also target the specific pollutants that feed it. The findings undermine global efforts to meet clean air targets, as smoke from mega-fires travels thousands of kilometers, crossing borders and undoing local gains. The agency calls for better cross-border monitoring and faster adaptation to a hotter, smokier future.