For the first time, using a chemical transport model designed to estimate the distribution and budget of isocyanic acid in the troposphere, Young et al. show that in several parts of the world, local emissions may increase the concentration of isocyanic acid in ambient atmosphere, thereby exposing large populations to potentially toxic levels of the acid. Their research shows that regions that experience large forest fires, such as tropical Africa, Southeast Asia, Siberia, Canada, and the Amazon, or are heavily polluted, like China, are particularly vulnerable. In these regions, concentrations of isocyanic acid in the atmosphere exceeded the 1 ppbv limit for about 7-90 days per year. Their model also predicts that doubling the rate of air pollutant emission, particularly in heavily polluted regions of China, could increase the exposure of humans in the region to more than 170 days per year to isocyanic acid levels exceeding 1 ppbv.