Smoke from burning biomass may contain enough isocyanic acid (HN=C=O) to cause physiologically significant levels of protein carbamylation, atmospheric chemists report (Proc. Natl. Acad. Sci. USA, DOI: 10.1073/pnas.1103352108). Carbamylation is part of an inflammatory response that has been linked to atherosclerosis and rheumatoid arthritis. Despite known health effects, HNCO has never been measured in atmospheric samples. Now, using negative-ion proton-transfer chemical-ionization mass spectrometry, James M. Roberts of NOAA in Boulder, Colo., and coworkers have measured HNCO in urban and wildfire air samples, as well as in laboratory-generated tobacco smoke. The researchers measured HNCO levels of 100 parts per trillion by volume (pptv) in Los Angeles air samples and 200 pptv in samples from Boulder after the 2010 Fourmile Canyon wildfire. HNCO levels in tobacco smoke were off scale when measured by these methods. Literature estimates suggest that tobacco smoke could contain as much as 140 parts per million by volume, the researchers note. The measurements show that humans can be exposed to detrimental levels of HNCO through multiple pathways, the researchers say.