Atmospheric aerosol particles (also known as particulate matter) are central to the cause of the two greatest threats to human security: air pollution (similar to 5 million premature deaths per year) and climate change (similar to 0.5 million per year). Addressing these threats requires an understanding of particulate matter sources responsible for both extreme air pollution immediately affecting human health and less extreme levels affecting climate over longer timescales. Here, extraordinary levels of air pollution, with submicrometre aerosol (PM1) mass concentration surpassing 300 mu g m(-3), were observed in a moderately sized European city and are attributed to emissions from residential solid fuel-specifically peat and wood, often promoted as 'slow-renewable', 'low-carbon' or 'carbon-neutral' biomass. Using sophisticated fingerprinting techniques, we find that consumption of peat and wood in up to 12% and 1% of households, respectively, contributed up to 70% of PM1. The results from this approach can better inform emissions reduction policies and help to ensure the most appropriate air pollution sources are targeted. Given the far greater abundance of solid fuels and concomitant emissions required to match the calorific benefit of liquid fuels, even modest increases in the consumption of 'green'-marketed solid fuels will disproportionally increase the frequency of extreme pollution events.