Toxicity and physicochemical characteristics of airborne particles found indoors remain not well understood, even though we spend the majority of our time indoors. There is a clear need to better understand effects of exposures in our homes. The aim of this study was to assess the differences in physicochemical properties and toxicity of particles found indoor and outdoor occupied residences in Sweden. Weeklong, simultaneous indoor and outdoor measurements were conducted in sixteen occupied residences in southern Sweden during winter 2016/17. Residences comprised of detached single-family houses and apartments with natural or mechanical ventilation. Two identical sets of instruments for indoor and outdoor measurements were used to assess physical characteristics of particles and collect particles for off-line chemical analysis and toxicological studies in mice. Measured time-resolved physical characteristics were number concentration and mean size, PM2.5 mass and black carbon concentrations. Off-line analysis comprised determination of polycyclic aromatic hydrocarbons, metals and soluble ions. Particles collected for toxicological studies were extracted in methanol and vacuum evaporated. Mice received a single intratracheal instillation of 18, 54 and 162 u.g of each of the pooled particle samples, namely indoor, outdoor and blanks. Lung inflammation and DNA damage in broncheoalveolar lavage cells, lung and liver tissue were evaluated 1, 3 and 28 days after intratracheal instillation. Average particle number concentration (10-300 nm) was higher indoors than outdoors. Gravimetrically assessed PM2.5 mass concentration was higher outdoors than indoors. Higher concentrations of metals Cr, Mn, Fe, Co, Ni, Cu, Cd, and Ba were measured indoors than outdoors. On average soluble ion levels CI-, N03-, S04- were higher outdoors than indoors. Measured endotoxin levels were generally low, but with higher concentrations indoors. Results of toxicological studies in mice showed higher toxicity of particles collected indoors compared to outdoors. Swedish Research Council FORMAS financed this project.
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