Organic extracts from airborne particles collected at various sites in Scandinavia have been tested for mutagenicity in the Ames Salmonella/microsome assay. Extracts from particles in the respirable size fraction (diameter less than 3 R±m) were mutagenic with and without metabolic activation. The mutagenic activity varied from day to day, mainly due to variations in meteorological parameters, especially wind speed and atmospheric stability. A seasonal variation could also be observed, with the highest average values in winter time.Samples collected in urban areas were considerably more mutagenic than samples from background areas. The results suggest that exhaust from motor vehicles are the most important source of mutagenic particles in urban areas.Comparison of roof top samples with street level samples indicated that atmospheric reactions cause transformation of nonpolar compounds in the primary emission to more oxygenated mutagenic compounds. It is, however, not known to which degree this causes an overall increase of the mutagenic activity.The mutagenic activity of emissions from stationary combustion sources have also been studied, and residential heating by burning solid fuels in small combustion units have been shown to be a major contributor to mutagens in the environment.

The mutagenicity of emission samples from three oil-fired and four coal-fired boilers have been compared by using the Salmonella/microsome assay. Very little or no mutagenic activity was observed in samples from five of these boilers. The sample from one oil-fired boiler showed mutagenic activity of about 500 revertants/MJ, and the sample from a coal-fired fluidized bed combustor had an activity of 58.000 revertants/MJ measured with strain TA 98 in the absence of metabolic activation. All samples contained substances that were cytotoxic to the test bacteria, thus making it difficult to obtain linear dose-response curves. Mutagenic activity at low levels may remain undetected due to this toxicity of the samples. Samples with mutagenic activity below the detection limit in the Salmonella test have also been tested for forward mutations at the HGPRT locus in V79 hamster cells. Weak mutagenic effects were detected in two of the samples, whereas the sample from one oil-fired boiler remained negative. In this test, as well as in the Salmonella test, a strong cytotoxic effect could be observed with all samples.