|Hazardous Constituents of
Rice Straw Smoke 83
Project Leader and Principal UC Investigators
D. P. H. Hsieh, J. N. Seiber, and Terryl Mast, Department of Environmental Toxicology, University of California, Davis
Objectives of this project were to identify mutagenic chemicals in rice straw smoke particles and to compare the chemical composition and mutagenicity of field collected rice smoke with rice smoke from the burning tower used at the University of California, Riverside, in air pollution studies. Another objective was to determine the effects exposing rice smoke to air and light have on mutagenicity. The mutagenicity of smoke from rice straw, barley straw, diesel fuel and cigarettes also was compared.
Whole extracts of particulate matter from combustion have been shown to be active in the Salmonella microsome mutagenicity test. This involves the use of the Salmonella bacteria for testing the possibility of cancer causing substances in the environment. Not all compounds that are active in this test are carcinogens or substances causing cancer, but most carcinogens are active, in that they cause mutations to occur. Therefore, the test is an indicator, not proof, that reactive components are carcinogens.
The complete range of organic compounds from rice smoke were extracted and fractionated into compound classes. These fractions were tested for mutagenic activity and as many as possible of the constituents in the active fractions were identified.
Field collected rice smoke extracts and rice smoke extracts from the Riverside burning tower were compared for mutagenic activity. No direct acting mutagenic activity was found from field collected rice smoke particulate matter, but a small amount of direct activity was noted from combustion of rice straw using the Riverside burning tower.
The Riverside burning tower smoke extracts had nearly four times more indirect mutagenic activity than the extracts from field collected smoke. Indirect mutagenic activity requires the use of an enzyme. This discrepancy between the field collected and burning tower smoke raises important questions regarding the extrapolation of results from simulated to real environments. The direct-acting mutagenicity of the tower samples may have been caused from environmental conditions that are not representative of field conditions.
Smoke extracts from wood stoves, cigarettes, and rice straw were compared. The wood stove smoke particulate matter extracts demonstrated a level of mutagenic potency higher than the extracts from rice straw smoke. Cigarette smoke particulate matter extracts demonstrated only the presence of indirect-acting mutagens, although at a level five times higher than that found in rice straw smoke extracts. Results obtained from the wood stove smoke particulate matter extract, although preliminary, indicate a potential health hazard during periods of atmospheric inversion. These conditions rarely occur with rice straw smoke, as burning is only allowed by the California Air Resources Board on days when there is sufficient air movement to rapidly dilute the smoke.
The mutagenic activity of rice smoke particulate matter collected downwind from a burning field and of smoke particulate matter extracts exposed to simulated sunlight for 24 hours were assessed and was less than the mutagenic activity of smoke collected adjacent to the field. These results indicate a diminishing mutagenic activity in the smoke particulate matter following field burning.