Project at a Glance
Title: Carcinogens and mutagens in ambient air particulate matter: sources and trends in Contra Costa County.
Principal Investigator / Author(s): Flessel, P
Contractor: Air and Industrial Hygiene Laboratory, California Department of Health Services
Contract Number: A1-162-32
Research Program Area: Atmospheric Processes
Many mutagens and carcinogens are known to be present in urban community air. Extensive chemical and biological characterization of these atmospheric pollutants is essential if accurate risk assessments are to be made and effective control strategies developed. This report describes progress in three areas of this complex environmental problem: 1) The development of more sensitive methods for measuring aerosol mutagens, 2) The identification of sources of mutagens and 3) The analysis of trends in mutagen and polycylcic aromatic hydrocarbon (PAH) levels in particulate organic matter (POM).
* A highly sensitive version of the Ames Salmonella test, called the microsuspension test, was applied to measure the mutagenic activity in organic extracts of community aerosols. Application of the microsuspension Ames test made possible high resolution diurnal studies of mutagenicity in small air samples of only two hours duration. Diurnal variations in mutagenic density (revertant/m3) of more than a factor of 10 were observed and these variations were highly correlated with fine fraction lead (Pb) in a pilot field study. The test can be applied in future studies where sample mass is a limiting factor.
* The origins of mutagens in POM were investigated further by sampling in Contra Costa County during six seasonal pollution episodes, each of 36 hours duration, in 1982-1984. Samples were collected at four locations ( Richmond, Martinez, Concord, Pittsburg) and analyzed for mutagenic activity in the Ames test, for PAH, oxyanions (NO3-, SO4=), pollutant gases (CO, NO, NO2, O3, SO2) and elemental source tracers (including Pb, Br, Ni, Fe, K). Diurnal, geographic and seasonal comparisons were made. Statistical techniques, including principal component (factor) analysis, were used to explore relationships between aerosol mutagens, PAH and source tracers. The results confirmed earlier observations and provided some new insights into the sources of aerosol mutagens.
o Several lines of evidence indicate that some mutagenic aerosols are primary automotive pollutants emitted directly into the atmosphere.
1. In this present and previous Contra Costa studies, mutagenic density and PAH were significantly positively correlated with fine fraction ( < 2.5 µmda ) Pb and/or Br, both derived primarily from motor vehicles.
2. Chemical analysis by other investigators has identified mutagens (various PAH and nitroarenes) in on-road vehicle particulate emissions, as well as other combustion source particulate matter.
3. Studies of upwind-downwind freeway data in Los Angeles by Statewide Air Pollution Research Center (SAPRC) scientists have demonstrated an incremental burden of direct mutagens in aerosol attributable to freeway traffic. The amount was comparable to the area-wide background mutagen density.
o Many results suggest that some mutagens behave as secondary aerosols. The hypothesis that some mutagenic aerosols are formed in the atmosphere is supported by the following evidence:
1. During pollution episodes in Contra Costa County, mutagens were positively correlated with NO3-, assumed to be a secondary aerosol tracer. The association of mutagenicity with NO3- occurred area-wide.
2. SAPRC scientists observed that ratios of mutagen densities (rev/m3) to CO were generally higher at Riverside, California, a downwind receptor site, than at El Monte, an intermediate receptor site in the Los Angeles basin. Since CO is an unreactive combustion emission, the mutagen density / CO ratio takes into account variations in emissions and atmospheric dispersion. Higher ratios at Riverside suggest atmospheric mutagen formation during the aerosol transport from Los Angeles.
3. The ratios of mutagenic densities to Pb which we have measured in Contra Costa County in this and a previous study were highest during summer episodes when the prevailing atmospheric conditions (i.e., hot, dry, stagnant) favored chemical transformations. Since Pb, like CO is an unreactive emission, the mutagenic density / Pb ratio should take into account variations in automotive emission profiles and dispersion. Thus the high ratios during episodes in August 1981 and September 1983 may reflect atmospheric mutagen formation.
4. Smog chamber studies have demonstrated the formation of nitro-PAH mutagens. Mutagenicity of some nirto-PAH's exceed the mutagenicity of the parent PAH by several orders of magnitude in laboratory analysis. Some of these highly mutagenic nitro-PAH's are known to be primary pollutants emitted by various combustion sources. However chamber studies have also shown that irradiation of mixtures of atmospheric hydrocarbons, nitric acid (HNO3) and reactive gases (NO2, O3) can lead to mutagen formation. Thus some hydrocarbons may be converted to secondary mutagenic products under simulated atmospheric conditions.
5. Measurements in a nitroreductase mutant indicate the likely presence of nitroarene mutagens. Less than 10% of the total mutagenicity in ambient air samples is due to identified PAH. Thus most of the mutagenicity remains to be explained in chemical terms. A substantial proportion of this excess mutagenicity may be due to highly mutagenic nitroarenes and derivatives, which are not only ubiquitious primary pollutants but may also be derived from secondary atmospheric transformations. We infer that such compounds were probably major contributors to the mutagenicity of Contra Costa aerosols from the fact that mutagenic activities of aerosol extracts were two to three times lower in a Salmonella strain (TA98NR) deficient in an enzyme required for some mononitroarene activation, than in the standard tester strain (TA98).
6. Finally, measurement artifacts confound the secondary mutagen hypothesis. The positive correlations of mutagenic density with NO3-, and the demonstration that mutagenic organic compounds can be formed under simulated atmospheric conditions support the hypothesis of secondary formation of mutagenic aerosols in the atmosphere. The association between mutagens and NO3- can be influenced by HNO3 artifacts produced by sampling on glass fiber filters. There are two concerns. Gas phase HNO3 can bind to glass fiber and artificially increase apparent particulate NO3- concentrations. More importantly, gas phase HNO3 may catalyze chemical transformations of PAH to produce highly mutagenic nitroaromatic compounds during sample collection on glass fiber. The significance of these potential artifacts cannot be assessed accurately at present.
* For the first time in Contra Costa County, industrial contributions to mutagenic aerosols were suggested by significant positive correlation between mutagenic density and S (both fine fraction S and SO2) at Richmond and Martinez. Sulfur oxides are major air pollutants in the vicinity of large oil refineries and chemical plants in Contra Costa County. The major industrial sources of SO2 are refineries in Richmond (Chevron), Martinez (Shell, Tosco) and Benicia (Exxon) and a chemical plant in Rodeo (Union).
Routine collection and analysis of 4 month seasonal composite filter samples was carried out in Contra Costa County between 1979-1984. The three periods were November - February, March - June and July - October. These periods approximate the three meteorological seasons in the area.
This monitoring effort demonstrated that levels of most aerosol pollutants including mutagens and PAH, were highest in the winter (Nov.-Feb.)
A prime goal of the monitoring was to detect any time trends which may have occurred. Monitoring did indeed reveal a positive trend in the concentration of mutagenic aerosols, despite decreasing or constant levels of the other pollutants measured. The annual average increased form 5 revertants/m3 in 1979-80 to 19 revertants/m3 in 1983-84. A three to four-fold increase in mutagenic density (from 8 revertants/m3 to 27 revertants/m3) was observed over the five winter seasons. Values in the spring increased form 2 to 18 revertants/m3 while summertime values increased by more than a factor of two from 5 to 13 revertants/m3 Further monitoring is needed to determine the persistence of these trends.
For questions regarding this research project, including available data and progress status, contact: Heather Choi at (916) 322-3893
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