Air pollution components are present as gases and as particulate matter. As particle deposition takes place in various parts of the respiratory system particulate matter may have other toxicological implications than gaseous pollutants, which all may penetrate in the lower part of the respiratory tract. In addition, suspended particulate matter represents a group of pollutants of variable physical as well as chemical composition. Therefore airborne particulate matter cannot be regarded as a single, pure pollutant.
This study deals with the mutagenic potency of airborne particulate matter bound organics and some of its toxicological implications.
The assessment of health hazards is not easily possible without knowledge of the chemical character of the particles. Risk assessment through a toxicological consideration of the individual constituents has serious drawbacks because of the large number of chemicals involved and the complexity of the mixture. Since only 30-40% of the organic compounds on airborne particles have been identified, the contribution of unidentified compounds to the toxicological risk may be significant. Therefore the assessment of the overall mutagenic or carcinogenic activity in air samples may provide a more realistic basis for the evaluation of the possible risks, than an evaluation on the basis of individual compounds.
The Salmonella /microsome assay has been a major assay used for monitoring the mutagenic potential of complex environmental mixtures such as airborne particulate matter. Results obtained with this test system may also be useful as a general air pollution parameter, representing particle bound extractable organics.
The Salmonella /microsome assay involves subsequently sample collection, extraction, exposure of the test strain and the quantitative assessment of the revertants. Extraction is carried out routinely with organic solvents, while generally liver homogenates are used to identify mutagens which require metabolic activation. In the first part of this study methanol and liver homogenates from Aroclor 1254-pretreated Wistar rats were used to study the occurrence of particle bound mutagens collected indoors and outdoors. In the second part of this study physiological fluids and lung homogenates, which are more representative for the environment particles encounter in lungs, are used as solvent and metabolizing system in the Salmonella /microsome assay. Also cytotoxicity of airborne particulate matter to rat alveolar macrophages was determined in order to study some toxicological implications of inhalation of particle bound organic compounds.
Mutagenic activity of extracts of airborne particulate matter from outdoor and indoor environments
Chapter 1 provides a summary of literature data on physical and chemical characteristics of airborne particulate matter.
The occurrence of mutagens in the environment and their sources are discussed.
Chapter 2 presents data on in vitro testing of airborne particles, collected outdoors. Much evidence exists on the mutagenicity of airborne particles at urban and industrial locations. This study shows that particulate matter at background (Terschelling) and rural locations (Wageningen) may also bear mutagenic compounds. In Wageningen the level of mutagenicity follows a yearly cycle, the highest activity being found in winter. Over the years considerable differences in mutagenicity were found. The correlation of mutagenicity of samples simultaneously collected on the same day at both locations and the relation between mutagenicity of air samples and wind direction and with air trajectories suggest that the mutagenic potential of suspended matter originates not from local sources but rather depends on large scale processes. This is supported by literature data which show that the mutagenic potential mainly occurs on the smallest particles which, as a result of their long residence time, may be transported over distances of thousands of kilometers.
The air, sampled at rural and background locations contains direct as well as indirect acting mutagens. The addition of liver microsomes gives variable effects on the level of mutagenic activity. Increases or decreases, if found are mostly relatively small. Furthermore, mutagenic activity was correlated with commomly measured air pollution parameters; multiple regression showed that SO 2 , NO 2 , NO, CO and 0 3 together account for 70% of the variation' in direct mutagenicity and 80% of the variation of indirect activity. SO 2 and NO 2 , and SO 2 , NO 2 and CO were significantly associated with the variation in direct and indirect activity respectively.
As variations in mutagenic activity can be explained to a large extent by commonly registered air pollutants, monitoring the mutagenic burden of aerosols does not contribute to our knowledge already obtained by monitoring SO , NO x and CO.
Chapter x 3 and chapter 4 deal with the comparison of particle bound mutagens collected indoors and outdoors. In chapter 3 the contribution of smoking and cooking and in chapter 4 the contribution of wood combustion to mutagenicity of indoor aerosols is established. The results show that not only extracts of outdoor particles, but also indoor particulate matter may contain mutagenic compounds. Comparison of the mutagenic activity indoors and outdoors indicates that outdoor extracts show a direct mutagenic activity which is clearly detectable in nearly all samples, whereas indoor samples mostly show a low or undetectable direct activity. It is found that the indirect mutagenic activity is generally larger in indoor samples than in outdoor samples. Morever in indoor extracts cytotoxic effects are more pronounced.
One of the sources of indoor mutagens may be penetration of outdoor particles. In the winter 82/83 no contribution of outdoor sources to mutagenic activity indoors was observed, while in the winter 84/85 a correlation between the extremely high levels of outdoor mutagenicity and indoor mutagenic activity was found. However, the important differences in composition of simultaneously collected indoor and outdoor samples which manifest themselves by the ratio - S9/+S9 justifies the conclusion that penetration of outdoor particles may be one but certainly not the only source of mutagenic activity indoors. Mostly the contribution of outdoor sources to indoor mutagenicity is only small.
From our studies it is obvious that cigarette smoking is the predominant source of airborne genotoxicity in homes. wood combustion appeared to be a second important factor producing genotoxic compounds as sometimes a 2-3 fold increase of mutagenic activity is found. Volatilization of cooking products represents a less important source of mutagens.
From these experiments it may be concluded that the removal of mutagens to the outdoor environment is not complete.
Exposure is a function of concentration and time. In The Netherlands people spend most of the time at home. As all indoor samples show a strong indirect mutagenic activity, it may be concluded that exposure to genotoxins will be determined to a large extent by the level of pollution inside homes, which implies that exposure to indirect acting mutagens is quantitatively of far greater concern than exposure to direct acting mutagens.
Biological availability of particle bound organic compounds
Chapter 5 reviews a number of studies which deal with some aspects of the biological fate of mutagenic compounds and their 'carrier' particles. Respirable airborne particles to which most potential mutagenic compounds, detected in the Ames assay, are adsorbed may deposit in various parts of the respiratory tract. one of the defence mechanisms with regard to possible harmful action of these particles is clearance, as it reduces residence time on potentially sensitive epithelial surfaces. Alveolar macrophages provide the initial defence of the lower respiratory tract towards particulate matter.
In chapter 6 results show a significant reduction of phagocytic activity in rat alveolar macrophages in vitro after exposure to extracts of airborne particulate matter, and the effect was greatest with indoor air. Metabolizing enzymes, together with transformed or not transformed particles may be released in the alveolar spaces as a result of damage to alveolar macrophages.
In chapter 7 dissolution of particle bound organics into newborn calf serum and lung lavage fluid from pigs is described. Although from our results it is clear that physiological fluids, especially when obtained from lung lavage are less efficient in removing mutagens than organic solvents, the suggestion seems to be justified that a certain elution of environmental chemicals into body fluids takes place.
Chapter 8 deals with metabolic activation of extracts of indoor and outdoor particulate matter. Drug metabolizing enzyme systems may also be seen as a defence mechanism towards chemicals invading the body. These enzyme systems enable the organism not only to convert lipid soluble harmful drugs into harmless water soluble metabolites, but also more toxic or mutagenic metabolites may be formed. Our results show that in addition to liver, lung homogenates of rat (Wistar) and mouse (Swiss) are also able to activate extracts of airborne particulate matter in a comparable way. Uninduced liver and lung homogenates showed only minor differences in activation capacity in the metabolism of airborne particles. In contrast to liver homogenates, Aroclor 1254- pretreatment of test animals did not give a strong induction in metabolic activation capacity of lung homogenates.
In liver, almost all cells contribute to metabolic capacity, whereas in the lung metabolic capacity is almost exclusively located in the Clara cells, which is only one of the 40 different lung cell types. In certain parts of the lungs, especially in the terminal bronchioles, in which Clara cells are located, a rather high metabolic activation may take place. Therefore these results suggest that the respiratory system may be an important site for in vivo bioactivation of respirable particles.