4.5.1. Outdoor air pollution

Molecular epidemiology studies have been conducted in occupationally exposed populations, the general population in different geographical areas, and susceptible populations of children. Occupational (e.g. traffic police, bus drivers, and mail carriers) or environmental (e.g. urban residents exposed to traffic and residents exposed to combustion sources) exposure studies have confirmed significantly increased levels of biomarkers of exposure of mutagenicity (e.g. urinary 1-OHP and 8-oxodG, urinary mutagenic activity, and protein adducts), biologically active internal exposures (e.g. DNA adducts and strand breaks in target tissues), cytogenetic damage (e.g. MN and CAs), mutations (in human newborns), and changes in gene expression. Table 4.19 provides an overview of the mechanistically important/relevant genetic effects in humans exposed to elevated levels of outdoor air pollution.

Table 4.19

Summary of genetic effects reported in molecular epidemiology studies of outdoor air pollution.

Increased susceptibility in humans to the effects of outdoor air pollution has been associated with genetic polymorphisms, for example GSTM1 null, alone or in combination with selected CYP1A1 genotypes (Hosgood et al. 2007).

A relatively small number of studies have shown that animals exposed to outdoor air pollution in situ have elevated levels of DNA damage, cytogenetic damage, and heritable mutations. Pedigree analyses of herring gulls collected from urban/industrial areas, as well as pedigree and male germ-cell analyses of both inbred and outbred mice housed in an area with elevated levels of outdoor air pollution (Yauk, 2004; Yauk et al., 2008), showed increased levels of germ-cell mutations associated with the polluted sites, and elimination of the effect via PM removal. The hypervariable repeat sequence loci examined are not associated with any known phenotype. Nevertheless, significant increases in paternal germline and germ-cell mutation rates in organisms exposed to outdoor air pollution confirm the ability of genetic damage induced by outdoor air pollution to be transmitted between generations. The phenotypic consequences of this transmission remain unknown. The most recent investigation of heritable germ-cell mutations in mice exposed to outdoor air pollution detected DNA damage in lung tissue, but not in gonadal tissue, suggesting a mechanism independent of bulky adduct formation. In situ exposures of cattle and mice to polluted outdoor air showed significant increases in cytogenetic damage in haematopoietic tissues. Numerous studies have also documented mutations and cytogenetic damage in plants exposed to elevated levels of outdoor air pollution.

In addition, there is relatively good evidence for the induction of genetic and related effects after controlled experimental exposures to components of outdoor air pollution (e.g. VOCs, SVOCs, PM, and PM extracts). Some of these components have been previously evaluated for their carcinogenic risk to humans (IARC, 2010c, 2012c, 2013).

4.5.2. Gases

The gaseous portion of outdoor air pollution contains well-known pulmonary irritants such as ozone and NO_x. Although no studies have investigated genetic and related effects induced by exposures to the gaseous portion of outdoor air pollution specifically, reviews of the scientific literature indicate that these agents can induce genetic effects in vivo (Victorin, 1994, 1996). Recent literature suggests that abnormal immune system regulation and chronic inflammation are key mechanistic features of obstructive pulmonary disorders that enhance lung cancer risk (Turner et al., 2007).

4.5.3. Volatile organic compounds

The VOC portion of outdoor air pollution can contain a wide range of substances, and the occurrence of these substances in outdoor air is reviewed in Section 1 of this Monograph. The types and concentrations of these substances (e.g. benzene, formaldehyde, 1,3-butadiene, and styrene) vary with respect to the type of sample, the atmospheric conditions, the physical–chemical properties of the compound, and the proximity to known sources. The carcinogenicity of these substances and the mechanisms underlying their carcinogenic activity are addressed in detail in IARC Monograph Volume 100F (IARC, 2012b).

4.5.4. Semivolatile organic compounds

The SVOC fraction of outdoor air pollution also contains a wide range of substances, and the occurrence of these substances is also reviewed in Section 1 of this Monograph. The types and concentrations of these substances in outdoor air samples, which can include PAHs and nitroarenes that are known mutagenic carcinogens (IARC, 2010c, 2013), vary with respect to the sample collection method, the physical–chemical properties of the compounds, the PM concentration and composition, the atmospheric conditions, and the proximity to known sources. Some components of outdoor polluted air, such as PAHs and nitro-PAHs, whether in the vapour phase or adsorbed to suspended PM, can be metabolically converted to reactive species that will bind covalently to DNA in human tissues and experimental systems. These substances have been previously reviewed by IARC (see Table 1.2 in Section 1) and several have been classified as Group 1, 2A, or 2B agents. The carcinogenicity of PAHs and selected nitro-PAHs and the mechanisms underlying PAH- and nitro-PAH-induced carcinogenesis are extensively reviewed in IARC Monographs Volumes 92 and 105 (IARC, 2010c, 2013). Volume 105 also provides an evaluation of diesel and gasoline engine emissions, important components of outdoor air pollution (IARC, 2013).

A small number of studies provide evidence that the SVOC portion of filtered outdoor air contains substances that induce mutations in bacteria and plants, DNA damage in bacteria, and mitotic recombination in Drosophila. Although the identity of the implicated substances and their mechanisms of action remain unclear, some studies have documented the presence of PAHs and nitro-PAHs that are known or suspected mutagenic carcinogens (Pyysalo et al., 1987; Sera et al., 1994; Gupta et al., 1996; Du Four et al., 2004; Škarek et al., 2007).

4.5.5. Airborne particulate matter

The adsorption of substances with a range of physical–chemical properties will influence the biological properties of PM in polluted outdoor air. Studies on model particles such as carbon black and titanium dioxide have shown inverse correlations between particle size and inflammogenicity (Duffin et al., 2007; Stoeger et al., 2006). Metals ions are involved in generating oxidative processes associated with particles deposited in the respiratory tract, and thus are a source of oxidative stress and inflammation (Tao et al., 2003; Li et al., 2008). Absorption onto carbonaceous particles of high-molecular-weight organic compounds, such as PAHs and nitro-PAHs, provides a mechanism whereby these semivolatile or non-volatile compounds can be delivered into the airways, where they can be absorbed and metabolically converted into reactive intermediates. For genotoxic organic compounds adsorbed to PM to manifest their genotoxic activity, they must be bioavailable. It is clear that organic solvents can effectively remove organic compounds from PM in outdoor air, and based on results obtained in vitro with bacteria and human cells, there is some evidence that biological fluids can effectively remove genotoxic compounds adsorbed to airborne PM. Ohsawa et al. (1983) and Takeda et al. (1983) showed that bovine serum extracts of airborne PM can induce mutations in Salmonella, but the potency of the extracts is low relative to organic solvent extracts. Yuan & Xun (1994) and Yuan et al. (1994) showed that PM extracts prepared using simulated lung fluid can cause DNA damage in cultured human amnion cells. Lei et al. (1993) showed that simulated lung fluid PM extracts can induce chromosomal damage in mouse haematopoietic cells.

Controlled short-term human exposures to concentrated airborne particles have been shown to be associated with pulmonary inflammation (Ghio et al. 2000; Samet et al., 2009). Nevertheless, the studies did not investigate the degree of sustained inflammation observed in rodents, most notably rats, at high lung PM burdens.

The information available indicates that inhalation exposure to PM promotes a pro-oxidant and pro-inflammatory milieu. Concomitant ROS production, together with exposure to mutagenic carcinogens such as PAHs, can be expected to give rise to a multitude of DNA lesions. If left unrepaired, these lesions can be expected to contribute to mutations and chromosomal damage that initiate and promote carcinogenesis.

Experimental exposures of rodents to organic PM extracts induced chromosomal damage. However, only a few studies used a route of exposure (inhalation) that is relevant to elevated human lung cancer risk attributable to PM exposures (Izzotti et al., 1996; Zhao et al., 2001). Intratracheal exposures of rats to PM suspensions induced DNA damage measured as strand breaks and oxidatively damaged DNA (Meng & Zhang, 2007; Lin et al., 2009; Danielsen et al., 2010; Zhang et al., 2011).

Exposure of Drosophila to PM extracts (larval exposure via feed) induced elevations in both somatic and germ mutation frequency. In addition, in vitro exposures to PM suspensions induced chromosomal damage in human lymphocytes and mutations in rat primary hepatocytes and Salmonella (Wei & Meng, 2006a, c; Alfaro-Moreno et al., 1997; Du Four et al., 2004). Collectively, these studies indicate that pulmonary exposure to PM or PM extracts causes genetic damage.

The bulk of published studies that assessed the induction of genetic and related effects in experimental systems were performed with organic solvent extracts of PM collected from polluted locations. Chemical analysis of extracts of PM collected from a wide range of locations clearly indicates that the matrix contains numerous PAHs and nitro-PAHs that are classified by IARC as known or probable human carcinogens (Yang et al., 1994; Durant et al., 1998; Pedersen et al., 1999, 2004, 2005; Brits et al., 2004; Calderón-Segura et al., 2004). Not surprisingly, organic solvent extracts of outdoor air can induce CAs, MN, SCEs, DNA strand breaks, unscheduled DNA synthesis, bulky adducts, and oxidative DNA lesions in cultured human cells. In addition, organic solvent extracts of PM can induce mutations, CAs, aneuploidy, MN, SCEs, DNA strand breaks, bulky adducts, and oxidative DNA lesions in cultured mammalian cells, as well as nuclear and mitochondrial DNA mutations and gene conversion in yeast, and mutations and DNA damage in bacteria. Finally, organic PM extracts can induce bulky adducts, DNA strand breaks, and oxidative lesions in naked DNA in solution.

In addition, effects induced by water and/or acid extracts of PM are also well documented. For example, acid extracts of PM induce MN and DNA strand breaks in cultured human lymphocytes (Yuan et al., 1999a, b; Jayasekher, 2009). Acid extracts contained transition metals, including nickel and chromium, which are known to participate in Fenton reactions that generate reactive peroxide and hydroxyl radicals, which contribute to the formation of ROS. Indeed, aqueous extracts of airborne PM have been shown to induce DNA strand breaks in rat lung, oxidative lesions on naked DNA in solution, the formation of ROS in vitro, and the formation of ROS in cultured mammalian cells.

Genetic and related effects of outdoor air pollution and other mechanistic events

Tables 4.20 and 4.21 summarize the genetic and related effects in humans and experimental systems induced by exposures to outdoor air pollution and samples derived from outdoor air pollution. A large body of evidence clearly indicates that humans exposed to elevated levels of outdoor air pollution have increased levels of chromosomal damage, including chromosome breaks and aneuploidy. Similar effects in experimental systems, both in vivo and in vitro, are also well documented. In addition, a variety of other genotoxic effects in humans and experimental animals exposed to elevated levels of outdoor air pollution or samples derived from outdoor air pollution (e.g. PM and PM extracts) are also well documented (e.g. mutations, DNA strand breaks, stable DNA adducts, and oxidized nucleobases). Sustained inflammation is induced in humans and experimental animals exposed to elevated levels of outdoor air pollution or samples derived from outdoor air pollution.

Table 4.20

Summary by end-point of genetic and related effects induced in humans and experimental systems by exposure to outdoor air pollution or samples derived from outdoor air pollution.

Table 4.21

Summary by exposure of genetic and related effects in humans and experimental systems induced by exposure to outdoor air pollution or samples derived from outdoor air pollution.

Cross-sectional studies of humans lend support to the contention that alterations in the pattern of DNA methylation in circulating lymphocytes can be induced by exposure to high levels of outdoor air pollution (Hou et al., 2011).

Polluted outdoor air can contain a wide range of agents, and the PM fraction is known to contain several substances that can initiate tumour formation via genetic damage and mutation (e.g. PAHs and transition metals), as well as less-harmful constituents that induce responses that contribute to tumour promotion (e.g. inflammation). In sum, there is compelling evidence across species and experimental systems that exposure to air pollution PM is associated with increased levels of DNA damage, mutations, and chromosomal damage. Other mechanistic events include sustained proliferative signalling, evasion of growth suppression, resistance to cell death, stimulation of angiogenesis, replicative immortality, activation of invasion, and metastasis (see Supplemental Figure S4).