Environmental transport, distribution and transformation.
Environmental transport and distribution.
NitroPAHs can be transported in the vapour phase or adsorbed onto particulate matter. Those with liquid-phase vapour pressures greater than 10-4 Pa at ambient air temperature (i.e., two- to four-ring PAHs and two-ring nitroPAHs) will exist at least partially in the gas phase.
Owing to their low aqueous solubility or insolubility, nitroPAHs are not expected to be transported in water. Data available give high values for sorption coefficients (log Koc), suggesting that nitroPAHs, similar to PAHs, adsorb onto soil and sediments. Leaching into groundwater is thought to be negligible. Some nitroPAHs may be slowly biodegradable under certain conditions.
The values for the n-octanol/water partition-coefficient (log Kow) range from 2.5 for 1-nitronaphthalene to 6.3 for 3-nitroperylene, suggesting a potential for bioaccumulation. There were no data avail-able on biomagnification.
Biotransformation.
Many anaerobic and aerobic bacteria reduce nitroPAHs to mutagenic aminoPAHs. Nitroreduction by intestinal microflora plays a major role in the metabolism of nitroPAHs in mammals. Although a wide variety of bacteria, fungi and algae have been shown to degrade the parent PAHs containing two to flve rings, nitro-substituted PAHs are only slowly degraded by indigenous microorganisms and may persist in soils and sediments. The recalcitrance of high molecular weight nitroPAHs is due in part to the strong adsorption to soil organic matter, low solubility, large molecular size and the polar character of the nitro group.
Time course studies in microcosms showed that 1-nitropyrene was degraded slowly under aerobic and anaerobic conditions in estuarine sediments.
Sphingomonas paucimobilis strain EPA 505 (a soil bacterium capable of utilizing fluoranthene as the sole source of carbon and energy) biodegraded 1-nitropyrene to 48.6% after 6 h.
The filamentous fungus Cunninghamella elegans has been shown to oxidatively metabolize, via a cytochrome P450 monooxygenase, a number of nitroPAHs (1-nitropyrene, 2-nitrofluorene, 2- and 3 nitrofluoranthene, 6-nitrochrysene, 1-nitrobenzo[e]pyrene and 6-nitrobenzo[a]pyrene) to products that are less mutagenic than the nitroPAHs themselves.
A plant cell culture derived from alligator weed (Alternanthera philoxeroides) detoxifled 1-nitropyrene and 1,3-, 1,6- and 1,8-dinitro-pyrene, all direct-acting mutagens, when incubated with them, as shown by mutagenicity response in the Salmonella typhimurium TA98 assay.
Abiofc degradation.
The photolysis of nitroPAHs has been studied under varied conditions of irradiation. The rate of photolysis depends not only on the conditions of irradiation but also on whether the nitroPAH is in the gaseous phase (e.g., 1-and 2-nitronaphthalene), in solution (type of solvent) or bound to solids/particles. In the latter case, the type and age of the particle seem to influence the photochemistry of the respective nitroPAH. The rate of photodecomposition, identification of photolysic products and resulting loss or gain of metabolic activity as determined by the S. typhimurium assay have been the main end-points studied.
Calculated atmospheric lifetimes of nitroPAHs due to photolysis and gas-phase reactions with hydroxyl and nitrate radicals and with ozone under atmospheric conditions show that the dominant loss process for nitroPAHs (e.g., 1- and 2-nitronaphthalene) is photolysis.
Particie oxidation of nitroPAHs by ozone may be the main loss pro-cess at night.
Environmental levels and human exposure
NitroPAHs that have been detected in ambient air include 1- and 2-nitronaphthalene and methylnitronaphthalenes (predominantly in the vapour phase), 2-nitrofluorene, 9-nitroanthracene, 9-nitrophenan threne, 2-, 3- and 8-nitrofluoranthene, 1- and 2-nitropyrene, 1,3-, 1,6-and 1,8-dinitropyrene and 6-nitrochrysene.
At remote and forest sites, nitroPAHs were either not detected or detected in the low picogram per cubic metre range (e.g., 17 pg/m3 for 2-nitrofluor?nthene; 4 pg/m3 for 1-nitropyrene). The concentration of nitroPAHs in the atmosphere of urban regions depends on the season, the type of heating used and the number and regulation of traffic vehicles. Reported levels in air do not usually exceed 1 ng/m3, although maxima of up to 13 ng/m3 have been reported.
Various studies have been performed monitoring certain isomeric nitroPAHs. Investigators have concentrated on the nitroPAHs that seem to be of quantitative/environmental (e.g., nitroPAHs of relative molecular mass 247: 1-nitropyrene, 2-nitropyrene, 2-nitrofluoran-thene) or carcinogenic (e.g., 1-nitropyrene, 2-nitrofluorene, dinitro-pyrenes) importance.
Studies of daytime/nighttime concentrations of specific isomeric nitroPAHs in certain regions (in particular California, USA) and parallel environmental chamber studies have led to an understanding of the atmospheric formation of certain nitroPAHs (2-nitrofluoran-thene and 2-nitropyrene). Concurrent studies of certain nitroPAHs (1-nitropyrene, dinitropyrenes) and traffic volume have confirmed that traffic emission is a source of nitroPAHs.
Most seasonal studies show higher winter/spring concentrations of marker nitroPAHs, which parallels the use of domestic heating, although this is not always the case.
Indoor air.
As nitroPAHs have been detected in the emissions of kerosene heaters, fuel gas and LPG burvers used for heating and cooking at home, as well as in the fumes of cooking oils, there is therefore a potential indoor exposure to nitroPAHs in poorly ventilated condi-tions.
Concentrations of polyaromatic compounds, including nitroPAHs, were measured in a study of indoor and outdoor air levels associated with 33 homes located in two US cities: Columbus, Ohio, and Azusa, California. The overall levels were much higher in homes occupied by smokers, but the use of natural gas heating and cooking appliances also appeared to increase the nitroPAH levels slightly.
1-Nitropyrene (4,2-25 600 ng/litre) was detected in 36 of 55 samples of wastewater from oil-water separating tanks of gasoline stations and in used crankcase oil.
1- and 2-nitronaphthalene and 1,3- and 1,5-dinitronaphthalene were detected in river water in Japan at concentrations of 1.3, 11.7, 1.7 and 3.2 ng/litre, respectively. In another water sample, 1-nitropyrene was identifed.
There are only limited data on the presence of nitroPAHs in samples of soil, sewage sludge, sediment and incinerator ash (e.g., for 1-nitropyrene, 0.03-0.8 ug/kg dry weight in soil, 0.68 ug/kg in sewage sludge, 25.2 ug/kg in sediment and <0.01-0.89 mg/kg in incinerator ash).
Food and beverages.
With the exception of spices, smoked and grilled foods and peanuts, the concentrations of nitroPAHs in foods are below 5 ug/kg. In a study in the United Kingdom, foodstuffs were monitored for the presence of 9-nitroanthracene and l-nitropyrene. Twenty-five out of 28 foods contained no detectable levels of there nitroPAHs. 9-Nitroanthracene was tentatively identified in peated malt, at 0.9 ug/kg, and 1-nitropyrene in two samplen of tea leaves, at 1.7 and 0.17 ug/kg. Another survey of nitroPAH levels in various foods in Austria showed mostly detectable levels of 2-nitrofluorene, 1-nitropyrene and 2-nitronaphthalene. The highest concentrations were found in spices, smoked foods and teas, in particular Mate tea, which is roasted. Nitro-PAHs were also detected in vegetables and fruits, probably due to atmospheric pollution.
1-Nitropyrene was detected in grilled corn, mackerel and (in considerable amounts) pork and yakitori (grilled chicken) grilled with sauce (up to 43 ng/g).
Other products.
In 1980, studies showed that extracts of selected xerographic toners and paper photocopies were mutagenic. The fraction of the carbon black B responsible for 80% of the mutagenicity contained 1 nitropyrene, 1,3-, 1,6- and 1,8-dinitropyrene, 1,3,6-trinitropyrene and 1,3,6,8-tetranitropyrene. As a result of this fmding, the manufacturers modified the production of carbon black B, substantially reducing the levels of nitropyrenes.
Occupational exposure.
Occupational exposure to nitroPAHs has been demonstrated in workplaces associated with the use of diesel engines. For example, concentrations of 1-nitropyrene in air were measured in various work places associated with the use of diesel engines. The highest levels (42 ng/m3) reported were determined in the breathing zones of the underground workers (drivers of diesel-powered excavators) at an oil shale mine in Estonia.
Effects on laboratory mammals and in vitro test systems.
Only six nitroPAHs have been tested for acute toxicity. In rats, an LD50 of 86 mg/kg of body weight (kg bw) after intraperitoneal (i.p.) application was reported for 1-nitronaphthalene; in mice, an oral LD, of 1300 mg/kg bw was reported for 2-nitronaphthalene. In further studies on both substances, systemic effects on the target organs lung and liver were observed after single high doses; however, 2-nitro-naphthalene seemed to be less toxic than 1-nitronaphthalene. 5-Nitro-acenaphthene at an i.p. dose of 1700 mg/kg bw was lethal to all treated rats. For 2-nitrofluorene, an oral LDSO of 1600 mg/kg bw in mice was reported, whereas gavaging with up to 5 000 mg 1-nitropyrene/kg bw resulted in no observable toxic effects. Local inflammation and ulceration were seen in rats after subcutaneous (sx.) injection of 8 mg 3-nitrofluoranthene/kg bw.
Data on Systemic or local non-neoplastic effects caused by short-term or long-term treatment with nitroPAHs are limited, as the end-point of most studies,has been carcinogenicity. In most cases, non neoplastic toxic effects were observed at doses at which carcinogenic responsen are also manifested. Systemic non-neoplastic toxic effects, such as reduced body weight or increased mortality, appeared presum-ably independently of carcinogenic effects in feeding studies with 5-nitroacenaphthene at a dose level of 500 mg/kg bw per day (rat) or 40 mg/kg bw per day (mice) and with 2-nitrofluorene at a dose of 25 mg/kg bw per day (rat). Medium-term exposure via inhalation to 1-nitropyrene resulted in metaplasia of the upper respiratory tract at concentrations of_0.5 mg/m3.
No data are available on skin and eye irritation, sensitization or reproductive toxicity.
Data on genotoxicity in vitro are available on 95 nitroPAHs; for 74 nitroPAHs, however, only one or two end-points, mainly in bacterial test systems, were investigated. A sufficient database, including eukaryotic test systems, has been found only with 21 nitroPAHs. Most of these substances (67 out of 95) showed positive results, but the results were derived from a small database. Clearly positive results were obtained for 19 nitroPAHs, and questionable results for 8 nitro-PAHs. With none of the nitroPAHs were clearly negative results obtained.
For 86 nitroPAHs, data on the S. typhimurium microsome test are available. In contrast to the parent PAHs, most nitroPAHs were clearly more effective in the Salmonella microsome test without metabolic activation. There are five nitroPAHs that showed exceptionally high mutagenic potency (>100 000 revertants/nmol) in this test system: 3,7-and 3,9-dinitrofluoranthene, 1,6- and 1,8-dinitropyrene, and 3,6-dinitrobenzo[a]pyrene.
Bacterial nitroreductase and acetyltransferase are involved in the metabolic activation of nitroPAHs, but not all nitroPAHs follow the same metabolic activation pathways. Furthermore, there is no uniform mutagenic effect of the different nitroPAHs, as they produce both frameshift and base pair substitutions in the S. typhimurium microsome test. There is evidence that nitroPAHs with nitro groups perpendicular to the aromatic ring are not as mutagenic as isomers having parallel nitro orientation.
Data on the in vivo genotoxicity of nitroPAHs are available for 15 nitroPAHs. All nitroPAHs that gave positive results in vivo were also positive in vitro. Four nitroPAHs that were positive in in vitro genotoxicity tests- revealed inconsistent or inconclusive genotoxicity (2-nitronaphthaene, 5-nitroacenaphthene and 3-nitrofluoranthene) or negative genotoxicity (2,7-dinitrofluorene; limited validity) results in vivo.
3-Nitrobenzanthrone, like 1,6- and 1,8-dinitropyrene, is highly mutagenic in bacteria through nitroreduction and O-esteriflcation. 3-Nitrobenzanthrone is also an effective gene mutagen and causes micronuclei formation in human cells in vitro and in mice in vivo.
2-Nitrodibenzopyranone was reported to be highly mutagenic in the S. typhimurium microsome test in strain TA98 (-S9), being more mutagenic than 2-nitrofluorene and 1-nitropyrene. 1- and 3-nitro-pyrene lactones have been found to be highly mutagenic in the S. typhimurium microsome test.
Studies on the in vitro genotoxicity of 2-nitrodibenzopyranone in forward mutation assays using two human B-lymphoblastoid cell lines are conflicting. Nitropyrene lactones were found to induce mutations at the tk and hprt loci in both cell lines. Further, they induced kineto-chore-positive and -negative micronuclei in the CREST modified micronucleus assay, which detects chromosomal loss and breakage events.
Data on carcinogenic effects are available for 28 nitroPAHs. Although inhalation is the main exposure route in humans, no long-term inhalation study on any nitroPAH is available. Most studies examined the carcinogenic effects of nitroPAHs by oral administration, topical application, pulmonary implantation or intratracheal administration.
Owing to the limitations in experimental design, none of the negative studies confirmed the absence of carcinogenic effects in animals. However, results showed carcinogenic effects in experimental animals for 5-nitroacenaphthene, 2-nitrofluorene, 3-nitrofluoranthene, 3,7- and 3,9-dinitrofluoranthene, 1- and 4-nitropyrene, 1,3-, 1,6- and 1,8-dinitropyrene and 6-nitrochrysene. Some carcinogenic effects in experimental animals were observed for 2-nitropyrene, 7-nitrobenz[a]-anthracene, 2- and 6-nitrobenzo[a]pyrene, 3,6-dinitrobenzo[a]pyrene, 7-nitrodibenz[a,h]anthracene and 3-nitroperylene. For the remaining 10 nitroPAHs tested, not enough data were available with which to evaluate their carcinogenicity in experimental animals.
Besides local effects at the site of injection, nitroPAHs induced mainly systemic tumours in mammary tissue, lung, liver and the haematopoietic system. 6-Nitrochrysene appears to be the most car cinogenic of the nitroPAHs considered here. With systemic effects after s.c. or i.p. injection, 1-nitropyrene was more carcinogenic than the dinitropyrenes. The carcinogenicity of 1-nitropyrene and dinitro-pyrenes varies, depending on the route of administration.
Nitrated benzo[a]pyrenes are generally less potent carcinogens than the parent compound BaP. However, the mono- or dinitrated pyrenes are more carcinogenic than pyrene. Similar results were presented for 3-nitroperylene compared with perylene and for 6-nitrochrysene compared with chrysene; with local effects after dermal exposure, however, 6-nitrochrysene was less active than chrysene.
Data were available on carcinogenic effects of sonve metabolites of 2-nitrofluorene, 1-nitropyrene and 6-nitrochrysene. Comparing 2-nitrofluorene with its metabolites in sats, the highest carcinogenic potency was shown by 2-acetylaminofluorene. 1-Nitropyrene was significantly more carcinogenic after oral application in rats than either 1-nitrosopyrene or 1-aminopyrene. In contrast, 1-nitrosopyrene induced a higher incidence of liver tumours in mice after i.p. appli-cation than 1-nitropyrene; no effects were observed with ring hydroxylated metabolites. 6-Nitrosochrysene and 6-aminochrysene were inactive, in contrast to the ring hydroxylated metabolites, which showed carcinogenic activity in the liver similar to that of the parent com-pound 6-nitrochrysene; this indicates that the metabolic activation of 6-nitrochrysene occurs by ring oxidation and/or a combination of ring oxidation and nitroreduction.
Effects on humans.
There are no reports on the effects of individual nitroPAHs on humans. As would be expected, since nitroPAHs occur in complex mixtures in the atmosphere and exhaust, the exact contribution of nitroPAHs to the adverse health consequences of exposure to polluted atmospheres and to exhaust cannot be elucidated.
At present, investigations on the effects of nitroPAHs on human health are being carried out using biomarkers of exposure. Several reports have described the development of methods for and provided data on the evaluation of 1-nitropyrene as a biomarker for occupa-tional exposure to diesel exhaust. Urinary metabolites of PAHs and nitroPAHs were determined in the urine of diesel mechanics using the enzyme-linked immunosorbent assay (ELISA). In another study, metabolites of l-nitropyrene (namely, N-acetyl-l-aminopyren-6-ol and N-acetyl-l-aminopyren-8-ol) were measured in the urine of workers in a shipping department. Several studies have focused on measuring the haemoglobin and plasma adducts of metabolites of 1-nitropyrene and other nitroPAHs and may provide appropriate biomarkers in future molecular epidemiological investigations.
Effects on other organisms in the laboratory and field.
Data on the acute toxicity of nitroPAHs to aquatic organisms are available only for 1-nitronaphthalene. An LC50 (96 h) of 9.0 mg/litre was reported for the fathead minnow (Pimephales promelas).
Furthermore, this ni troPAH inhibited the growth of the ciliate Tetra-hymena pyriformis, with an EC50 (60 h) of 17.3 mg/litre.
Some studies have been concerned with the effect of nitroPAHs on the metabolism of some aquatic species - for example, the sub-cellular and tissue distribution of two- and one-electron NAD(P)H dependent nitroreductase activity in marine invertebrates from three phyla: mussel (Mytilus edulis), crab (Carcinus maenas) and starfish (Asteria rubens). NADPH-dependent two-electron nitroreductase activity, occurring only under anaerobic conditions, was detected in the microsomal and cytosolic fractions of the major digestive tissues of mussel (digestive gland) and crab, but not in the gills of either species. 1-Aminopyrene was the only metabolite identified. No activity was detectable in the pyloric caeca or stomach region of the star-fish. NAD(P)H-dependent one-electron nitroreduction was present in all subcellular fractions of the major digestive tissues of the three species.
In the presence of calf thymus DNA, adducts derived from 1-nitropyrene were detected in vitro using hepatic S9 fractions prepared from fish. The ability of 1-nitropyrene to form DNA adducts was also established in vivo using brown trout (Salmo trutta) and turbot (Scoph-thalmus maximus). These DNA adducts were comparable to those obtained in Wistar rats treated with 1-nitropyrene.