Polycyclic Aromatic Hydrocarbons Overview
Polycyclic aromatic hydrocarbons (PAHs) are a class of more than 100 chemicals generally produced during the incomplete burning of organic materials, including coal, oil, gas, wood, garbage, and tobacco. PAHs are composed of up to six benzene rings fused together such that any two adjacent benzene rings share two carbon bonds. Examples include phenanthrenes, naphthalene, and pyrene. Important PAH sources include motor vehicle exhaust, residential and industrial heating sources, coal, crude oil and natural gas processing, waste incineration, and tobacco smoke. The emitted PAHs can form or bind to particles in the air, and particle size depends in part on the source of the PAHs. The smaller or fine particulates (e.g., PM2.5 or smaller) have higher concentrations of PAHs than the larger or coarse particulates (Bostrom et al., 2002; Rehwagen et al., 2005). Ambient air PAH concentrations show seasonal variation (IPCS, 1998; Rehwagen et al., 2005). Smoking, grilling, broiling, or other high temperature processing leads to PAH formation in meat and in other foods, as well. Uncooked foods and vegetables generally contain low levels of PAHs but can be contaminated by airborne particle deposition or growth in contaminated soil. With the exception of naphthalene, the PAHs described here are not produced commercially in the U.S.
Human exposure usually occurs to PAH mixtures rather than to individual chemicals, and PAH mixture composition varies with the combustion source and temperature (ATSDR, 1995). For persons without occupational exposure, important sources of PAHs include ambient air pollution (especially motor vehicle exhaust), smoke from wood or fossil fuels, tobacco smoke, and foods. PAH exposure can occur in workplaces where petroleum products are burned or coked, such as coke production, coal gasification and gas refining, iron or steel production, roofing tar and asphalt application, waste incineration, and aluminum smelting. Coal tar ointments containing PAHs are used to treat several inflammatory skin conditions.
PAHs are lipid soluble and can be absorbed through the skin, respiratory tract, and gastrointestinal tract. PAH metabolism is complex and occurs primarily in the liver, and to a lesser extent, in other tissues. PAH elimination occurs via urine and feces, and urinary metabolites are eliminated within a few days (Ramesh et al., 2004). PAHs and their urinary hydroxylated metabolites measured in at CDC are shown in the table. The metabolic pathways and enzyme-inducing effects of specific PAHs, such as benz[a]pyrene, have been actively studied to elucidate cancer potential and causal mechanisms (Ramesh et al., 2004). Although immunologic, kidney and brain toxicity have been seen in animals after high doses were administered, it is unclear if similar effects may occur in humans. Lung, bladder, and skin cancers have been reported in occupational settings following high PAH exposures (Bosetti et al., 2007; Bostrom et al., 2002; Lloyd, 1971). Exposure to fine particulates has been associated with fetal growth retardation, respiratory disorders, and cardiovascular disease, but it is unknown whether PAHs contained within fine particulates are etiologic (ATSDR, 1995; Choi, 2006).
IARC classifies naphthalene as a possible human carcinogen. NTP determined that naphthalene is reasonably anticipated to be a human carcinogen. Many other PAHs are considered to be probable or possible human carcinogens. IARC and NTP have classified specific PAH-containing chemical mixtures (e.g., soot, coke oven emissions, coal tars and coal tar pitches) as human carcinogens. OSHA has developed criteria on the allowable levels of these chemicals in the workplace.
Information about external exposure (i.e., environmental levels) and health effects is available in reviews (Bosetti et al., 2007; Bostrom et al., 2002; Brandt and Watson 2003) and from ATSDR at https://www.atsdr.cdc.gov/toxprofiles/index.asp.
PAH Metabolites in the National Biomonitoring Program
|Polycyclic Aromatic Hydrocarbon (CAS number)||Urinary hydroxylated metabolite (CAS number)|
|Fluorene (86-73-7)||2-Hydroxyfluorene (2443-58-5)
|Naphthalene (91-20-3)||1-Hydroxynapthalene (90-15-3)
|Phenanthrene (85-01-8)||1-Hydroxyphenanthrene (2433-56-9)
|Pyrene (129-00-0)||1-Hydroxypyrene (5315-79-7)|
Measurement of urinary metabolites reflects recent exposure to PAHs. Some of the parent PAHs can produce more than one measurable urinary metabolite, as shown in the Table. The hydroxylated metabolites of PAHs are excreted in human urine both as free hydroxylated metabolites and as hydroxylated metabolites conjugated to glucuronic acid and sulfate. Urine metabolite profiles can vary depending on the PAH source(s), but also have been found to vary between individuals experiencing similar exposures within the same workplace (Grimmer et al., 1997; Jacob and Seidel 2002).
Finding a measurable amount of one or more metabolites in the urine does not imply that the levels of the PAH metabolites or the parent PAH cause an adverse health effect. Biomonitoring studies of urinary PAHs provide physicians and public health officials with reference values so that they can determine whether or not people have been exposed to higher levels of PAHs than are found in the general population. Biomonitoring data can also help scientists plan and conduct research on exposure and health effects.
CAS No. 85-01-8
Phenanthrene is used in manufacturing dyestuffs and explosives and in biological research. Sources of phenanthrene include diesel fuel exhaust, coal tar pitch and tobacco smoke. Phenanthrene has been found in particle emissions from natural gas combustion and municipal incinerator waste, and in the particulates present in ambient air pollution near high vehicular traffic and industrial or urban areas (ATSDR, 1995; Fang et al., 2006; Rehwagen et al., 2005). IARC determined that phenanthrene was not classifiable with respect to human carcinogenicity.
Urinary levels of 1-hydroxyphenanthrene, 2-hydroxyphenanthrene, 3-hydroxyphenanthrene, and 4-hydroxyphenanthrene reflect recent exposure. Geometric mean and median urine concentrations of 1- and 3-hydroxyphenanthrene in a 1998 sample of German adults were about 2-fold higher than levels in the NHANES 2001–2002 and 2003–2004 subsamples (Becker et al., 2003). Children and adults in housing where coal tar flooring glue was applied had similar urinary 1-, 2-, 3-, and 4-hydroxyphenanthrene levels compared to residents in houses without the glue; mean levels of these metabolites were higher than levels for similar age groups in the Fourth Report (Heudorf and Angerer, 2001a). Smoking increases levels of urinary 2-, 3-, and 4-hydroxyphenanthrene (Becker et al., 2003; Elovaara et al., 2006; Heudorf and Angerer 2001b; Jacob et al., 1999). Occupational PAH exposures have been associated with median urinary phenanthrene metabolite concentrations that range from 10 to 100 times higher than median values in the general population (Elovaara et al., 2006; Gundel et al., 2000).
Finding a measurable amount of one or more urinary phenanthrene metabolites does not imply that the level causes an adverse health effect. Biomonitoring studies on levels of phenanthrene metabolites provide physicians and public health officials with reference values so that they can determine whether people have been exposed to higher levels of phenanthrene than are found in the general population. Biomonitoring data can also help scientists plan and conduct research on exposure and health effects.
Agency for Toxic Substances and Disease Registry (ATSDR). Toxicological profile for polycyclic aromatic hydrocarbons [online] 1995. Available at URL: https://www.atsdr.cdc.gov/toxprofiles/tp69.html. 5/26/09
Becker K, Schulz C, Kaus S, Seiwert M, Seifert B. German environmental survey 1998 (GerES III): environmental pollutants in the urine of the German population. Int J Hyg Environ Health 2003; 206:15-24.
Elovaara E, Mikkola J, Makela M, Paldanius B, Priha E. Assessment of soil remediation workers’ exposure to polycyclic aromatic hydrocarbons (PAH): Biomonitoring of naphthols, phenanthrols, and 1-hydroxypyrene in urine. Toxicol Lett 2006;162:158-163.
Fang G-C, Wu Y-S, Chen J-C, Chang C-N, Ho T-T. characteristic of polycyclic aromatic hydrocarbon concentrations and source identification for fine and coarse particulates at Taichung Harbor near Taiwan Strait during 2004–2005. Sci Tot Environ 2006;366:729-738.
Gundel J, Schaller KH, Angerer J. Occupational exposure to polycyclic aromatic hydrocarbons in a fireproof stone producing plant: biological monitoring of 1-hydroxypyrene, 1-, 2-, 3- and 4-hydroxyphenanthrene, 3-hydroxybenz(a)anthracene and 3-hydroxybenzo-(a)-pyrene. Int Arch Occup Environ Health 2000;73(4):270-274.
Heudorf U, Angerer J. Internal exposure to PAHs of children and adults living in homes with parquet flooring containing high levels of PAHs in the parquet glue. Int Arch Occup Environ Health 2001a;74(2):91-101.
Heudorf U, Angerer J. Urinary monohydroxylated phenanthrenes and hydroxypyrene–the effects of smoking habits and changes induced by smoking on monooxygenase-mediated metabolism. Int Arch Occup Environ Health. 2001b 74(3):177-83.
Jacob J, Grimmer G, Dettbarn G. Profile of urinary phenanthrene metabolites in smokers and non-smokers. Biomarkers 1999;4(5):319-327.