Biomonitoring Summary
Chlorobenzenes
Chlorobenzene (Monochlorobenzene) CAS No. 108-90-7
1,2-Dichlorobenzene (o-dichlorobenzene) CAS No. 95-50-1
1,3-Dichlorobenzene (m-dichlorobenzene) CAS No. 541-73-1
1,4-Dichlorobenzene (p-dichlorobenzene, Paradichlorobenzene) CAS No. 106-46-7
General Information
Chlorobenzene (monochlorobenzene) and the three dichlorobenzenes are halogenated aromatic hydrocarbons pirmarily used in industrial and chemical synthetic processes. Chlorobenzene has been used to produce DDT, phenol, and nitrobenzene. The dichlorobenzenes are also chemical intermediates in synthesis of dyes, pesticides, and other industrial products. The chlorobenzenes have sometimes been used as solvents for pesticides and auto parts degreasers (ATSDR, 2007). 1,4-Dichlorobenzene (1,4-DCB; paradichlorobenzene) is used also as a moth repellent and as a deodorizer (ATSDR, 2007).
Ambient air is the primary source of chlorobenzene exposure for the general population. Indoor air levels of 1,4-DCB may exceed outdoor levels when moth repellents or deodorizers are in use (Wallace et al., 1987, 1991). Dietary sources are negligible (Schaum et al., 2003), and chlorobenzenes generally are not detected in drinking water or groundwater in the United States (USGS, 2006), but may be detected where industrial waste containing these chemicals has been discharged (IPCS, 2004). Chlorobenzenes volatilize from soil and water (ATSDR, 2007, 2008). People involved in the production or use of chlorobenzenes may be exposed by inhalation or dermal contact. Chlorobenzenes are well absorbed after inhalation and ingestion. 1,4-DCB is not appreciably absorbed through intact skin. Within a few hours following exposure, these chemicals are eliminated from tissues via oxidative hepatic metabolism followed by conjugation or oxidation. The major urinary metabolites are dichlorophenols (ATSDR, 2007, 2006).
Human health effects from chlorobenzenes at low environmental doses or at biomonitored levels from low environmental exposures are unknown. In humans, high air levels of 1,2- or 1,4-dichlorobenzenes cause eye and nasal irritation, and prolonged or repeated contact with concentrated solutions of either chemical may cause skin irritation or sensitization (Elovaara, 1998). Asthma and reduced pulmonary function have been associated with recent exposure to aromatic chemicals, including 1,4-DCB, but causation is unclear (Arif and Shah, 2007; Elliott et al., 2006). Laboratory animals exposed to high levels of chlorobenzene may demonstrate liver enlargement and serum transaminase elevations, renal tubular cell damage, and central nervous system depression. High doses of 1,2- or 1,3-dichlorbenzenes can result in centrilobular liver necrosis and decreased thyroid hormone levels and, among male animals, renal tubular degeneration (ATSDR, 2007; Elovaara, 1998; NTP, 1987). 1,4-DCB is not as acutely hepatotoxic or thyrotoxic as the other dichlorobenzene isomers (den Besten et al., 1991, 1992; Stine et al., 1991). In animal studies, 1,4-DCB is not considered to be a reproductive or developmental toxicant (ATSDR, 2007, 2008; Elovaara, 1998). Animals fed high doses of 1,4-DCB demonstrated an increased incidence of renal and hepatic tumors, but no evidence was found of mutagenicity or genotoxicity in vitro (NTP, 1987).
The U.S. EPA and the FDA regulate the levels of 1,2- or 1,4-dichlorobenzene in air and water and in bottled drinking water, respectively. U.S. EPA regulates the monochlorobenzene level in drinking water. NIOSH and ACGIH provide workplace guidelines for 1,2- and 1,4-dichlorobenzenes and monochlorobenzene levels in air. IARC classifies 1,4-dichlorobenze as a possible human carcinogen, and NTP determined that it was reasonably anticipated to be a human carcinogen. However, IARC determined that the human carcinogenicity of 1,2-dichlorobenzene and 1,3-dichlorobenzene was unclassifiable. Additional information about external exposure (i.e., environmental levels) and health effects is available from ATSDR at https://www.atsdr.cdc.gov/toxprofiles/index.asp.