Dichlorobenzene (C₆H₄Cl₂) occurs as three isomers (1,2-DCB, 1,3-DCB and 1,4-DCB) which vary in the relative sites of attachment on the benzene ring of the two chlorine groups. 1,2-DCB and 1,3-DCB are liquids at room temperature and pressure, whilst 1,4-DCB is solid. Solubilities in water are 147, 106 and 83 mg l^-1 for 1,2-DCB, 1,3-DCB and 1,4-DCB, respectively, and log Kow values (preferred values) are in the range 3.28 to 3.62. Dichlorobenzenes tend to volatilise.

Crane and Fawell (1989) and Hobbs et al. (1996) both report production and usage volume data for dichlorobenzenes in the 1980s, but more recent figures do not seem to be available in the literature. Hobbs et al. (1996) reported that 1,4-dichlorobenzene was no longer produced in the UK, and that imports were probably about 750 tonnes per annum, i.e. a significant (75%) decrease compared to the 1980s as a result of increasing use of replacement chemicals in toilet blocks and other space deodorants.

Industrially, 1,2-DCB and 1,4-DCB are more significant than 1,3-DCB.

Notable uses of 1,2-DCB include as a solvent and degreasing agent in various applications, a chemical intermediate, a fuel additive, a dye carrier in the textiles industry and an active ingredient and production chemical in some pesticides and wood preservative (along with 1,4-DCB as a more minor ingredient). Crane and Fawell (1989) report the principal uses in the UK as production of agrochemicals (50%) and as a dye solvent (50%), but more recent data are not available.

The primary applications of 1,4-DCB are as a space deodorant and as a moth repellent, with more minor uses including a precursor in the production of certain chemicals, as a catalyst in the production of mercapto acids, as a dye carrier, for mould and mildew control and in wood preservative (along with 1,2-DCB). Crane and Fawell (1989) report the principal uses in the UK as moth repellent and space deodoriser (90% combined) and as a chemical intermediate (10%). However, all uses, including as a deodorant and a moth repellent, have declined significantly in the UK. Hobbs et al. (1996) indicated that all 1,4-DCB now imported in to the UK were used to manufacture toilet and air deodorants.

The only notable use of 1,3-DCB is as an intermediate in chemical synthesis.

Potential industrial point sources will include aqueous effluent from any industry where dichlorobenzenes are made, formulated, used as process solvents or used as intermediates. A particularly significant diffuse source of 1,4-DCB would be its use as a space deodorant in lavatory systems, with direct release to sewers or other sewerage disposal routes, and the relative importance of this source will have increased with the decline in other uses of 1,4-DCB.

Dichlorobenzenes can also be produced in water by chlorination of raw drinking waters, and a number of authors have reported the consequent presence of all three isomers in supply (although overall Crane and Fawell 1989 concluded that significant production of dichlorobenzenes by chlorination was unlikely).

Production of dichlorobenzenes can result from the biodegradation of higher-chlorinated benzenes already in the environment, either in aquatic systems or with the potential for input into aquatic systems. Dichlorobenzenes have also been reported to be produced during incineration of municipal waste, sewage sludge, fossil fuels and, in particular, chlorinated polymers, such as polyvinyl chloride and chlorinated polyethylenes. In each case, input of dichlorobenzenes to the atmosphere may subsequently result in depositional input to aquatic systems.

The widespread occurrence of dichlorobenzenes in the atmosphere confers the potential for contamination of all associated surface waters, even in the absence of manufacturing or use-related inputs. Based on a theoretical population centre of 1 million people in 100 km² using typical amounts of dichlorobenzene, a concentration of 2 µg l^-1 1,4-DCB in surface waters was predicted by Rippen et al. (1984) (reported in Crane and Fawell 1989). Generally, it was the 1,4-DCB isomer which had been reported at the highest concentrations in surface waters up to 1989, at concentrations from 0.004 to 310 µg l^-1, but the 1,3-isomer may be more significant in aquatic sediments (Crane and Fawell 1989).

Studies of fresh and saline waters reported by the Foundation for Water Research (FWR) (1990) similarly indicated concentrations were below the µg l^-1 level, although concentrations associated with suspended sediment had been reported as high as tens of mg kg^-1.

The fate and behaviour of dichlorobenzenes has been summarised by Hedgecott et al (1998).

Dichlorobenzenes do not demonstrate extremes of aqueous solubility or lipophilicity, but are volatile. Models describing their environmental partitioning indicate that dichlorobenzenes in surface waters will be prone to removal from the water column by volatilisation and also by sorption to particulates which settle out into sediments. Models for environmental volatilisation, and real monitoring data, suggest 50% or more may volatilise from flowing waters in 8 hours to 3 days, whilst this may increase from 3 to 100 days (but mostly less than 30 days) for lakes and seawater mesocosms.

In the atmosphere, dichlorobenzenes may be degraded by chemical- or sunlight-catalysed reactions, and may also sorb to particulates which are subsequently deposited (whilst losses of dichlorobenzenes dissolved in rain are not expected to be significant). A tendency to sorb to organic solids is also suggested by the log Kow values of 3.28 to 3.62 and log Koc values of 2.2 to 3.0.

Different standard aerobic biodegradability tests indicate that dichlorobenzenes can be classified as 'readily biodegradable' through to 'resistant to biodegradation', depending on the test type and conditions. There do not appear to be any reports of standard anaerobic biodegradability tests. Biodegradation of dichlorobenzenes in aerobic aqueous and soils systems has been widely reported. However, its environmental significance is probably limited, except where volatilisation is impeded. Studies of anaerobic systems (e.g. sediment cores) provide no evidence of biodegradation.

Sorption to suspended solids in surface waters also occurs, and has been reported by Hobbs et al. (1996) to result in significant contamination of settled solids by 1,4-DCB following sedimentation.

An exhaustive literature review on the toxicity of dichlorobenzenes to marine organisms has not been carried out for the purposes of this profile. The information provided in this section is taken from existing review documents (Hedgecott et al 1998). The most sensitive groups of organisms have been identified.

The available dataset was found to be limited. However, no particular taxa appear to be significantly sensitive to dichlorobenzenes, with the toxicity of the three isomers appearing to be similar.

Crane and Fawell (1989) found the mysid shrimp Mysidopsis bahia to be the most sensitive of those species considered with acute LC50 values of 2.0 to 2.9 mg l^-1 for the three isomers of DCB.

More recent data summarised by Hedgecott et al (1998) included a 24 hour LOEC for growth of 1 mg l^-1 1,2-DCB for the pacific oyster and with 96 hour >toxic= effects at 1.28 mg l^-1 1,2-DCB, for plaice. However, four species of algae did not grow when exposed for 10 days to 13 mg l^-1 1,2-DCB, and it is reasonable to assume that the threshold effects concentration for these species might also be around, or even below, 1 mg l^-1.

In an echinoderm reproduction study previously considered by Crane and Fawell (1989), adverse effects were seen at 0.15 mg l^-1. However, the ecological implications of the observed pattern of effects were unclear and, therefore, a possible safe concentration could not be determined.

No data could be located for sediment dwelling organisms.

Crane and Fawell (1989) reviewed both fresh and saltwater studies and concluded that bioaccumulation of dichlorobenzenes resulted in a maximum BCF value of 1,400 in freshwater organisms (in rainbow trout exposed to 1,4-DCB), with no experimental information for saltwater species (although tentative BCFs between 36 and 280 might be deduced from one particular field study).

Hedgecott et al (1998) found that few conventional bioaccumulation data had become available since Crane and Fawell's review, but BCFs can also be determined from a few freshwater tests investigating lethal and non-lethal body burdens of dichlorobenzenes. These data indicate low BCFs (<100) for aquatic plants exposed to 1,2-DCB, higher values (c.600) for Daphnia (the only water column invertebrate investigated) exposed to 1,2-DCB and values ranging from 13 to 741 (and 1,800 based on lipid weight) for fish exposed to any of the three isomers.

Generally, these data suggest a similar range of freshwater BCFs as those previously identified by Crane and Fawell (1989). Data for saltwater species are limited to a single study with a species of crab, with a lipid-based BCF of 1,445 determined for 1,4-DCB, implying a similar extent of accumulation as seen in freshwater fish.

Potential effects include:

• toxicity of dichlorobenzenes (sum of all isomers) to invertebrates at concentrations above the EQS of 20 microg l^-1 (annual average) and 200 microg l^-1 (maximum allowable concentration) in the water column;
• potential for bioaccumulation in saltwater organisms based on information for freshwater organisms.

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