Entry into the marine environment
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
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
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
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
Recorded levels in the marine
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.
Fate and behaviour in the marine
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
biodegradable' through to 'resistant
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.
Effects on the marine environment
Toxicity to marine organisms
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
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
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 on interest
features of European marine sites
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.