Guidelines for Canadian Drinking Water Quality: Guideline Technical Document – Dichlorobenzenes
Table of Contents
The maximum acceptable concentrations (MAC) and aesthetic objectives (AO) for 1,2- and 1,4-dichlorobenzene in drinking water are:
(In cases where total dichlorobenzenes are measured and concentrations exceed the most stringent value [5 µg/L], the concentrations of the individual isomers should be established.)
Health Canada has withdrawn the MAC for 1,2 dichlorobenzene. Please see Withdrawal of Select Guidelines for Canadian Drinking Water Quality (2022) for more information.
Dichlorobenzenes (DCBs) are chlorinated aromatic compounds. There are three DCB isomers: 1,2-DCB, 1,3-DCB and 1,4-DCB. 1,2- and 1,3-DCB are liquids at room temperature, whereas 1,4-DCB is a solid with a melting point of 53°C. Their vapour pressures are moderate, ranging from 90 to 270 Pa at 25°C. They are moderately soluble in water, with solubilities ranging from 30.9 to 124.5 mg/L at 20°C (for 1,4- and 1,3-DCBs, respectively). Their log octanol-water partition coefficients (Kow) are moderately high, around 3.0 for all three isomers.Footnote 1 Dichlorobenzenes are not manufactured in Canada, but several thousand tonnes of both the 1,2- and 1,4-isomers are imported each year from the United States for use in degreasing and paint removal formulations (1,2-DCB), as chemical intermediates (1,2-DCB), in moth crystals (1,4-DCB) and in urinal or space deodorants (1,4-DCB). Emissions of DCBs to the environment are believed to be small; however, the 1,4-isomer may be released into water from urinal deodorants.Footnote 1
It is unlikely that DCB levels are reduced significantly during conventional drinking water treatment processes. However, removal of volatile organic compounds by packed tower aeration and granular activated carbon adsorption has been estimated to be 90 to 99% effective, and it is likely that concentrations of DCBs below 1 µg/L can be achieved in Canadian drinking water supplies using these methods.Footnote 7
The carcinogenicity of 1,2-DCB has been investigated in a recently completed NTP study.Footnote 19 Doses of 0, 60 and 120 mg/kg bw were administered daily by gavage in corn oil, five days per week for 103 weeks, to groups of 50 male and 50 female F344/N rats or B6C3F1 mice. In both rats and mice, there were no differences in survival rates of treated and control animals, and there was no evidence of compound-related neoplastic or non-neoplastic lesions. It should be noted, however, that these doses were probably less than the maximum tolerated dose (MTD).
Dichlorobenzenes are not mutagenic in bacteria, but mutations in mould and plant cultures and chromosomal aberrations in plants and in human workers (1,2-DCB) have been observed.Footnote 10 There have also been isolated reports of leukaemia in workers exposed to DCBs.
1,2-Dichlorobenzene: There has been no evidence of carcinogenicity of 1,2-DCB in two species. However, the doses administered in the studies conducted to date were probably below the MTD, thereby reducing the sensitivity of the assays. The data available are onsidered, therefore, to be inadequate to classify 1,2-DCB with respect to its potential carcinogenicity; it has, therefore, been included in Group VA (inadequate data for evaluation).
For compounds classified in Group VA, the MAC is derived on the basis of division of the NOAEL or LOAEL in an animal species by an uncertainty factor. For 1,2-DCB, the acceptable daily intake (ADI) is derived as follows:
Long description - The equation used to calculate the acceptable daily intake (ADI) of 1,2-dichlorobenzene.
- 30 mg/kg bw per day is the LOAEL observed in the most sensitive species (rats) in the most recent and well-documented subchronic study19
- 1000 is the uncertainty factor (×10 for less-than-lifetime study; ×10 for LOAEL rather than NOAEL; and ×10 for extrapolation from animals to humans [generally 100; however, end point for the LOAEL is much more sensitive than traditional end points, and NOAEL in chronic studies is greater than that in subchronic studies])
- 5/7 is the conversion of five days per week of dosing to seven days per week.
1,3-Dichlorobenzene: There are no data available to serve as a basis for establishing an MAC for 1,3-DCB in drinking water.
1,4-Dichlorobenzene: For 1,4-DCB, there was clear evidence of carcinogenicity in the most sensitive bioassay (NTP) conducted to date.Footnote 23 (It should be noted, however, that the increased incidence of renal tubular cell adenocarcinomas in male rats was observed at dose levels that caused damage to the kidneys.) 1,4-Dichlorobenzene has, therefore, been classified in Group II -- probably carcinogenic to man (sufficient evidence in animals; inadequate data in man) -- and cancer risks have been estimated on the basis of the results of the NTP carcinogenesis bioassay in F344/N Footnote 23 Incorporating a rats and B6C3F1 mice (gavage). surface area correction and using the robust linear extrapolation model for each of the significantly increased tumour types, one can calculate that unit lifetime risks associated with the ingestion of 1 µg/L 1,4-DCB in drinking water range from 1.2 × 10-7 (based on hepatocellular adenomas in male mice) to 4.3 × 10-7 (based on phaeochromocytomas of the adrenal gland in male mice).* The estimated ranges of concentrations in drinking water corresponding to lifetime risks of 10-5,
10-6 and 10-7 for these same tumour types based on the model described above are as follows:
|Lifetime risk||Concentrations in drinking water (µg/L)|
|10-5||23 - 83|
|10-6||2.3 - 8.3|
|10-7||0.23 - 0.83|
Average adult body weight = 70 kg; average daily intake of drinking water = 1.5 L.
1,2-Dichlorobenzene: Because 1,2-DCB is classified in Group VA (inadequate data for evaluation), the MAC in drinking water is derived from the ADI as follows:
Long description - The equation used to calculate the maximum acceptable concentration (MAC) for 1,2-dichlorobenzene in drinking water.
- 0.021 mg/kg bw per day is the ADI, as derived above
- 70 kg is the average body weight of an adult
- 0.20 is the proportion of total intake ingested in drinking water; limited available data indicate that the amount of DCBs ingested in drinking water is 20% of that ingested in food
- 1.5 L/d is the average daily consumption of drinking water for an adult.
1,4-Dichlorobenzene: Because 1,4-DCB is classified in Group II (probably carcinogenic to man), the MAC is based on consideration of available practicable treatment technology and estimated lifetime cancer risks. Because the MAC must also be measurable by available analytical methods, the PQL is also taken into consideration in its derivation.
An MAC of 0.005 mg/L (5 µg/L) was established, therefore, on the basis of the following considerations:
- The estimated unit lifetime cancer risks associated with the ingestion of 1 µg/L 1,4-DCB in drinking water range from 1.2 × 10-7 (based on hepatocellular adenomas in male mice) to 4.3 × 10-7 (based on phaeochromocytomas of the adrenal gland in male mice). Therefore, the estimated lifetime risks associated with the ingestion of drinking water containing 5 µg/L 1,4-DCB (i.e., 6.0 × 10-7 to 2.2 × 10-6) are within a range that is considered to be "essentially negligible."
- is likely that concentrations of DCB below 1 µg/L can be achieved in Canadian drinking water supplies by packed tower aeration and granular activated carbon adsorption.
- The PQL (based on the ability of laboratories to measure DCBs within reasonable limits of precision and accuracy) is 5 µg/L.
The aesthetic objective (AO) for 1,4-DCB is ≤ 0.001 mg/L, based on the threshold odour value.Footnote 24
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