الكلور وتطهير المياه

Chlorine in Drinking-water

Background document for development of
WHO Guidelines for Drinking-water Quality
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GENERAL DESCRIPTION

Identity
Element or
compound
CAS no. Molecular formula
Chlorine 7782-50-5 Cl2
Hypochlorous acid 7790-92-3 HOCl
Sodium hypochlorite 7681-52-9 NaOCl

Physicochemical properties of chlorine (1,2) [Conversion factor in air: 1 ppm = 2.9 mg/m3]
Property Value
Boiling point -34.6 °C
Melting point -101 °C
Density 3.214 g/litre at 0 °C and 101.3
kPa
Vapour pressure 480 Pa at 0 °C
Water solubility 14.6 g/litre at 0 °C

Organoleptic properties

The taste and odour thresholds for chlorine in distilled water are 5 and 2 mg/litre,
respectively. In air, chlorine has a pungent and disagreeable odour (2).

Major uses

Large amounts of chlorine are produced for use as disinfectants and bleach for both domestic
and industrial purposes, and it is also widely used to disinfect drinking-water and swimmingpool
water and to control bacteria and odours in the food industry (3,4).

Environmental fate

In water, chlorine reacts to form hypochlorous acid and hypochlorites. All three species exist
in equilibrium with each other, the relative amounts varying with the pH. In dilute solutions
and at pH levels above 4.0, very little molecular chlorine exists in solution. The
concentrations of hypochlorous acid and the hypochlorite ion are approximately equal at pH
7.5 and 25 °C. Chlorine can react with ammonia or amines in water to form chloramines
(4,5).

ANALYTICAL METHODS

A colorimetric method can be used to determine free chlorine in water at concentrations of
0.1–10 mg/litre. Other methods allow for the determination of free chlorine, chloramines,
other chlorine species, and total available chlorine, and are suitable for total chlorine
concentrations up to 5 mg/litre. The minimum detectable concentration of chlorine is about
0.02 mg/litre (6).

ENVIRONMENTAL LEVELS AND HUMAN EXPOSURE

Air
A mean ambient air level of 1 mg/m3 was reported for chlorine (7).

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Water
Chlorine is present in most disinfected drinking-water at concentrations of 0.2–1 mg/litre (3).

Food
Cake flour bleached with chlorine contains chloride at levels in the range 1.3–1.9 g/kg.
Unbleached flour may contain small amounts of chlorite (400–500 mg/kg) (8).

Estimated total exposure and relative contribution of drinking-water
The major routes of exposure to chlorine are through drinking-water, food, and contact with
items either bleached or disinfected with it.

KINETICS AND METABOLISM IN LABORATORY ANIMALS AND HUMANS

Most studies on the pharmacokinetics of chlorine, hypochlorous acid, or hypochlorites
employ reactive 36Cl-labelled compounds and probably reflect the fate of the chloride ion or
other reaction products generated from the parent molecules. In rats, hypochlorous acid was
readily absorbed through the gastrointestinal tract, distribution being highest in the plasma;
smaller amounts were found in bone marrow, kidney, testes, lung, skin, duodenum, spleen,
liver, and bone (9,10). In vivo, sodium hypochlorite was metabolized to trichloroethanoic
acid, dichloroethanoic acid, chloroform, and dichloroacetonitrile (11). Hypochlorous acid
administered to rats was excreted primarily in the urine and faeces, mostly in the form of
chloride ion (10). None was excreted in expired air (9).

EFFECTS ON LABORATORY ANIMALS AND IN VITRO TEST SYSTEMS

Acute exposure
Calcium hypochlorite has an oral LD50 in the rat of 850 mg/kg of body weight (2).

Short-term exposure

No consistent effects on organ weights or histopathology of tissues were noted in Sprague-
Dawley rats (10 per sex per dose) given chlorine in drinking-water at 0, 25, 50, 100, 175, or
200 mg/litre (males: 0, 2, 7.5, 12.8, or 16.7 mg/kg of body weight per day; females: 0, 3.5,
12.6, 19.5, or 24.9 mg/kg of body weight per day) for 90 days (12) or in rats fed flour
containing 1257 or 2506 mg of chlorine per kg (62.5 or 125 mg/kg of body weight per day)
for 28 days (13).
Enhanced weight gain was observed in all male rats (10 per dose) given drinking-water
containing chlorine at 0, 20, 40, or 80 mg/litre (0, 4.1, 8.1, or 15.7 mg/kg of body weight per
day) for 6 weeks (14). The results of a 4-week study in which female C57BL/6N mice were
given hyperchlorinated tapwater (4.8–5.8 mg/kg of body weight per day) suggested an
adverse effect on the macrophage defence mechanisms of mice. The LOAEL in this study
was 4.8 mg/kg of body weight per day (15).
In a study in which male CR-1:CD-1 mice (30 per dose) received chlorinated drinking-water
(0.02, 0.2, 2.9, or 5.8 mg/kg of body weight per day) for 120 days, none of the mice showed
evidence of a statistically significant change in humoral or cell-mediated immune response. A
NOAEL of 5.8 mg/kg of body weight per day was identified (16).
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Long-term exposure

F344 rats (50 per sex per dose) were administered sodium hypochlorite in drinking-water
(males: 0.05% or 0.1%, 75 or 150 mg/kg of body weight per day; females: 0.1% or 0.2%, 150
or 300 mg/kg of body weight per day) for 2 years. Effects included a dose-related depression
in body weight gain in all groups, depressed liver, brain, and heart weights in males given a
0.05% dose, decreased salivary gland weights in both female groups, and decreased kidney
weights in females given 0.2% (17).
In a 2-year bioassay, F344 rats and B6C3F1 mice were given chlorine in drinking-water at
levels of up to 275 mg/litre (up to 24 mg/kg of body weight per day for male rats and male
mice, 15 mg/kg of body weight per day for female rats, and 22 mg/kg of body weight per day
for female mice). There was a dose-related decrease in water consumption for both mice and
rats. No effects on body weight or survival were observed in any of the treated animals (18).
Wistar rats were fed cake prepared from flour treated with 1250 or 2500 mg of chlorine per
kg (males: 12.8 or 25.3 mg/kg of body weight per day; females: 17.0 or 35.0 mg/kg of body
weight per day) for 104 weeks. A dose-related reduction in spleen weight was seen in
females, and dose-related haematological effects were observed in both sexes. A LOAEL of
12.8 mg/kg of body weight per day was identified in this study (19).

Reproductive effects, embryotoxicity, and teratogenicity

C3H/HeJ and C57BL/6J mice administered drinking-water containing 10 mg of residual
chlorine per litre (1.9 mg/kg of body weight per day) for 6 months showed no adverse
reproductive effects (20). In a seven-generation study in which rats were given drinking-water
chlorinated at 100 mg/litre (10 mg/kg of body weight per day), no treatment-related effects on
fertility were found (21).
Oral administration of hypochlorite ion or hypochlorous acid at 100, 200, or 400 mg of
chlorine per litre (1.6, 4.0, or 8.0 mg/kg of body weight per day) resulted, in the case of
hypochlorite, in dose-related increases in the amount of sperm-head abnormalities in male
B6C3F1 mice. A NOAEL of 8.0 mg/kg of body weight per day was identified for
hypochlorous acid and a LOAEL of 1.6 mg/kg of body weight per day for hypochlorite ion
(22).

Mutagenicity and related end-points

Sodium hypochlorite has been found to be mutagenic in Salmonella typhimurium TA1530
and TA100 but not TA1538 (23,24). Calcium and sodium hypochlorite both produced
chromosomal aberrations in Chinese hamster fibroblast cells without metabolic activation
(24). Hypochlorite ion and hypochlorous acid were negative in the in vivo erythrocyte
micronucleus assay and in bone marrow aberration studies (22).

Carcinogenicity

F344 rats (50 per sex per dose) were given sodium hypochlorite in drinking-water (males:
0.05% or 0.1%, 75 or 150 mg/kg of body weight per day; females: 0.1% or 0.2%, 150 or 300
mg/kg of body weight per day) for 2 years. Experimental groups did not differ from controls
with respect to the total tumour incidences or mean survival times, and most of the tumours
found were of types that commonly occur spontaneously in F344 rats. The authors concluded
that sodium hypochlorite was not carcinogenic in rats (17).
In a seven-generation toxicity study, the incidence of malignant tumours in rats consuming
drinking-water with a free chlorine level of 100 mg/litre (10 mg/kg of body weight per day)
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did not differ from that in controls (21). The incidence of tumours in treated animals was not
significantly elevated in F344 rats and B6C3F1 mice (50 per sex per dose) given solutions of
sodium hypochlorite (70 or 140 mg/kg of body weight per day for male rats, 95 or 190 mg/kg
of body weight per day for female rats, 84 or 140 mg/kg of body weight per day for male and
female mice) in their drinking-water for 103–104 weeks (25).
In a 2-year bioassay, F344 rats and B6C3F1 mice were given chlorine in drinking-water at
levels of 0, 70, 140, or 275 mg/litre (8, 13, or 24 mg/kg of body weight per day for male rats;
5, 7, or 15 mg/kg of body weight per day for female rats; 8, 15, or 24 mg/kg of body weight
per day for male mice; and 1, 13, or 22 mg/kg of body weight per day for female mice).
Although there was a marginal increase in mononuclear-cell leukaemia in the groups of
female rats given 140 and 275 mg/litre, it was considered to be equivocal evidence of
carcinogenic activity because the incidence was significantly elevated compared with controls
only for the middle dose and the incidence of leukaemia in the concurrent controls was lower
than the mean in historical controls (18).

EFFECTS ON HUMANS

Exposure to chlorine, hypochlorous acid, and hypochlorite ion through ingestion of household
bleach occurs most commonly in children. Intake of a small quantity of bleach generally
results in irritation of the oesophagus, a burning sensation in the mouth and throat, and
spontaneous vomiting. In these cases, it is not clear whether it is the sodium hypochlorite or
the extremely caustic nature of the bleach that causes the tissue injury.
The effects of heavily chlorinated water on human populations exposed for varying periods
were summarized in a report that was essentially anecdotal in character and did not describe
in detail the health effects observed (26). In a study on the effects of progressively increasing
chlorine doses (0, 0.001, 0.014, 0.071, 0.14, 0.26, or 0.34 mg/kg of body weight) on healthy
male volunteers (10 per dose), there was an absence of adverse, physiologically significant
toxicological effects in all of the study groups (27). It has been reported that asthma can be
triggered by exposure to chlorinated water (28). Episodes of dermatitis have also been
associated with exposure to chlorine and hypochlorite (29,30).
In a study of 46 communities in central Wisconsin where chlorine levels in water ranged from
0.2 to 1 mg/litre, serum cholesterol and low-density lipoprotein levels were higher in
communities using chlorinated water. Levels of high-density lipoprotein (HDL) and the
cholesterol/HDL ratio were significantly elevated in relation to the level of calcium in the
drinking-water, but only in communities using chlorinated water. The authors speculated that
chlorine and calcium in drinking-water may interact in some way that affects lipid levels (31)
An increased risk of bladder cancer appeared to be associated with the consumption of
chlorinated tapwater in a population-based, case–control study of adults consuming
chlorinated or non-chlorinated water for half of their lifetimes (32).

GUIDELINE VALUE

In humans and animals exposed to chlorine in drinking-water, specific adverse treatmentrelated
effects have not been observed. IARC has concluded that hypochlorites are not
classifiable as to their carcinogenicity to humans (Group 3) (17).
The guideline value for free chlorine in drinking-water is derived from a NOAEL of 15 mg/kg
of body weight per day, based on the absence of toxicity in rodents that received chlorine as
hypochlorite in drinking-water for up to 2 years (18). Application of an uncertainty factor of
100 (for inter- and intraspecies variation) to this NOAEL gives a TDI of 150 μg/kg of body
weight. With an allocation of 100% of the TDI to drinking-water, the guideline value is 5
5
mg/litre (rounded figure). It should be noted, however, that this value is conservative, as no
adverse effect level was identified in this study. Most individuals are able to taste chlorine or
its by-products (e.g. chloramines) at concentrations below 5 mg/litre, and some at levels as
low as 0.3 mg/litre.

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