Sodium hypochlorite

Endpoint summary

Description of key information

Short description of key information on bioaccumulation potential result: 
Animal data suggest that after exposure via oral route, HOCl is absorbed and excreted mainly through urine as chloride (36.43% + 5.67 of the administered dose after 96h); a lesser extent of HO36Cl-derived radioactivity not necessarily associated with absorption was detectable in the faeces 96h after exposure (14.8% + 3.7). Once in the body, it reacts directly with organic molecules to form some organochlorinated compounds, characterised by their own toxicity. No data are available for other routes of exposure, including dermal and inhalation. Human data are very scant and indirect. Absorption is suggested by some transient and not severe systemic symptoms following ingestion, although the possibility they are secondary to a local effect could not be ruled out with certainty.

Additional information

Toxicokinetics, metabolism and distribution

Sodium hypochlorite dissolved in water exists as a mixture of different chlorine species, whose relative amounts depends mainly on the pH. In the biological systems, characterised by pH values in the range 6-8, the most abundant active chemical species are HOCl and ClO-, in equilibrium. The latter is predominant at alkaline pH values, while Cl2 is mainly present at pH below 4 (see paragraph 2.4). Sodium hypochlorite readily interacts with organic molecules and cellular components, leading to the formation of chlorinated organic compounds possessing their own inherent toxicity (BIBRA, 1990). Very few data have been produced on ADME for HOCl and are limited to the oral route. No information is available on dermal exposure or inhalation. However, due to its polarity very limied skin absorption can be expected for the intact skin. Exposure via inhalation is limited due to the low vapour pressure and the crystaline structure of sodium hypochlorite.

Endogenous occurrence

Hypochlorous ions are physiologically present in the human body, being formed by white blood cells (neutrophils and monocytes) as a powerful antimicrobial agent during inflammation processes. When the recognition of “non-self” proteins in an invading microorganism triggers the immune response, the enzyme myeloperoxidase located in mammalian neutrophils catalysed hypochlorous acid formation trough the oxidation of chloride ion in combination with hydrogen peroxide (Weiss, 1989; Babior, 1984; IARC, 1991). The endogenously formed hypochlorous acid plays a key role in the process of phagocytosis through which bacteria are killed. Due to its potent cytotoxic action, hypochlorite is also responsible for neutrophil-mediated tissue damage associated with the inflammatory response. Its high efficiency as antimicrobial agent is associated with the lack of a catalytically active detoxifying mechanism for HOCl in both bacteria and mammalian cells. Although it has been suggested that HOCl-induced cytotoxicity can be associated to the degradation of a number of functionally important molecules (Weiss, 1989; Bernofsky, 1991) the primary mechanism of action is still not fully elucidated.

Besides being an oxidant itself, HOCl can react with H2O2 and superoxide anion to generate other highly reactive oxidizing molecules (singlet oxygen and hydroxyl radical), which very likely contribute to the onset of toxicity. In addition, hypochlorite can react with a number of cellular components, such as amino-acids, thiolic compounds, nucleotides and lipoproteins, forming organochloride species (Weiss, 1989; Bernofsky, 1991; Fleming, 1991), some of which endowed with their own toxicity. As well as many N-chloramines, derived from reaction with both nucleotides and amino acids, chlorohydrins of unsatured fatty acids (Winterbourn, 1992) and chlorinated sterols (Hazen et al., 1996 a ,b) have been identified as by-products of in vitro reactions of the myeloperoxidase/peroxidase/chloride system.

Based on a mean HOCl production rate of 3.15 x 10-8 μM/cell-h for the myeloperoxidasecatalysed reaction and assuming that about 0.1% of total neutrophils are triggered at any one time, Haas (1994) estimated a production ratio of 16 μM/day HOCl from the human immune system. Considering a possible 1-5% yield, the total amount of hypochlorite corresponds to a total generation of organochlorine compounds in the human body in the range of 5.7 to 28 μg/day (equal to 0.16 - 0.8 μM/day).

Animal data

Absorption, distribution and excretion

Abdel-Rahman et al. (1983) studied the toxicokinetics of hypochlorous acid (HOCl). Three groups of 4 Sprague-Dawley rats were orally administered with different quantities of HO³⁶Cl solution (range of specific radioactivity 1340-2190 dpm/μg³⁶Cl): the first group of 4 nonfasted rats received 3 ml of 250 mg/l HO³⁶Cl aqueous solution (0.75 mg per animal); the second group of 4 fasted rats received 200 mg/l HO³⁶Cl aqueous solution (0.60 mg per animal). Blood samples were taken from animals of these two groups at different times (0- 96hr) and tissue specimen were prepared at sacrifice for ³⁶Cl content assessment. The third group of fasted rats receiving 200 mg/l HO³⁶Cl aqueous solution (0.60 mg per animal) were housed in metabolic cages in order to collect urine, faeces and expired air at different times for ³⁶Cl radioactivity measurement.

³⁶Cl is readily absorbed and found into the bloodstream: a peak of radioactivity in rat plasma occurred 2 hours after HO³⁶Cl administration in group I (fasted rats) (7.9 μg/ml) and 4 hr after administration in group II (non-fasted rats) (10.7 μg/ml). The half-life of³⁶Cl in group II resulted 2-fold higher (88.5 h) than the one measured in group I (44.1 h), very likely due to the different fasting conditions of animals (Abdel-Rahman et al., 1983). Indirect indication of rapid absorption through the GI tract was given by the occurrence of blood GSH depletion evidenced soon after (15-120 min) the acute treatment of Sprague Dawley male rats with 3 ml aqueous solution containing 10, 20, 40 mg/l HOCl by gavage (Abdel-Rahman and Suh, 1984).

³⁶Cl radioactivity was distributed throughout the major tissues, 96 hr after HO36Cl administration. The higher levels were found in plasma (1.92 μg/g), whole blood (1.59 μg/g), bone marrow (1.55 μg/g), testis (1.26 μg/g), skin (1.20 μg/g), kidney (1.13 μg/g) and lung (1.04 μg/g). The lowest levels were found in the liver (0.51 μg/g), carcass (0.40 μg/g), and fat tissue (0.09μg/g) (Abdel-Rahman et al., 1983). The distribution of ³⁶Cl in plasma and whole blood studied 24 hr after treatment showed that plasma ³⁶Cl content was 4-fold higher than radioactivity measured in packed cells. In plasma about 20% of total ³⁶Cl was bound to protein, while in red cells a high percentage of ³⁶Cl was loosely bound to the erythrocyte membrane or exchangeable with chloride in saline. The subcellular distribution of ³⁶Cl in the liver, showed that the main fraction of the radioactivity recovered in hepatic homogenate was localised in the cytosol, and only 4% was bound to proteins (as measured in the TCA precipitate) (Abdel-Rahman et al., 1983).

HO³⁶Cl-derived radioactivity was not detected in expired air throughout the 96 hr study. During the same period, 36.43% + 5.67 (mean + S.E.) of the administered dose was excreted through the urinary route, while 14.8% + 3.7 was recovered in the faeces, giving a poor total recovery of 51.23% + 1.97 (Abdel-Rahman et al., 1983).

Metabolism

As previously indicated, HOCl is not enzymatically metabolised and its (bio)tranformation readily occurs through direct reactions with organic compounds or with other chemicals present in the cellular environment, including hydrogen peroxide. Results from the toxicokinetic study carried out by Abdel-Rahman et al. (1983), showed that the chloride ion accounted for >80% ³⁶Cl radioactivity present in rat plasma.

When Sprague-Dawley rats were administered HClO at 0, 1, 10 or 100 mg/l daily in drinking water for one year, no significant chloroform concentrations, were observed in rat blood at any time (4, 6, 9, 12 months) during the treatment (Abdel-Rahman and Suh, 1984).

The formation of organochlorinated compounds was tested in the stomach content and in the blood samples of four groups of three Sprague-Dawley rats each: fasted/non-fasted control group, fasted / non-fasted dosed group. The dosed groups were administered by gavage with 7 ml of a 8 mg/l solution of sodium hypochlorite at pH 7.9 (about 140 mg/kg bw) and sacrificed after one hour: the results were expressed as detectable or not-detectable for the very low levels of reaction products (detection limit range: 0.06-1.3 μg/ml plasma). Qualitatively it resulted that acetic acid was found in all the blood and stomach content samples from all the 4 groups, including controls. Trichloroacetic acid, dichloroacetic acid and chloroform were detected only in the stomach content of dosed animals (fasted and not fasted), suggesting its formation independently from the presence of food content in the gut. On the contrary, dichloroacetonitrile detection was limited to gut samples from non-fasted rats. Some plasma samples of dosed animals resulted positive to the presence of trichloroacetic acid (Mink 1983).

In the same laboratory, a multiple dose study was also carried out dosing rats for 8 days orally with 8 and 16 mg/kg bw/day NaOCl, a much lower concentration with respect to the acute study by Mink et al (1983) and more consistent with drinking water intake. Following the final dose rats were placed in metabolism cages, and urine was collected in water-cooled vials. No organo-chlorinated compounds were detected in urine extract by means of GC/MS analyses (Kopfler et al, 1985). The presence of Cl- was not assessed.

Human data

No specific studies on humans have been conducted so far. Nevertheless, it is possible to obtain some information from some reported cases of accidental ingestion. Reported effects included some systemic symptoms, such as laboured breathing, decreased blood pressure, increased sodium levels in the blood and acidosis, probably due to the formation of hypochlorous acid and Cl2 gas at the low pH typical of the gastric environment (Done, 1961; Ward and Routledge, 1988). The systemic effects could suggest the absorption and distribution of NaOCl, although it is not possible to exclude they are secondary to its local irritating and/or corrosive action producing tissue damage.

Intoxications caused by the direct inhalation of hypochlorite vapours have never been reported; it is generally due to misuse of bleaching solutions, when mixed with ammonia or acids, responsible for dramatic pH changes.

Conclusions

Animal data suggest that after exposure via oral route, HOCl is absorbed and excreted mainly through urine as chloride (36.43% + 5.67 of the administered dose after 96h); a lesser extent of HO36Cl-derived radioactivity not necessarily associated with absorption was detectable in the faeces 96h after exposure (14.8% + 3.7). Once in the body, it reacts directly with organic molecules to form some organochlorinated compounds, characterised by their own toxicity. No data are available for other routes of exposure, including dermal and inhalation. Human data are very scant and indirect. Absorption is suggested by some transient and not severe systemic symptoms following ingestion, although the possibility they are secondary to a local effect could not be ruled out with certainty.

Discussion on bioaccumulation potential result:

Studies on the absorption, distribution, metabolism and excretion were conducted with [³⁶Cl]-radio-labelled test substance in rats. Fasted and unfasted Sprague-Dawley rats were administered 3 mL of a radiolabelled hypochlorus acid solution orally. Heparinised blood samples were collected at several time points from 5 minutes to 72 hours after treatment (study termination) by orbital sinus puncture. Blood and tissue samples were collected and examined for radioactivity content at study termination. Another group of fasted rats was housed in modified Roth all-glass metabolism chambers for the collection of expired air and faecal and urine samples at 8 to 96 h.

The results from the metabolism study in rats with [³⁶Cl]-hypochlorus acid demonstrate, that the compound was absorbed after oral administration with a constant of 0.157 h-1 and excreted with an elimination constant of 0.009 h-1. The half-life obtained for absorption and excretion was 4.42 hours and 77. 0 hours, respectively. 76 % of the recovered dose was excreted in the urine at 72 hours (faeces 24 % and expired CO₂ not detectable). The metabolism study revealed the HO³⁶Cl is converted and eliminated in the chloride form. Hypochlorite was widely distributed in the body with highest values in plasma, followed by bone marrow, kidney, testes, lung, skin, duodenum, spleen, stomach, liver, carcass and ileum. The distribution scheme of ³⁶Cl, after HO³⁶Cl administration, revealed the highest ³⁶Cl activity in the plasma and whole blood, while the lowest activity was measured in the liver, ileum and adipose tissue. The plasma carried four times the activity of ³⁶Cl than packed cells. The decrease of total ³⁶Cl after washing packed cells with cold saline suggested that a high percentage of total ³⁶Cl was loosely bound to the erythrocyte membrane or exchangeable with the chloride in saline. Approximately 20 % of the ³⁶Cl in the plasma was bound to protein and in the same time this concentration was higher (five-fold) than the amount which was bound to the liver protein.