(NOTICE:  The project that is the subject of this report was approved by the Governing Board of the National Research Council, whose members are drawn from the councils of the National Academy of Sciences, the National Academy of Engineering, and the Insti- tute of Medicine.  The members of the committee responsible for the report were chosen for their special competences and with regard for appropriate balance.
This project was supported by Contract No. DAMD17-99-C-9049 between the National Academy of Sciences and the U.S. Department of Defense and Contract No. 68-C-03-081 between the National Academy of Sciences and the U.S. Environmental Protection Agency.   Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the or- ganizations or agencies that provided support for this project.
International Standard Book Number-13: 978-0-309-10358-9
International Standard Book Number-10: 0-309-10358-4)

1. Introduce.
Chlorine dioxide (ClO2) is a yellow to reddish-yellow gas at room temperature. It has an unpleasant odor, similar to the odor of chlorine and reminiscent of nitric acid. It is very reactive and a strong oxidizing agent. Pure chlorine dioxide is stable in the dark and unstable in light (Budavari et al. 1996). 

Table 1.1: AEGL summary of values for chlorine dioxide (ppm [mg / m3])


10 min

30 min 1 h  4 h 8 h

End Point (Reference)













Slight salivation, slight lacrimation,

and slight chromodacryorrhea

in rats exposed to 3 ppm for 6 h (DuPont 1955)














salivation,, dyspnea, weakness, andpallor in rats exposed to 12 ppm for 6 h (DuPont 1955)













No lethality in rats exposed to 26 ppm for 6 h (DuPont 1955)

Inhaled (airborne) chlorine dioxide acts primarily as a respi- ratory tract and ocular irritant. In air chlorine dioxide gas readily decom- poses both thermally and photochemically. Thermal decomposition is characterized by a slow induction period followed by a rapid autocata- lytic phase that may be explosive if the initial concentration is above a partial pressure of 76 mm Hg. Unstable chlorine oxide may be formed as an intermediate, and the presence of water vapor is hypothesized to ex- tend the duration of the induction period by reacting with the chlorine oxide intermediate. When water vapor concentrations are high, explosiv- ity is minimized and all decomposition occurs in the induction phase; the water vapor inhibits the autocatalytic phase. The products of thermal de- composition of gaseous chlorine dioxide include chlorine, oxygen, hy- drogen chloride, HClO3, and HClO4. The proportions of products formed depend on the ambient temperature and concentration of water vapor (Kaczur and Cawfield 1993). Photochemical decomposition of gaseous chlorine dioxide initially  involves homolytic scission of  the  chlorine oxygen bond to form ClO and O. These products then generate secon- dary products including chlorine peroxide, chlorine, oxygen, and chlo-rine trioxide (Griese et al. 1992; Kaczur and Cawfield 1993). The chlo- rite ion does not persist in the atmosphere either in ionic form or as chlo- rite salt and is not likely to be inhaled.

In aqueous media, chlorine dioxide is relatively unstable and disso- ciates in water into chlorite and chloride, and to a lesser extent into chlo- rate (Budavari et al. 1994. Chlorine dioxide is prepared from chlorine and sodium chlorite or potassium chlorate and sulfuric acid (Budavari et al. 1996). Chlorine dioxide is always made at the place where it is used because of the risk of rapid decomposition. The production volume of chlorine dioxide was estimated from the total sodium chlorate consump- tion for chemical pulp bleaching, as this use accounts for > 95% of all chlorine dioxide production. The annual production of chlorine dioxide in the United States was estimated to be 79, 81, 146, 226, and 361 kilo- tons for the years 1970, 1975, 1980, 1985, and 1990, respectively (ATSDR 2002). As stated above, the major use of chlorine dioxide is for chemical pulp bleaching. Other uses include drinking water disinfection and the  bleaching of  textiles, flour, cellulose, leather, fats, oils, and beeswax; taste and odor control of water; as an oxidizing agent; and in the manufacture of chlorite salts (ACGIH 2001). In 2001, chlorine diox- ide was used to decontaminate public buildings in the United States after the release of anthrax spores (ATSDR 2002). Chemical and physical properties are listed in Table 1.1.

2. Human toxicity data. 

  •  Nonlethal Toxicity.
    Elkins (1959) reported that 5 ppm chlorine dioxide was “definitely” irritating to humans. No other details were reported. Three odor thresholds have been reported for chlorine dioxide: 0.1 ppm (Ellenhorn and Barceloux 1988), 9.4 ppm (Amoore and Hautala 1983), and 15 ppm (Vincent et al. 1946). However, there are no reliable data to support these values.

Table 1.2. Chemical and Physical Data.

Parameter Value Reference
Synonyms Chlorine peroxide IPCS, 1993
  Chlorine oxide; Chlorine  
  (IV) oxide  
Molecular formula ClO2 Budavari et al. 1996
Molecular weight 67.45 Budavari et al. 1996
CAS Registry Number 10049-04-4 ACGIH, 2001
Physical state Gas Budavari et al. 1996
Color Yellow to reddish-yellow Budavari et al. 1996
  bluish-white liquid  
Solubility in water 3.01 g/l at 25°C and 34 Budavari et al. 1996
  mmHg (decompose)  
Vapor pressure 760 torr at 20°C ACGIH, 2001
Vapor density (air = 1) 2.3 ACGIH, 2001
Specific gravity 1.642 at 0°C (liquid) ACGIH, 2001
Melting point −59°C ACGIH, 2001
Boiling point 11°C ACGIH, 2001

Bronchitis and emphysema were reported in a 53-year-old chemist repeatedly exposed to low concentrations of chlorine dioxide over a pe- riod of several years and to higher concentrations in conjunction with three explosions (Petry 1954). Dyspnea of increasing severity and asth- matic bronchitis were reported apparently after cessation of the expo- sures. No exposure concentration was reported.

A 49-year-old woman was exposed to an unknown concentration of chlorine dioxide accidentally generated while bleaching dried flowers (Exner-Friesfeld et al. 1986). She initially noticed a sharp, pungent smell and experienced coughing, pharyngeal irritation, and headache. Seven hours after exposure, she was hospitalized due to a worsening cough and dyspnea. Clinical findings included tachypnea, tachycardia, and rales on auscultation. Clinical chemistry revealed marked leukocytosis. The chest x-ray was normal. The vital capacity and forced expiratory volume in 1 sec were decreased, to 73% and 70% of normal, respectively, and airway resistance was correspondingly increased. Blood gas  examination re- vealed hypoxia despite alveolar hyperventilation. Symptoms resolved with corticosteroid treatment, and a follow-up examination two years post-exposure showed normal pulmonary function.
In another case report, Meggs et al. (1995; 1996) evaluated 13 adults (12 females and 1 male) 5 years after an occupational exposure to chlorine dioxide associated with a leak in a water purification system pipe. No exposure concentration or duration data were presented. Ob- served long-term effects included sensitivity to respiratory irritants (13 people), disability accompanied by  loss  of  employment (11  people), chronic fatigue (11 people), and nasal abnormalities, including talangec- tasia, paleness, edema, and thick mucus (13 people). Nasal biopsies from the exposed workers showed chronic inflammation with lympho- cytes and plasma cells in 11 of the 13 people. This inflammation was described as mild in two persons, moderate in eight persons, and severe in one person. Nasal biopsies of three control subjects showed mild in- flammation in one subject. The number of nerve fibers in biopsies from the exposed workers was greater than in biopsies from the control group.
Golemme and Lundgren (1957) studied 12 male employees who reported symptoms after they began work with chlorine dioxide at a sul- fite-cellulose production factory. Spot samples of chlorine and chlorine dioxide during normal operations were generally <0.1 ppm. Occasional leaks from faulty vacuum lines would result in “high” levels of chlorine, chlorine dioxide, and/or sulfur dioxide. Chronic bronchitis was diag- nosed in  7  of  the  12  workers. The  workers reported breathlessness, wheezing, irritant cough, and ocular discomfort associated with the leak- ages.
Ferris et al. (1967) examined 147 men employed at a pulp mill; the length of employment was not reported. The workers were exposed to sulfur dioxide or chlorine and chlorine dioxide, with average chlorine dioxide concentrations ranging from 0 to 0.25 ppm and average chlorine concentrations ranging from 0 to 7.4 ppm. (Peak chlorine dioxide con- centrations reached 2 ppm, and peak chlorine concentrations reached 64 ppm.) Shortness of breath, excess phlegm, and bronchitis were noted in the workers, with workers exposed to chlorine or chlorine dioxide exhib- iting more severe symptoms than those exposed only to sulfur dioxide.
Kennedy et al. (1991) compared health effects in 321 pulp mill workers exposed to chlorine dioxide and chlorine with 237 control work- ers at a rail yard. Personal time weighted average concentrations at the pulp mill were 5 to 14 ppm chlorine and <0.1 ppm chlorine dioxide. No air monitoring data from the rail yard were provided. Additionally, chlo- rine or chlorine dioxide “gassing” exposures from accidental releases were reported by 60% of the pulp mill workers. There were increased incidences of wheezing and breathlessness reported by pulp mill workers compared to the rail yard workers; however, pulmonary function tests did not reveal any significant differences between the pulp mill workers and the rail yard controls. Airflow obstruction, as measured by FEV1, was increased (p < 0.05) in the pulp mill workers experiencing “gassing” incidents compared with those not experiencing “gassing” incidents.

Developmental/Reproductive Effects.

No data concerning developmental or reproductive effects of chlo- rine dioxide inhalation in humans were identified in the available litera- ture; however, epidemiological studies of populations consuming chlo- rine dioxide- treated drinking water were located. A retrospective study was conducted using 1940s birth records from Chicopee, Massachusetts; this community utilized “relatively high” levels of chlorine dioxide for water disinfection (Tuthill et al. 1982). The morbidity and mortality ex- perience of infants born in Chicopee was compared to that of infants born in Holyoke, Massachusetts, a geographically contiguous community that utilized traditional chlorination practices. There was no difference in fetal, neonatal, or infant mortality; or in birthweight, sex ratio or birth condition between infants born in the two communities. There was an apparent increase (p < 0.05) in the number of infants judged as premature by physician assessment in the chlorine-dioxide-exposed population (7.8%) compared with the control community (5.8%). However, there was no increase in prematurity when data were evaluated controlling for the age of the mother.
In another study, Kanitz et al. (1996) conducted an epidemiological study comparing 548 infants born to mothers (Genoa, Italy) who had consumed water disinfected with chlorine dioxide (<0.3 mg/mL) and/or sodium hypochlorite with 128 infants born to mothers (Chiavari, Italy) who had consumed primarily untreated well water. There was a higher frequency of infants with small (≤49.5 cm) body length in mothers ex- posed to chlorinated water (chlorine dioxide adjusted odds ratio [OR] = 2.0 [95% CI = 1.2-3.3]; sodium hypochlorite OR = 2.3 [95% CI = 1.3-4.2]) compared with those exposed to well water. There was also a higher frequency of infants with small (≤35 cm) cranial circumference in mothers exposed to chlorinated water (chlorine dioxide adjusted OR =2.2 [95% CI = 1.4-3.9]; sodium hypochlorite OR = 3.5 [95% CI = 2.1-8.5]) compared with those exposed to well water. There was also an ap- proximate doubling of cases of neonatal jaundice in infants of mothers who consumed the disinfected water. The conclusions that can be drawn from this study are confounded by lack of quantitative exposure informa- tion, possible exposure to other chemicals in the water, and lack of con- sideration of nutritional and smoking habits and maternal age distribution between the two populations.

  • Genotoxicity.

No data concerning the genotoxicity of chlorine dioxide in humans were identified in the available literature.

  • Carcinogenicity

No data concerning the carcinogenicity of chlorine dioxide in hu- mans were identified in the available literature.

  • Summary

Deaths from chlorine dioxide exposure have occurred but exposure concentrations are unknown. Exposures that failed to result in mortality suggest that chlorine dioxide is a respiratory irritant causing wheezing, cough, dyspnea, decreased pulmonary function, and  nasal pathology. Specific exposure levels and/or durations for specific symptoms were not available and were confounded by concurrent exposures to other chemi- cals. Information on developmental/ reproductive effects was available only for the oral route of exposure from disinfected drinking water and these studies contain many confounding variables, making it impossible to definitively attribute the effects to chlorine dioxide. No genotoxicity or carcinogenicity data were located.

3. Animal toxicity data.

  – Nonlethal Toxicity.

  • Rats

DuPont  (1955)  conducted a  series  of  repeated-exposure experi- ments in male Sprague-Dawley rats. Chlorine dioxide was generated by adding a sodium chlorite solution dropwise at  a constant rate into a heated flask containing 85% phosphoric acid. Metered air was passed through the flask and then into a bell jar containing 2 or 4 rats. The chlo- rine  dioxide concentration was  determined analytically  at  least  three times during each exposure period. A group of four rats was exposed to12 ppm chlorine dioxide, 6 h/day for 6 or 7 days. Clinical signs observed on the first day of the study included lacrimation, salivation, dyspnea, weakness, and pallor. These signs increased in severity with repeated exposures. All of the rats survived through the sixth exposure. Two of the rats died after the sixth exposure, and two were sacrificed for pathol- ogy after the seventh exposure. Necropsy revealed acute bronchitis and emphysema, but no evidence of pulmonary edema, in all four rats.
DuPont (1955) also similarly exposed a group of four rats to 3 ppm chlorine dioxide, 6 h/day for 10 days. Clinical signs observed on the first day of the study included slight salivation, slight lacrimation, and slight chromodacryorrhea. These signs increased in severity with repeated ex- posures. No animals died, and no gross or microscopic pathology was observed when rats were sacrificed immediately after the tenth exposure.
As mentioned in Section 3.1.1, Dalhamn (1957) conducted a series of four experiments to examine both lethal and nonlethal effects of chlo- rine dioxide inhalation in an unspecified sex and strain of rats. In one experiment, three rats were exposed once a week for 3 min to decreasing concentrations of chlorine dioxide; the animals were exposed to 3435 ppm chlorine dioxide on day 1, to 1,118 ppm on day 8, and to 760 ppm on day 16. A group of three rats was exposed to compressed air and served as controls. Respiratory distress was observed and mean body weight of the exposed rats was 10% below that of controls on day 16. Necropsy revealed bronchopneumonia and hyperemia of the renal corti- comedullary junction in two of the exposed rats; however, the lungs and kidneys of the third exposed rat were normal. The lungs were normal in all three control rats; however, renal hyperemia was noted in two.
In another experiment, Dalhamn (1957) exposed groups of five rats to 0 or 0.1 ppm chlorine dioxide 5 h/day for 10 weeks. No clinical signs were observed during treatment and no treatment-related effects were noted at necropsy.
Paulet and Desbrousses performed a series of repeated-exposure studies in rats. Unfortunately, for the purposes of AEGL derivation, data after single exposures were not reported. Groups of five male and five female rats were exposed to 10 ppm chlorine dioxide, 2 h/day for 30 days. Groups of 10 male and 10 female rats were exposed to 5 ppm, 2 h/day for 30 days, and groups of 10 male and 10 female rats were ex- posed to 2.5 ppm, 7 h/day for 30 days (Paulet and Desbrousses (1970). The strain of rat was not specified; control groups with equal numbers of rats were used for each exposure scenario. Nasal discharge, red eyes, and bronchopneumonia accompanied by desquamation of alveolar epithelium were observed at 5 and 10 ppm, with effects being more severe at 10 ppm. Increased erythrocyte and leukoctye counts were noted in animals exposed to 10 ppm chlorine dioxide. Rats exposed to 2.5 ppm exhibited lymphocytic infiltration of the alveolar spaces, alveolar vascular conges- tion, hemorrhagic alveoli, epithelial erosions, and inflammatory infiltra- tions of the bronchi. No effects were reported in control animals. In an- other report, Paulet and Desbrousses (1972) exposed a group of eight Wistar rats (sex not reported) to 1 ppm chlorine dioxide, 5 h/day, 5 days/week for 2 months. Vascular congestion and peribronchiolar edema were observed at necropsy.

In another study, Paulet and Desbrousses (1974) exposed groups of 10-15 rats (sex and strain not reported) to 0, 5, 10, or 15 ppm chlorine dioxide, 15 min, 2 or 4 times/day for 1 month. At 15 ppm, mortality was observed in 1/10 rats exposed 2 times/day and in 1/15 rats exposed 4 times/day. Decreased body weight, nasal and ocular inflammation and discharge, bronchitis, and peribronchiolar lesions were observed at 15 ppm; effects were more severe in animals exposed 4 times/day. Alveolar irritation and decreased body weight were observed at 10 ppm, and no effects were reported at 5 ppm.

  • Rabbits.

A group of four rabbits was exposed to 5 ppm chlorine dioxide, 2 h/day for 30 days, and a group of eight rabbits was exposed to 2.5 ppm, 4 h/day for 45 days (Paulet and Desbrousses 1970). The strain and sex were not specified; control groups with equal numbers of rabbits were used for each exposure scenario. Nasal discharge, red eyes, and broncho- pneumonia accompanied by desquamation of alveolar epithelium were observed at 5 ppm. Rabbits exposed to 2.5 ppm exhibited hemorrhagic alveoli and congested capillaries in the lungs at study termination. Pul-monary effects had resolved in animals sacrificed 15 days after exposure termination (2.5 ppm group).

  • Developmental/Reproductive Effects.

No  information  regarding  developmental/reproductive effects  of chlorine dioxide in animals via the inhalation route was located in the available literature. However, several oral studies were available. No developmental or reproductive effects were noted in Long-Evans rats administered daily gavage doses of 0, 2.5, 5, or 10 mg/kg chlorine diox- ide in water (Carlton et al. 1991). Groups of 12 males were treated for 56 days  prior  to  mating  and  throughout the  10-day  mating  period,  and groups of 24 females were treated 14 days prior to mating, during the mating period and during gestation and lactation.
In another gavage study, Toth et al. (1990) administered daily doses of 0 or 14 mg/kg chlorine dioxide to four male and four female Long- Evans rat pups on postnatal days 1-20. Body weight of treated rats was decreased on days 11, 21, and 35, and forebrain weight was decreased on days 21 and 35. Decreased protein content and DNA content of the brain were also observed on days 21 and 35.
Taylor and Pfohl (1985) administered 0 or 100 ppm chlorine diox- ide to female Sprague-Dawley rats in the drinking water 14 days prior to gestation and throughout gestation and lactation. Decreased brain weight, due mainly to a decrease in cerebellar weight, was observed in 21-day- old pups from treated dams. A decrease in total cerebellar DNA content was also noted in these pups. A decrease in exploratory behavior was observed in the 60-day-old pups of treated dams.
Taylor and Pfohl (1985) also administered 0 or 14 mg/kg chlorine dioxide to male Sprague-Dawley rat pups from untreated dams via ga- vage on postnatal days 5 to 20. Decreased body weight, absolute and relative whole brain and forebrain weights, and forebrain DNA content were observed in 21-day-old treated pups. Decreased home cage activity was observed on days 18-19, and wheel-running activity was decreased on day 10. No other effects were reported.
In another study, groups of six to eight female Sprague-Dawley rats were administered 0, 1, 10, or 100 ppm chlorine dioxide in the drinking water for 2.5 months prior to mating and during gestation days 0-20 (Suh, et al. 1983). There was a trend for decreasing number of implants per litter and number of live fetuses per dam. Total fetal weight and male fetal weight were increased at 100 ppm.
In another drinking water study, groups of 12 female Sprague- Dawley rats were administered 0 or 100 ppm chlorine dioxide for 10 days prior to mating and during the gestation and lactation periods (Mobley et al. 1990). The litter weight of treated animals was lower than controls at birth. Also, chlorine dioxide treated pups exhibited decreased exploratory activity on postconception days 36-38 but not on days 39-40.

  • Genotoxicity.

No information regarding the genotoxicity of chlorine dioxide in animals via inhalation was located in the available literature. Chlorine dioxide was positive in an in vivo micronucleus assay in mice after an i.p. injection of 3.2-25 mg/kg chlorine dioxide (Hayashi et al. 1988). Meier et al. (1985) administered 0.1 to 0.4 mg chlorine dioxide by ga- vage to Swiss CD-1 mice for 5 consecutive days; there was no evidence of increased incidences of micronuclei or bone marrow chromosomal aberrations and no effect on sperm head morphology. In an in vitro study, chlorine dioxide was negative for chromosome aberrations in Chi- nese hamster fibroblast cells (Ishidate et al. 1984). It was negative in the Salmonella typhimurium reverse mutation assay without activation and positive with activation (Ishidate et al. 1984); however, water samples disinfected with chlorine dioxide were negative both with and without activation (Miller et al. 1986).

  • Chronic Toxicity/Carcinogenicity.

No information regarding the carcinogenicity of chlorine dioxide in animals via the inhalation route was located in the available literature. In a dermal exposure study, the dorsal area of groups of five female SENCAR mice were shaved and the mice were placed in chambers con- taining 0, 1, 10, 100, 300, or 1000 ppm chlorine dioxide dissolved in wa- ter, 10 min/day for 4 days (Robinson et al. 1986). The chambers were designed to prevent inhalation of vapors. An increase in epidermal thick- ness, suggesting epidermal hyperplasia, was noted at 300 and 1000 ppm.
Miller et al. (1986) tested the carcinogenic potential of concentrates prepared from chlorine dioxide disinfected drinking water in  several short-term assays. The concentrates did not increase the incidence of lung adenomas in strain A mice, skin tumor frequency in SENCAR mice, or gammaglutamyl transpeptidase positive foci in rat livers.

  • Summary

Lethality data are very limited; no LC50 values were available. Re- ports of lethality were available for rats, mice, guinea pigs and rabbits; however, experimental details were generally poorly reported. Pulmo- nary congestion and edema were noted at necropsy in some animals after lethal exposure to chlorine dioxide. Sublethal studies were also limited and most used repeat exposure protocols; however, limited data were available describing clinical signs observed after the first exposure. Chlo- rine dioxide is an irritant as evidenced by lacrimation, salivation, dysp- nea, weakness, and pallor, and by difficulty breathing, nasal discharge, ocular irritation and pneumonia observed in rats during or after exposure to sublethal concentrations of chlorine dioxide. Developmental delays were observed in animals following ingestion of chlorine dioxide in wa- ter. Genotoxicity studies with chlorine dioxide yielded both positive and negative results and no long-term carcinogenicity studies were available.


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