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Carcinogenicity

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Animal data: taking together the results from the three studies on sodium and potassium metabisulphite (Tanaka et al., 1979; Til et al., 1972; Feron and Wensfoort, 1972) there was no indication that metabisulphite had any carcinogenic effect. The drinking water study by Tanaka et al. (1979) on potassium metabisulphite in mice is considered as most suitable for the assessment of carcinogenicity (read-across info, see `discussion`). 
Human data: 4 reliable studies on pulp and paper mill workers were available ((Milham and Demers, 1984; Robinson, et al. 1986; Anderson, et al. 1998; Rix, et al. 1997), see section 7.10.2., based upon which no crcinogenic activity must be expected for the sulfites.

Key value for chemical safety assessment

Additional information

Read-across concept for sulfites, hydrogensulfites, metabisulfites, dithionites and thiosulfates:

A comprehensive read-across concept has been developed for sulfites, hydrogensulfites and metabisulfites, based on the pH-dependant equilibrium in aqueous solutions which is summarised in the following equations:[1],[2]

           SO2+ H2O <->`H2SO3´         H2SO3<->H++ HSO3-<->2H++SO32-    2HSO3-<->H2O +S2O52-

Since the nature of the cation (i.e., sodium, potassium, ammonium…) is not assumed to contribute substantially to differences in toxicity and solubility (all compounds are very soluble in water), only the chemical and biological properties of the anion are considered as relevant determinants. Based on the described equilibrium correlations, unrestricted read-across between the groups of sulfites, hydrogensulfites and metabisulfites is considered justified.

 

Additionally, it is known that sodium dithionite disproportionates in water to form sodium hydrogen sulfite and sodium thiosulfate (equation II)2,[1], so that this substance can also be considered to be covered by the read-across concept described above. Since it can easily be anticipated that the substance is not stable enough under physiological conditions to fulfil the requirements of study guidelines, instead the products of decomposition have to be considered:

 

       2 S2O42-+ H2O→2HSO3-+ S2O32-

 

Not fully covered by this read-across concept is the substance class of thiosulfates: although the thiosulfates are also well known to disproportionate in aqueous solution to form polythionic acids and SO2(HSO3-), this requires somewhat different, more acidic conditions. Therefore, read-across to sulfites is primarily restricted to appropriate physiological conditions, i.e. oral administration where the gastric passage with the strongly acidic conditions in the stomach will facilitate the chemical disproportionation described above:

 

       HS2O3-+ H2S2O3HS3O3- + SO2+ H2O

 

[1]Hollemann Wiberg, Lehrbuch der Anorganischen Chemie, 101.Auflage

[2]Handbook of Chemistry and Physics, Ed. Lide, DR, 88thedition, CRC Press

Oral route:

Animal data:

Taken together the results from the three studies on sodium and potassium metabisulphite (Tanaka et al., 1979; Til et al., 1972; Feron and Wensfoort, 1972), there was no evidence that metabisulphite had any carcinogenic effect. The drinking water study by Tanaka et al. (1979) on potassium metabisulphite in mice is considered as most suitable for the assessment of carcinogenicity, because the size of the experimental groups (50 animals of each sex) corresponded to that recommended in the OECD Guidelines for Carcinogenicity studies (451, 453).The highest concentration of 2% potassium metabisulphite, corresponding to an estimated dose of 2500 mg/kg bw/d K2S2O5, did not indicate any carcinogenic potential. However, there is some indication for a possible tumour-promoting potential of metabisulphite in glandular stomach carcinogenesis (Furihata et al., 1989; Takahashi et al., 1986).

 

Human data:

Only a few detailed studies of pulp and paper mill workers (Milham and Demers, 1984; Robinson, et al. 1986; Anderson, et al. 1998; Rix, et al. 1997) are available which analyzed whether the workers might be at an increased risk for several site-specific malignancies. However, analyses of exposure-response relationships were not possible, because no exposure levels were available in any of the studies.

The retrospective cohort study in Danish sulphite pulp mill workers had a 2-fold increased risk for stomach cancer and pancreatic cancer (Rix et al., 1997). Other cancers with elevated risks were leukaemia (SIR 1.84) and soft-tissue sarcomas (SIR 2.37). The increased risk for stomach cancer found in this study was in accordance with that of other studies from sulphite pulp mills. The stomach cancer risk was increased in a retrospective cohort mortality study among workers in American sulphite mills (Robinson et al., 1986). They found 11 cases out of 523 observed deaths (SMR 149) with an increasing risk by time since the first employment. When process-specific analyses were conducted, the risk of lymphosarcoma and reticulosarcoma was increased only for men who had worked in sulphate mills. A proportionate mortality study among pulp and paper workers in the United States and Canada indicated a statistically significant excess risk of stomach cancer (Milham and Demers, 1984). Higher proportionate mortality ratios (PMRs) for lymphosarcoma (statistically significant) and kidney, pancreatic and rectal cancers were associated with jobs in the sulphite process. Hodgkin´s disease deaths occurred primarily in sulphate (Kraft) process workers.

 

IARC:

The International Agency for Research on Cancer (IARC, 1992) has evaluated the evidence for carcinogenicity and concluded: There is limited evidence for sulphur dioxide carcinogenicity in experimental animals. There is however inadequate evidence for sulphites, bisulphites and metabisulphites for carcinogenicity in experimental animals.

 

Inhalation and dermal route:

There are no reliable studies regarding cancer in experimental animals after inhalation or dermal exposure to any of the sulphite compounds under consideration. Taking into account the negative oral carcinogenicity data it can be predicted that chronic inhalation or dermal exposure to the various sulphite compounds would not result in remote site carcinogenicity. A certain uncertainty may exist with respect to the possibility of the formation of local tumours in the respiratory tract following long-term inhalation. However, SO2 inhalation for 21 weeks did not result in tumour formation in the lung as shown in a study on the potential co-carcinogenic role of SO2 in the induction of lung carcinoma by benzo(a)pyrene.

 

Summary:

The available data on long-term oral exposure of experimental animals to sodium and potassium metabisulphite allow an evaluation of the carcinogenic risks of sulphite compounds for humans exposed via the oral route. There was no indication that metabisulphite had any carcinogenic effect itself. However, there is evidence of a tumour-promoting potential in glandular stomach carcinogenesis in animals and stomach cancer in pulp mill and paper workers.

 

Taking into account the applicability of the read-across approach between the different sulphites, the carcinogenity assessment of the sulphites and hydrogensulphites (group 1), sodium dithionite (group 3), and thiosulphates (group 4) can be based on the negative findings of the above mentioned study on potassium metabisulphite (read-across group 2) in mice.

Justification for classification or non-classification

The available data on long-term oral exposure of experimental animals to sodium and potassium metabisulphite allow an evaluation of the carcinogenic risks of sulphite compounds for humans exposed via the oral route. There was no indication that metabisulphite had any carcinogenic effect itself.

Taking into account the applicability of the read-across approach for the different sulphites, the carcinogenity assessment of the sulphites and hydrogensulphites (group 1), sodium dithionite (group 3), and thiosulphates (group 4) can be based on the negative findings of the above mentioned study on potassium metabisulphite (read-across group 2) in mice. No classification of the substance as carcinogenic is required.