DrsNet expert panel response regarding the potential link between chlorine and bowel cancer risk : Group 3 'Not classifiable as to its carcinogenicity to humans'.

Question sent to Cancer Research UK and the expert panel + Response. questionsfromtheaudience@doctors.org.uk Question : Could a simple health message reduce the incidence of bowel cancer ? Until chlorine in tap water is proven NOT to increase the risk of bowel cancer, should the government advise the public to either boil, or leave their tap water to stand overnight, to allow evaporation of chlorine before consumption. At least one study has suggested a link between consumption of chlorine in tap water and bowel cancer : http://www.ncbi.nlm.nih.gov/pubmed/10952098 Cancer Epidemiol Biomarkers Prev. 2000 Aug;9(8):813-8. Case-control study of colon and rectal cancers and chlorination by-products in treated water. King WD, Marrett LD, Woolcott CG. 'Males exposed to an estimated THM level of 75 microg/liter for > or = 35 years had double the risk of those exposed for < 10 years (odds ratio, 2.10; 95% confidence interval, 1.21-3.66)' Chlorinated water and bowel cancer are both largely western phenomena providing an interesting epidemiological association. From a logical point of view, chlorine kills bacteria very effectively, and probably many mucosal cells in the gut as it comes out of solution as tap water warms up inside our gut. These dead mucosal cells need to be replaced by other dividing cells. I've always felt that anything that increases mitotic rates increases the risk of cancer. It would be interesting if chlorinated tap water was a major aggravating factor in inflammatory bowel disease, a condition already known to be associated with bowel cancer. Of course should a link be proven, then alternatives such as ozonation of tap water might be a more permanent solution and reduce any risk from inhalation of chlorine from showers and swimming pools as well . It could be a significant aggravating factor in asthmatics, and may increase the risk of lung cancer via the mechanism described above. RESPONSE Professor Sir Mike Richards, 23/03/2012 12:16 This is an interesting hypothesis – and like so many hypotheses it is difficult to prove or disprove. The International Association for Research on Cancer (IARC) classifies agents into one of the following categories: Group 1 - carcinogenic to humans (107 agents) Group 2A - Probably carcinogenic to humans (61) Group 2B - possibly carcinogenic to humans (269) Group 3 - Not classifiable as to its carcinogenicity to humans (508) Group 4 - Probably not carcinogenic to humans (1) Chlorinated drinking water is classified as Group 3. I am also very grateful to you for drawing my attention to the case control study from Ontario Canada undertaken between 1992 and 1994. I note that although higher exposure to Chlorinated water appeared to increase the risk of colon cancer in males, the same effect was not observed in females. No relationship was observed for rectal cancer. The authors themselves state that the results of the study should be interpreted with caution because they are only partially congruent with the limited amount of literature addressing this issue. Based on these findings I think it is very unlikely that any government would wish to give advice such as you have suggested, which at the very least would cause inconvenience. There are several other approaches to reducing the risk of colorectal cancer (e.g. related to lifestyle factors) for which there is strong evidence and for which the government does provide advice to the public.