A new study has warned that regulatory limits set on a byproduct of a common disinfectant are insufficient to reduce the risk of colorectal and bladder cancer in the United States and the European Union (EU).
The study published in the Environmental Health Perspectives warned that chlorine — a common disinfectant used since the 1900s — reacts with natural organic matter to form a chemical compound called trihalomethane (THM), the most prevalent class of by-products in chlorinated drinking water. The four most common THMs (chloroform, bromoform, bromodichloromethane and chlorodibromomethane) may damage the DNA.
Their study highlighted that the THM likely increases the risk of bladder cancer above concentration of 41 microgrammes per litre (80μg / L).
The sum of all four THMs is currently regulated in drinking water to not exceed 80 μg / L in the US and 100 μg / L in the EU. In India, the acceptable limits are 200 μg / L for chloroform, 100 μg / L for bromoform, 100 μg / L for dibromochloromethane and 60μg / L for bromodichloromethane.
“Our findings suggest that these limits may fail to protect against cancer in the general population,” the researchers wrote in their study.
Previous studies have raised concerns that THMs may have carcinogenic properties. In 1974, scientists observed that THMs were formed during the disinfection step of drinking water if free chlorine was the disinfectant. In 1991, the World Health Organization (WHO) International Agency for Research on Cancer classified chloroform as possible human carcinogens.
This indicated that there is limited evidence of carcinogenicity in humans, sufficient evidence of carcinogenicity in experimental animals and strong evidence to show that the agent exhibits key characteristics of human carcinogens.
Studies suggested that chloroform induced renal and liver tumours in both rats and mice while bromoform and bromodichloromethane cause aberrant crypts and otherwise rare tumours of the large intestine in rats.
The researchers from Sweden conducted a review and meta-analysis of observational studies to summarise the evidence of exposure to THMs in drinking water with 14 different cancers.
They analysed 29 papers that assessed residential THM concentrations in relation to cancer of 14 different organs: Bladder cancer, colorectal cancer (including separate studies on colon or rectal cancer), pancreatic cancer, kidney cancer, female breast cancer, leukaemia, endometrial cancer, ovarian cancer, prostate cancer, lung cancer, cancer of the upper digestive tract, non-Hodgkin’s lymphoma and malignant melanoma.
Of the 29 studies, the researchers included 16 and analysed them for two outcomes: Bladder cancer and colorectal cancer, comprising 5,860 and 9,262 cases, and 84,371 and 90,272 participants.
The overall results showed a statistically significant 33 per cent and 15 per cent increased risk of bladder cancer and colorectal cancer, respectively, when comparing the highest with the lowest category of THM exposure.
Their analysis also showed that the association between THM and colon cancer was statistically significant at THM concentrations greater than 19μg / L up to 47μg / L. For rectal cancer, no statistical significance was reached. “For other cancer outcomes evaluated, there were insufficient data available to draw any firm conclusions,” the researchers wrote.
The researchers also observed sex differences in the association between THM and bladder cancer and colorectal cancer. The overall increased risk of bladder cancer and colorectal cancer, it found, was more pronounced in men.
However, the researchers acknowledged that the evidence presented in the study was limited-suggestive. This means that the proof is mainly based on findings from cross-sectional studies (a type of research study that involves observing a group of people or collecting specific information at a single point in time or over a short period of time).
For example, a survey may be done to collect information about the total number of people in a group who have or had a certain disease (such as cancer) or risk factor (such as THM, smoking or obesity). In this example, the survey can provide some information about whether there is an association between the THM (risk factor) and the cancer (disease) but does not prove that they are linked.
Limited-suggestive evidence also indicates that there are insufficient experimental studies or inconsistent results on the subject.
The researchers called for further research on disinfection by-products and cancer, particularly some cancers for which are less-studied such as female hormone-related cancers and malignant melanoma.