The Drinking Water Inspectorate (DWI) has published the results of a research study to assess levels of NDMA (N-nitrosodimethylamine) in drinking water in England and Wales. The DWI commissioned the study on behalf of Defra following emerging evidence from North America that very low levels of NDMA (N-nitrosodimethylamine) may occur in drinking water as a disinfection by-product.

According to the DWI, overall the research findings are reassuring. At over 90% of the treatment works, samples of final water were free from detectable concentrations of NDMA (limit of detection 0.9ng/l). At three treatment works, trace levels of NDMA were found in the final water, all final water concentrations were below <10 ng/l, as compared to the proposed WHO guideline value of 100ng/l .

Early in 2008, DWI commissioned from the Health Protection Agency (HPA) a comprehensive toxicological risk assessment for NDMA in drinking water. The key conclusions of this assessment were that NDMA is a potent animal carcinogen by several routes of exposure and genotoxic both in vitro and in vivo. The HPA advised that there is no identifiable threshold for adverse effects therefore exposure should be reduced to as low as reasonably practicable. However the advice also recognised that drinking water is not a major route of exposure and was reassuring in that it did not recommend immediate public health action in respect of the very low concentrations of NDMA ( in final water) reported in the above study.

The DWI notified the manufacturer of the products which appear to contain NDMA of the findings of the research and encouraged it to take whatever steps are necessary to ensure that NDMA is not present in the products. According to the DWI, the manufacturer has now completed an investigation of NDMA levels in its products which broadly confirmed the Inspectorate's findings. It found NDMA arose in its production process as a result of the use of a particular additive, which it has now eliminated from its process. Following the completion of a successful works scale operations trial without using the additive which demonstrated a permanent reduction in the levels of NDMA in its products, the company has now implemented these changes permanently.

Both the literature element and the testing phase of the research project identified a number of possible risk factors for the presence of NDMA in drinking water generally (including the first evidence that ferric coagulants may contain NDMA). This new finding is now the subject of a follow up research project commissioned by DWI to establish whether water treatment coagulants are a widespread source of NDMA. In light of the  findings and health advice, the DWI has also published guidance to the water industry on NDMA.