A quantitative risk assessment of exposure to nitrates in drinking water and thyroid disorders in East Anglia, United Kingdom

Review of animal and epidemiological studies suggest that exposure to nitrates in drinking water is associated with thyroid disorders, including mild - to - moderate iodine deficiency; hyperthyroidism; hypothyroidism; thyroid hypertrophy (goitre) and thyroid cancer. However, the weight of evidence following a meta – analysis is strongest for goitre; weak for subclinical hypothyroidism and weakest for clinical hypothyroidism and hyperthyroidism (clinical and subclinical). The effect estimate for goitre is, OR = 3.13 (95%Cl: 2.35-4.16); I2 = 24.9%, p = 0.28. Although causality was not firmly established between nitrates in drinking water and goitre, the risk assessment framework was used to estimate lifetime excess risk of thyroid cancer in East Anglia given widespread nitrate contamination of drinking water sources in the region. Thyroid cancer was used as a proxy for goitre given that malignancy can result from goitre which is usually benign and there is no register for goitre and/or benign thyroid tumours in the UK. Risk estimates suggests that 20 cases or 13 per cent of the 154 thyroid cancer cases calculated in a population of 2,849,918 in East Anglia in 2014, can be attributable to nitrates in drinking water and this would have been eliminated from the population if there was no nitrates in drinking water. The lifetime excess risk of thyroid cancer at nitrate levels below and equal to the drinking water standard of 50mg/l, is 0.02 – 0.28. This is above the range (1x10-6 to 1x10-5) considered negligible and suggests that the current drinking water standard for nitrates, originally set to protect against infantile methaemoglobinemia is unlikely to protect against thyroid cancer and warrants a review. The review should include a consideration of lowering the drinking water standard; reduction of nitrates in drinking water sources and/-or introducing iodine prophylaxis in the UK given that the effect of nitrates on the thyroid gland is dependent on the amount of dietary iodine intake. Although there were a lot of uncertainties and assumptions in the risk assessment process, the recommendation is based on the precautionary principle.