Selenium
Cities measured
87
Detected in
87 / 87
EU limit
20 μg/L
Highest
2.8 μg/L — Jakarta
Overview
Selenium is an essential trace element required for glutathione peroxidase (an antioxidant enzyme) and thyroid hormone metabolism. It has a narrow margin between deficiency and toxicity — the difference between a deficient and a toxic intake is approximately 10-fold.
Health Relevance
Selenium deficiency causes Keshan disease (cardiomyopathy) and Kashin-Beck disease (joint disease), documented in selenium-poor regions of China. Selenium toxicity (selenosis) causes hair and nail loss and neurological damage.
Regulatory Limits
Drinking Water Directive 2020/2184
20 μg/L (WHO: 40 μg/L).
Controversy & Contested Science
Early observational studies suggested selenium supplementation reduced cancer risk, driving widespread selenium supplement use. The SELECT trial (Selenium and Vitamin E Cancer Prevention Trial, ~35,000 men) found no protective effect and some evidence of elevated diabetes risk at supplementation doses in selenium-adequate populations. This exemplifies the 'more is not better' problem in micronutrient research — beneficial at deficiency levels, harmful at excess, with a narrow optimal window.