The classic adage of toxicology is that “the dose makes the poison” – a phrase that captures the idea that everything has a threshold between safe and unsafe.
Selenium is a nutritionally essential trace element that occurs naturally in the soil, water and air, and is commonly consumed in a diet including foods like fish and nuts.
Selenium can have beneficial or toxic effects on human health, depending on the dose. People who often eat fish or aquatic plants that live in waterways with elevated levels of selenium are exposed to the health risks of chronic selenium intoxication, because fish and plants can accumulate high levels of selenium from the environment.
A summary of the report’s information about the nutritional value of selenium and the thresholds for toxicity are available in two infographics. One called “Selenosis: Signs, Symptoms and Causes of Toxic Selenium Exposure" is to guide health practitioners in identifying selenosis. Another, “Selenium: Too Much of a Good Thing," provides information about selenium intake levels and current limits.
Unlike other elements and chemicals, there is a miniscule difference between the recommended dietary allowance of selenium per day that is healthy (55 micrograms a day) and that which can be unhealthy (more than 400 micrograms a day). For reference, one microgram is 0.001 milligrams. Chronic selenium intoxication, called selenosis, can cause fatigue, damage to nails and hair, and some neurotoxic effects.
The amount of selenium in a person’s diet can depend on the level of selenium in the environment surrounding the food source. Selenium can be introduced to soil, water or air as a byproduct of various industrial uses like mining coal and metals.
The health risk of consuming selenium above recommended limits comes from long-term overexposure. Therefore, there are lower limits on the recommended dietary allowance of selenium per day for subsistence fishers because they eat more fish more often than recreational fishers or other populations. In comparison, drinking water supplies a fraction of the total allowable selenium intake, so there are less stringent guidelines for selenium in drinking water.
Allison Voglesong Zejnati is public affairs specialist at the IJC’s Great Lakes Regional Office in Windsor, Ontario.