Natures Aid® Selenium provides 200mcg Selenium in combination with vitamins A, C & E plus Zinc. Selenium contributes to: Normal spermatogenesis, Maintenance of normal hair and nails, Normal function of the immune system and Normal thyroid function. Selenium also protects cells from oxidative damage. Oxidative stress occurs when there are high levels of free radicals in the body. Free radicals are caused by frying foods, alcohol, tobacco smoke, pesticides and air pollutants. Free Radicals can cause damage to cells, through a process called oxidation. Some nutrients, including Selenium can help to protect the cells against that damage.