One of the key roles that Selenium plays is in protecting cells from oxidative stress, which occurs when there are high levels of free radicals in your body. Your body comes into contact with free radicals every day, their levels can be increased by a wide variety of common activities such as frying foods, drinking alcohol and smoking tobacco, even pesticides and air pollutants cause free radicals to be released into the air you breath. Free radicals can cause damage to your cells, through a process called oxidation. What?s more, Selenium also contributes to normal sperm production (also known as spermatogenesis), the maintenance of normal hair and nails, the normal function of the immune system and even the normal function of the thyroid gland.