Selenium sulfide has not caused cancer in animals when it is placed on the skin, and the use of anti-dandruff shampoos containing selenium sulfide is considered safe. The EPA believes that other selenium compounds are not classifiable with regard to their carcinogenicity. Studies of laboratory animals and people show that most selenium compounds probably do not cause cancer. Selenium can be measured in blood, urine, and nails of exposed individuals. However, since selenium is essential and normally present in foods, low levels are always found in body tissues and urine.
Tests are most useful for people who have recently been exposed to high levels.
Urine is used to determine short-term exposure. Nail clippings can be used to determine longer-term exposure. These tests aren't available at most doctors' offices, but can be done at special laboratories that have the right equipment. Animal testing is sometimes necessary to find out how toxic substances might harm people and how to treat people who have been exposed. Laws today protect the welfare of research animals and scientists must follow strict guidelines.