In most applications the source needs to be replaced when the activity has decreased by a factor of two or slightly more. Industrial radiography sources containing 192Ir, which has a half-life of only 74 days, usually become spent because of decay two or three months after purchase, while 60Co sources used for either industrial radiography or teletherapy need to be replaced every 5–10 years. Although the sources may not be useful, they may be still highly radioactive, posing a potential danger to people and the environment even after several half-lives have elapsed.