The release of radioactive material into the environment either from an accidental release from a nuclear installation such as a power plant, or from a nuclear explosion. The Windscale nuclear accident in 1957, Chernobyl in 1986, and Fukushima Daiichi in 2011 led to wide-scale contamination of the environment. The most harmful isotopes to human health include iodine-131 and strontium-90. Both isotopes are taken up by animals such as grazing cows and transferred to humans through the consumption of milk. Iodine-131 accumulates in the thyroid gland while strontium-90 accumulates in the bones.