Sheep in Scotland, UK, contaminated with radioactive fallout from the 1986 Chernobyl reactor explosion, have been declared permissible to market for consumption. The news was a relief to farmers in the area. However, permissible does not mean safe. The European Commission in 1986 declared 1,000 bequerels of Cesium-137 per kilogram of sheep meat permissible for humans to consume. But as the U.S. National Academy of Science has confirmed for decades now, any exposure to radioactivity, no matter how small, carries a health risk, and cumulative exposures add to the risk. Chernobyl's radioactive cesium-137 fallout has a half life of 30 years, meaning its hazard in the environment will persist for 300 to 600 years.