Fallout from nuclear weapons tests in the atmosphere is mainly ⁹⁰Sr and ¹³⁷Cs, which have 28.6-and 32.2-y half-lives, respectively. Atmospheric tests were terminated in most countries in 1963, although China only did so in 1980. It has been found that environmental activities of these two isotopes are decreasing faster than their half-lives. Why might this be?