An article in the December 2007 edition of Biophotonics International Magazine summarizes  a recent study published in Nature Biotechnology by researchers at MIT and Boston’s Beth Israel Deaconess Medical Center that determined quantum dots must be less than 5.5 nm in order to be rapidly excreted from the body. According to the article, this finding was unexpected because "the threshold for renal filtration and urinary excretion of globular proteins is estimated between 6.2 and 7.1 nm." The quantum dots tested by the researchers had CdSe cores and ZnS shells, and were tested in mice after receiving neutral charges. Interestingly, the study found that quantum dots 5.5 nm and below in diameter had an approximate four-hour half life in the body. The scientists also maintain "the ability of nanoparticles to pass the kidney filters is a function of their shape, size and surface charge."