A new assessment report by the World Health Organization says there is no evidence that microplastics found in most drinking water are harmful to humans. The WHO did warn against complacency, though, saying that more study was needed on the matter. The report also said larger microplastics, about the size of the diameter of human hair, are of least concern because “they pass straight through the human body.” The smaller particles could possibly get stuck in human digestive tracts, but researchers are confident they are “unlikely to accumulate in harmful quantities.”