The latest study to look at the long-term effects of Roundup, a popular weed killer developed by Monsanto in the 1970s, raises questions about the herbicide's possible contributions to poor health in certain communities. The study, published Tuesday in JAMA, tracked people over the age of 50 in southern California from 1993-1996 to 2014-2016, with researchers periodically collecting urine samples during that time. The percentage of people who tested positive for a chemical called glyphosate, which is the active ingredient in the herbicide Roundup, shot up by 500% in that time period. The levels of glyphosate also spiked by 1208% during that time. One trial from the UK, in which rats were fed low levels of glyphosate throughout their lives, found that the chemical contributed to a higher risk of nonalcoholic fatty liver disease, a condition in which fat accumulates in the liver and contributes to inflammation and scarring of the tissue. [Researcher Paul] Mills says that the levels of glyphosate documented in the people in his study were 100-fold greater than those in the rats. While Roundup was developed to eliminate most weeds from genetically modified crops " and thus reduce the amount of pesticides sprayed on them " recent studies have found that many weeds are now resistant to Roundup. That means growers are using more Roundup, which could only exacerbate potential negative health effects on people who consume those products.