MTBE was detected in about 5 percent of ground-water samples collected by NAWQA across the Nation. The concentrations typically were low, well below the U.S. Environmental Protection Agency (EPA) drinking-water consumer advisory concentration of 20 to 40 micrograms per liter, which is based on taste and odor thresholds. In fact, less than 1 percent of samples included in the USGS studies exceeded the EPA consumer advisory concentration of 20 micrograms per liter. Still uncertain, however, are possible human health effects. EPA has tentatively classified MTBE as a possible human carcinogen, but because of insufficient toxicity studies has not instituted a drinking-water health advisory or standard. EPA is using NAWQA findings, along with other investigations and research, to investigate possible drinking-water and human-exposure issues.