Question: at what level is arsenic harmful?
Answer: The U.S. Environmental Protection Agency (U.S. EPA) set an arsenic maximum contaminant level (MCL) for public water supplies at 0.010 mg/L. This is equivalent to 0.010 parts per million (ppm), 10 micrograms/liter (µg/L), or 10 parts per billion (ppb). The EPA also sets the MCL Goal (MCLG) for drinking water.