https://doi.org/10.1351/goldbook.15758
In the United States, nonregulatory health-based reference level of chemical traces (usually in ppm, i.e., \(\pu{mg L-1}\)) in drinking water at which there are no adverse health risks when ingested over various periods of time.
Note: Such levels are established for 1 day, 10 days, long-term and life-time exposure periods. They allow for a wide margin of safety.