The endpoint concentrations for regulated toxic substances under the risk management program rule (40 CFR Part 68 Appendix A) are listed in units of milligrams per liter (mg/L). Is this equivalent to parts per million (ppm)?
No, mg/L is not always equivalent to ppm. Whereas ppm is a volume-to-volume or mass-to-mass ratio, mg/l is a mass-to-volume relationship. To convert from units of mg/L to ppm, use the following equation. Endpoint (ppm) = [Endpoint (mg/L) x 1000 x 24.5] / [Molecular Weight] EPA has included the RMP toxic endpoints in both ppm and mg/L in Appendix B of the Risk Management Program Guidance for Offsite Consequence Analysis (EPA550-B-99-009, April, 1999).