For toxic endpoints, are milligrams per liter (mg / L) equivalent to parts per million (ppm)?

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).