"The Clinical Toxicology of Commercial Products, 5th Edition (1984), gives lead a toxicity rating of 3 to 4 (3 = moderately toxic, 4 = very toxic) and the EPA has set 0.015 milligrams per liter as the MCL for lead in drinking water - with a goal of 0.0 milligrams per liter. The toxicity rating for fluoride is 4, yet the MCL for fluoride is currently set at 4 parts per million, over 250 times the permissible level for lead." From today's Letter to the Editor. Juneau Empire


This is an easy question for most folks, I imagine, but not for me. So: How does one measure parts per litre against parts per million?