 Imagine you're eating a juicy swordfish steak and your friend tells you it contains one part per million of the metal mercury. What does that mean? If we're talking about stuff that you don't want to be eating like mercury, parts per million or PPM for short tells you how much of the bad stuff is in the good stuff, especially if there's just a small amount of the bad stuff there. It's a measure of concentration of one substance in another. The trouble with PPM though is that the value you get depends on what you're measuring. Imagine for instance that you have a rather large pool containing 1 million sea monsters and within this writhing mass there are four and a half skunks. Four and a half because one of them had an unfortunate interspecies encounter. What then is the concentration of skunks here in parts If you measure the number of animals there are approximately four and a half parts per million skunk to sea monster. But what if you're interested in the relative mass of skunks rather than their number? Assuming that each sea monster weighs 15 metric tons there are only 0.004 parts per million skunk to sea monster on a mass per mass basis. All that's changed is the way we're measuring things but the quantity we get in PPM is very different. On the other hand if we do the math based on smell and you'll have to trust me here on how much a sea monster smells compared to an enraged skunk because believe me these skunks will be enraged. There will be approximately 4,500 parts per million skunk to sea monster. So this is the challenge with using parts per million to measure concentration. The quantity only makes sense if you know what is being measured. Fortunately going back to the swordfish steak concentrations of hazardous substances are usually measured on a mass for mass basis. So one part per million mercury in swordfish is the same as saying one milligram of mercury for every kilogram of a fish. But it does pay to know what is being compared to what just in case. For more insights into the science of risk don't forget to subscribe to Risk Bites