Help me think through Lead in drinking water-- math, chemistry, biology

• We're buying a house (still negotiating, hence the anonymity), and we have learned that the level of lead in the water is slightly above the expected range. Not knowing whether this is a big deal, I've tried to do some math to figure out whether being over the threshold is an object of legitimate concern (or whether the range is the fruit of a hyperactive regulatory state) and the process has led me to even more questions. The water test returned a level of 17ppb versus an "action level" of 15ppb. While not an order of magnitude over the limit (the old "action level" was ~3x the current reading, even) or anything, clearly this isn't ideal (and if we didn't have small children, I might even blissfully ignore it as "close enough" and go about my day) so I decided to perform some back-of-the-envelope math and subsequently I've begun to wonder whether this standard is too permissive. Please follow my math below: A lead concentration of 15 parts per billion (I presume that the standard is set by volume) is roughly equivalent to 171 parts per billion by weight (lead is 11.4x as dense as water.) A toddler drinks 1.3l of water per day, which weighs 2.86 lb. At 171 parts per billion of lead by weight, 2.86 lb of water contains .000008 ounces of lead. 365 days of this (with full retention) gets us to .0029oz, .08g,or 81,141 micrograms of lead. Meanwhile, a healthy lead level for a toddler is 2 micrograms/deciliter of blood. Figure a toddler contains 10 deciliters of blood, and conclude that the bloodstream can safely can hold 20 micrograms. Leading me to conclude that if the blood standards and the water standards are consistent, the permitted annual drinking intake is 4000x the permitted point-in-time blood content. Or I have misplaced a decimal point. Now, I understand that lead isn't really metabolized and that it settles in the bones. So while I'm clearly over my head here (for starters, I'm comparing a stock and a flow), I'm not satisfied that 15ppb is safe and 17ppb is unsafe and I am rather alarmed. I wonder about the following: 1. I think I got all the conversions right. Did I? 2. Are either the water standard or the blood standard set with rigor? Or have the authorities simply designated some arbitrary percentile cutoffs? 3. Can the blood standard and the water standard be reconciled in a bottom-up fashion using known rates of lead metabolism and absorption? Or is it all top-down slicing? 4. By the time a child reaches 18 years of age, it's conceivable that despite never consuming "unsafe" water, he will have ingested 100,000x the weight of lead that his bloodstream can safely hold. It's quite alarming from a headline perspective (especially given all the "lead is really, really bad for kids" research generated over the past few years). If I knew what I was talking about, would I still be alarmed?

• Answer:

  I work in the environmental industry. The 15 ppb standard is totally reasonable and I personally would like to see it lower. (I would also like to see the blood lead level lower.) Lead is like radon -- there is no "safe" value of a carcinogen. We have to set a level to move towards, but it doesn't make the level we've set "safe". I'm not getting into your back of the envelope math in detail -- there's a bunch more about absorbtion rates etc. that goes into this kind of calculation and that you don't have info on. The piece of information we're missing here is what kind of drinking water sample this was. There is a big difference between 17 ppb in a first-flush sample and 17 ppb in a sample once the water has been running for 10 minutes. If I knew what I was talking about, would I still be alarmed? I work in the industry. If I were in your situation, I'd start with the following: First: Google [name of town] 2011 Annual Drinking Water Quality Report. It should come right up, and it will have info about whether your town's supply lines have known issues with lead. (If it doesn't come up, call the local water department and they can get you a copy.) If the issue is a known thing in the area, there's probably a local department (maybe Water, maybe Health) who can help you deal with it; call them and ask what they recommend. There may be local funding or programs for this. If there isn't a known supply pipe issue: It may be worth finding out more about the sampling methodology used by whoever sampled this for you. Usually a single-sample test is what's called a "first flush" sample -- they sample the very first water that comes out of the sink after it's been unused for eight hours or so. That means they get the water that has been sitting in the pipes overnight, so it's a worst-case scenario. It may be that the water has much lower levels of lead once it's been running for 30 seconds. You can call a local consulting firm (or ask your local Health dept etc.) to help you do more testing to see if it's just the first-flush water that's been sitting, or if it's an issue even once the water has been running. If it's only the water that's been sitting that has high lead levels, you may be able to deal with the problem by letting your water run for 30 seconds prior to using it. Also, lead sampling is generally done on the cold water supply; if they sampled the domestic hot water, well, there's your problem. If that's what they did, you'd need to resample the cold water (ideally by sampling both the first flush sample and a sample once the faucet has run for 30 seconds). As a general best-practice measure, nobody should be drinking hot domestic water. tl;dr: The US drinking water standard for lead is reasonable, even arguably too high. There are a lot of local resources that may be able to help you with this. Also, even if you do have lead in your first-flush water, you may be able to significantly reduce any exposure by always running the water before drinking, and never drinking water from the hot tap. (Running the water before drinking is also a great idea with drinking water fountains, esp. the old ones.) A filtration system may end up being the way to go, but there are several steps before that point.