Help me think through Lead in drinking water-- math, chemistry, biology
January 29, 2013 7:37 AM Subscribe
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.
posted by anonymous to Science & Nature (14 answers total) 1 user marked this as a favorite
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?