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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.

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?
posted by anonymous to Science & Nature (14 answers total) 1 user marked this as a favorite
 
They talked a bit about this in the recent front page article about leaded gasoline but a lot of public health recently seems to tilt in the direction of "there is no safe level of detected lead" when it comes to small children.

Fortunately it's a good idea in a lot of ways, and cheaper than you might think, to install some heavy duty water filtration for your house such as reverse osmosis.
posted by hobo gitano de queretaro at 7:47 AM on January 29, 2013 [3 favorites]


Don't fuck with lead. At all, ever.

Presumably there are other homes in your area with the same problem; local plumbers will have experience in installing filtration systems.
posted by tel3path at 7:52 AM on January 29, 2013 [3 favorites]


FWIW, Canada's limit is 10ppb.

A lot of the time the lead is coming from inside the house, from old lead pipes or lead solder between slightly less old pipes. You might want to subtract the cost of replacing all of the house's plumbing from your offer.
posted by Sys Rq at 7:54 AM on January 29, 2013 [3 favorites]


Nthing that public health experts believe there is no safe level of lead for children (and it's not good for adults either). The regulations in place are considered out of date.
posted by ClaudiaCenter at 8:09 AM on January 29, 2013 [2 favorites]


As mentioned above, the legislated maxima for lead in drinking water go down frequently as more and more studies get published. Personally, I'd want to see some remediation done, particularly with children or potential for children in the house.
posted by bonehead at 8:32 AM on January 29, 2013


Clarification: in water contamination terminology, ppm = mg/L and ppb = ug/L.

I agree with the other commenters. The EPA lowered the action level for valid public health reasons.

Also, as Sys Rq said, the vast majority of lead contamination cases in developed countries come from old piping. The city might be responsible for any lead pipes up to your property, and the rest would be yours to replace.

If it's coming from the water supply, your city has a maaajor problem.
posted by Paper rabies at 8:35 AM on January 29, 2013 [1 favorite]


BTW, on your math: chemical concentrations are typically written in laws or regs as either mass of lead/volume water or as mass of lead/mass of water. It can be either way, and is dependent on your applicable statutes. You don't need to do that extra mass conversion step. You just need to multiply that concentration by the volume of exposure.

In terms of the risk assessment, it's rather more complicated by what the uptake of lead from the water supply as well---it's not totally transferred, which is partly why you're seeing an exceedance. Still, as a precautionary approach, your simple exposure model gives you an idea of the problem.
posted by bonehead at 8:40 AM on January 29, 2013


Just a note to say that in your calculations the definition of ppb is pretty much microgram/liter. See also here.
So this changes your calculations quite a bit - most of which are overestimates.
posted by vacapinta at 8:46 AM on January 29, 2013


I grew up in an old house with lead pipes in a neighborhood with serious lead in the water issues. I agree with the suggestions to ask about any kinds of pipe replacement and/or neighborhood water issues, and if you have young kids and this is an older house, I would ask about lead contamination in the ground from past coats of lead paint. Neither of these is insurmountable; my parents never replaced the internal pipes (we drank bottled water almost exclusively,) the neighborhood has provided a lot of filters and replacement pipes owned by the city, they added topsoil and we were never allowed to eat anything that we grew, which are not huge steps. However, the seller owes you that information!
posted by jetlagaddict at 8:53 AM on January 29, 2013


I agree with the above that there is no safe level of lead. The standards are all based on the levels in blood going down as we've gotten rid of leaded gas, so now it's not excusable to say that folks will be getting lead from other sources Anyway.

This is actually pretty rare. Most chemicals do have a "safe" level. If you're interested in how agencies figure this stuff, out, this is a state page about lead in drinking water. It's different from the others, because there is no "reference dose" for a safe level. If you're curious about how calculations are often done, (potential self link?) these spreadsheets include toxicological and exposure information (ie, how much water it's assumed children drink, etc) used for calculated "safe" levels when there is a chemical spill. You would be interested in the residential water spreadsheet (sf12rw.xlsx). While there is a lead "reference dose" in those spreadsheets, it is not based on toxicological studies like most chemicals, it's a back-calculation from the drinking water standard.

This is all a long way of saying that yes, lead is always a concern. However, home filtration is a great option in most such cases.
posted by ldthomps at 8:59 AM on January 29, 2013


Find out the cost of the optimum filtration system, and adjust your offer by that much.
posted by Sidhedevil at 9:08 AM on January 29, 2013


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.
posted by pie ninja at 9:20 AM on January 29, 2013 [10 favorites]


Lead bioaccumulates in the body. Available lead in the bloodstream is mistaken by the body as calcium, and added to bones & brain tissue.

In the brain, calcium acts in the gap between neurons-- it flashes a spark. When lead is substituted for calcium, the spark does not flash, and that neural pathway becomes inactive.

The result of "lead poisoning" in the brain is aggression and poor impulse control, in addition to reduced intellect.

Lead poisoning-- Not good for adults, very bad for children, catastrophic for societies.
posted by ohshenandoah at 11:05 AM on January 29, 2013


pie ninja is exactly right.
posted by gjc at 5:12 PM on January 29, 2013


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