Why hot water OK in the shower or dishwasher, but not to drink?

September 26, 2023 12:30 AM

My partner recently discovered this piece of advice from the EPA, recommending against hot water for cooking, drinking or making baby formula, due to elevated levels of lead. So why is it OK to rub all over ourselves, our clothes and our dishes?

Apparently hot water can cause more leaching of lead from old, corroded pipes than cold water. Makes sense.

According to random other Googling, soap and water is not enough to wash away lead residue. So, what? Is hot water / lead-y not dangerous in general? Also, does it have to get to a certain temperature to be dangerous? The EPA also recommends washing dishes in warm water, so I dunno.

Is this just CYA behavior by our wonderful government that will not in any way address the root cause? Or is there something that makes it less dangerous in these scenarios?

posted by stoneandstar to Home & Garden (28 answers total)

Response by poster: If this is actually very dangerous I would imagine the dishwasher is just a giant lead depositing machine… ?
posted by stoneandstar at 12:31 AM on September 26, 2023

I mean- to state the obvious, there is a difference in magnitude between the amount of lead you interact with ingesting/digesting the water and merely touching it (even if you immerse yourself in it). I imagine a large part of that recommendation is CYA/abundance of caution, but babies have really tiny and developing brains so I think it’s probably err-on-the-side-of-caution territory there with the formula, at the very least.
posted by charmedimsure at 12:44 AM on September 26, 2023 [8 favorites]

Best answer: Washing machines and dishwashers take water from the cold-water pipe and heat it up themselves. They don't have lead inside them, so hot water never touches lead.
posted by dudekiller at 12:45 AM on September 26, 2023 [4 favorites]

Response by poster: That makes sense, thank you 🙏🏻

To state the obvious re: touching vs. ingesting, I’m quite sure that being covered in lead and eating off of lead-covered dishes and baby bottles would lead to quite a lot of lead exposure!
posted by stoneandstar at 12:48 AM on September 26, 2023

Best answer: (My washing machine definitely takes hot water from our hot water heater, fwiw. Most I've ever encountered do, that's they have cold and hot water hookups in the back)
posted by stray at 1:40 AM on September 26, 2023 [26 favorites]

I think this will depend on the age and location of your property..

Virtually all of the places I've lived at over last few decades don't have any lead pipes nor a lead hot water cylinder.
Most of the new water pipes installed seem to be plastics (not even copper for the most part). And hot water has pretty typically been from infinity style instantaneous gas heaters that only heat the water on-demand. I've not even had a cold water tank for a while, it's all been mains water fed directly.

So if you're living in a newish house with new plastic or copper pipes, and particularly with an on-demand water heating (rather than stored).. and so long as the main public water supply is free of lead, I don't really see how you could be getting lead into your hot water tap.

But I've primarily lived in the UK and in Aotearoa NZ., so your circumstances could be different
posted by many-things at 2:46 AM on September 26, 2023

Well, like, you can swim in seawater but you can’t drink it (or wash your clothes on it, for that matter). Skin is pretty good at keeping stuff out. Mouths are good at letting stuff in.
posted by mskyle at 4:41 AM on September 26, 2023 [22 favorites]

As a data point, I grew up with a "don't drink hot water" rule for this reason. (I was born in the mid-1980s in Illinois.) At some point, my mother (born mid-1950s in Yorkshire) mentioned that she was pretty sure it was unnecessary, just habit.

Interestingly, my mother now puts hot water in the kettle (on the grounds it requires less to boil) and I don't, precisely because we didn't growing up.

I'll note too that the big concern with lead paint is kids ingesting it if it flakes off (or they lick the walls, because kids). Granted, we have much more casual contact with our clothes and dishes than with walls.
posted by hoyland at 5:03 AM on September 26, 2023 [2 favorites]

With the EPA’s examples, you’re directly putting lead inside the body. A lot more water flows through the pipe when you run the dishwasher or washing machine, probably diluting any leached lead below levels where the EPA considers it a concern. Any lead did end up on the plates or on your clothes has to enter your body through contact with food, or your skin, reducing exposure again. According to the EPA, human skin does not absorb lead in water.
posted by zamboni at 5:18 AM on September 26, 2023 [2 favorites]

Best answer: Dishwashers are typically connected to the hot water intake (the heating element just heats it further and keeps the circulating water hot throughout the cycle) so that is not the reason.

With anything toxic, dose is critical. Lead and other heavy metals are found in all kinds of vegetables and other foods, leeched naturally from the soil, but usually at tiny levels that aren't a concern unless you're eating very large amounts of that particular food (this is another reason why a balanced diet is important).

They give a blanket warning not to drink hot water because some people drink large amounts of water (and for babies, it's a high proportion of formula relative to their body size). If lead was in their hot water, they could be drinking enough to be harmful, especially for babies. The amount of lead left in the tiny amount of water that dries onto your dishes or clothes is minimal (if any at all), assuming an uncontaminated water source, so the risk is minimal.
posted by randomnity at 5:22 AM on September 26, 2023 [4 favorites]

Wouldn't this only be a concern if there were lead either in your water heater or in the pipes between there and your tap?

So if the water heater and the pipes are new enough to be lead free then I think the EPA's statement is overly broad. Possibly intentionally, but still.
posted by Awfki at 5:41 AM on September 26, 2023 [1 favorite]

Best answer: The EPA offers this to help you figure out if you have lead in your water.
posted by mareli at 6:28 AM on September 26, 2023

I think what makes it less dangerous is that plates in a dishwasher don’t absorb the small amounts of lead in the hot water, whereas if you drank that water, you would absorb the lead.
posted by Vatnesine at 6:33 AM on September 26, 2023 [1 favorite]

Best answer: IMO: it's total nonsense CYA advice. The number of people who cook with hot water has to approach 75%+ of the population, for things like spaghetti or soups- basically any food that you put water from the tap and heat up. Fine, babies shouldn't drink water anyways. But everyone else? They are using hot water to cook with.

A lot more water flows through the pipe when you run the dishwasher or washing machine,


A modern dishwasher uses about 3-5 gallons of water, I think your average washing machine less than 20. So not really all that much water flows though them. And do none of you have plastic cups that flip or bowls that retain a bit of water from the dishwasher? You rinse them for like 1 second, dry and put in the cupboard.
posted by The_Vegetables at 7:30 AM on September 26, 2023 [1 favorite]

Dishwashers are typically connected to the hot water intake

In the US and Canada, the standard is for dishwashers take in hot water, but in many other places (Australia, Japan, UK, EU…), standard dishwashers take in cold water.
posted by mbrubeck at 7:31 AM on September 26, 2023 [1 favorite]

Best answer: Note that American appliances are connected to hot water and European appliances to cold water (and heat the water with their internal elements) which is why you're getting two distinct groups of answers. This is because US houses are wired for 120V generally with split-phase 240V only on select outlets for electric cookers and dryers - American washing machines and dishwashers therefore only have 110V to play with for water heating.
posted by atrazine at 7:33 AM on September 26, 2023 [9 favorites]

A modern dishwasher uses about 3-5 gallons of water, I think your average washing machine less than 20. So not really all that much water flows though them.

Assuming 3/4" pipe and a water content measurement of 0.023 gallons/ft:

3 gallons: ~130 ft
5 gallons: ~217 ft
20 gallons: ~869 ft

Even at 3 gallons, a dishwasher is likely to completely purge the line of standing water, particularly if it's plumbed off the nearest sink, and you follow the advice to run the sink faucet until it's hot before starting the dishwasher.
posted by zamboni at 8:16 AM on September 26, 2023

Response by poster: Thanks everyone. Obviously I understand that lead is not absorbed through the skin, but if it’s on dishes… it’s on food. And if it’s on clothes, it’s probably quite easy to get from the clothes into your eyes, ears and mouth. Not to mention showering in it and getting it all over your face, skin and hair. Children also bathe in it, which I can’t imagine is wonderful? I know the dose matters but it doesn’t sound like they’re being terribly precise about what dose is a problem, probably because they know it’s impractical to act on.

We filter our drinking water, no idea if that matters, but I did ordinarily run hot water into the pot or kettle while cooking. I guess I’ll stop and just get my lead exposure from my hair, skin, dishes and clothes. 🙄
posted by stoneandstar at 9:04 AM on September 26, 2023 [1 favorite]

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4961898/

Quote from that article: "There is no threshold value for the level of lead present in blood below which its concentration can be considered safe. Extremely low yet permanent levels of lead exposure were found to reduce the cognitive capacity of children (Needlemann et al., 1990)."

Another: "However there is no threshold value below which lead exposure can be considered safe. It has been found to impair development and have harmful effects even at lower levels (Rossi, 2008; Barbosa et al., 2005). "

So no one will tell you a certain dose that is not harmful. There's no such thing as a such small dose that it isn't harmful. Even if you only have low exposure, less is still less harmful.
posted by starfishprime at 9:19 AM on September 26, 2023

It’s not just lead specifically, this is not really some “EPA CYA” - it’s just that hot water will tend to dislodge more “gunk” (technical term) than cold. So whatever is in your pipes is more likely to be sloughed off and end up in hot water from the tap. In most modern homes this isn’t lead per se, it’s unlikely to cause significant problems (unless you’re a baby), but you’re probably also better off not ingesting it.

If your plumbing contains galvanized piping (usually found on older homes), that is a source of contaminants, particularly when heated water travels through it.

Most modern homes use plastic (PVC) pipes - heating plastic can release various compounds, so running hot water from the tank through plastic pipes and then drinking it seems not ideal to me - it would be similar to heating water in a plastic container in the microwave. Some people are OK with that, some aren’t.

If your home is piped with copper piping from the hot water heater to your taps that would be the ideal scenario, but the hot water would still dislodge mineral residue that may build up from hard water, if you have it.

All of this is why pouring boiling water down a slow drain is a quick way to open it back up - the hot water removes a lot of the built up sludge. I wouldn’t want to drink that!
posted by annie o at 10:18 AM on September 26, 2023 [5 favorites]

There's lead in a lot of copper pipe solder. Lead is really bad for kids. I wouldn't want to swim in leaded water, washing isn't a huge concern unless the concentration is high. The amount on dishes dishes or in clothes (rinsed w/ cold water) would be negligible. I use cold water for cooking because it's okay to be prudent and I think it tastes better.

I know an adult who was harmed by lead paint in their old house. Long-term effects even after chelation. Lead from gas and paint is still in soil. testing is useful.
posted by theora55 at 10:58 AM on September 26, 2023 [1 favorite]

Response by poster: My point isn’t really that they need to say lead is safe over X, but that there seems to be a serious health hazard way beyond the bounds of the recommendation, which they are not addressing. To say that no lead exposure is safe, but the amount in showers, dishwashers and laundry machines is negligible… aren’t those contradictory statements?
posted by stoneandstar at 1:05 PM on September 26, 2023

Response by poster: The reason I say “CYA” is because they are not giving sound principles— perhaps because they would seem excessive to Americans? or perhaps because lead in washing machines, baths, etc. is so negligible as to be virtually safe, but they think that’s too hazardous a claim to make due to behavioral & environmental variance?— not because I think lead is safe.
posted by stoneandstar at 1:08 PM on September 26, 2023

Best answer: TLDR: hot water has measurably more lead in it across more systems so rinsing with it remains fine but direct consumption is not considered safe.

We had lead issues at our previous residence. Relative to the level in the city supply* our cold water tested much much lower compared to the hot. We asked about still using the hot and was told it was fine - even though it was elevated. The old limit for lead was 15 parts, the new limit is 9, ours was 6. The hot water was deemed fine because lead doesn't magically get pulled from the water, onto the dish/laundry, then through your skin or onto your food and into your body. Lead, which is heavy and will not (generally) be chemically bonded during either washing or rinsing, so it doesn't stay stuck to your dry item. Each step is a significant reduction in how much lead remains to get ingested or absorbed. To be extra cautious at our old place I would often double rinse the kids stuff, but that was also to deal with sensitive skin, in addition to generalized paranoia.

That is very much not the same situation as drinking lead straight in your water - there your body becomes the accumulator and will absorb all the bio-available lead it can. Glass after glass, year after year.

I think a significant factor is that many residences in the US and Canada have hot water that is produced and stored in a tank. This hot water tank provides ample time for any lead that is available in that system to leach into the water. Generally the cold water supply does not have a tank so the level of lead does not have time to accumulate. I am also super adamant about flushing pipes after service has been turned off or disuse. It takes water over minute to move from the street to our farthest tap, so that is how long I run it.

Large spaces like apartments, condos and most large commercial buildings will also have hot water systems that are not completely purged even with regular with use and lead will also accumulate in those systems as well.

Someone noted that PVC is widely used - many jurisdictions, like mine, still limit Pex and PVC. And lead will still be found in so many solder joints. Because plumbers resisted using lead free long after it was banned in the 80's. In fact, you can still buy it on Amazon.


*which is above negligible. And that is too damn high. So all our drinking water is filtered.
posted by zenon at 1:52 PM on September 26, 2023 [1 favorite]

To say that no lead exposure is safe, but the amount in showers, dishwashers and laundry machines is negligible… aren’t those contradictory statements?

Well, firstly the idea that there's no safe dose of lead does not contradict the idea that the dose makes the poison. More lead is still more harmful than less lead.

There might be no such thing as a harmless dose of lead, but there also is no such thing as a person who can totally avoid ingesting lead, and there never was. At some point you have to draw a line and say that this very low probability thing is not worth worrying about, because the resources that you'd putting into avoiding lead are taking away from the resources that you could use to avoid more likely problems.

The amount of lead in contaminated water is tiny. It is only worth worrying about if you're ingesting lots of this water. The tiny amount of lead that is in the tiny amount of water that dried off of your dishes is tiny tiny, and how much of that is transferring from the dishes to the food? I'm sure it's less than was in the food before you put it on the dish. The only activity you cite where you're going to ingest any amount of leady water that we should even mention is in the shower or bath, because you'll swallow or inhale a small amount--but is that really anything approaching a glass of water a day?
posted by polecat at 5:46 PM on September 26, 2023 [2 favorites]

From EPA

Can I shower in lead-contaminated water?
Yes. Bathing and showering should be safe for you and your children, even if the water contains lead over EPA’s action level. Human skin does not absorb lead in water.

This information applies to most situations and to a large majority of the population, but individual circumstances may vary. Some situations, such as cases involving highly corrosive water, may require additional recommendations or more stringent actions. Your local water authority is always your first source for testing and identifying lead contamination in your tap water. Many public water authorities have websites that include data on drinking water quality, including results of lead testing. Links to such data can be found on the EPA Consumer Confidence Report website.

My paraphrase: even if you're over the limit, showering is probably OK, but for those small number of households that are waaaaaay over the limit, then maybe not.
posted by polecat at 5:51 PM on September 26, 2023 [1 favorite]

Best answer: Hearing things like "there is no known safe level of lead exposure" tends to freak us out. But in deciding what is safe and reasonable here, we have to do something thinking about relative exposure levels. How much lead are we actually exposed to in drinking water - and drinking it from the cold tap vs the hot tap - and how does that compare to the exposure levels from dishes washed in that same water, clothes washed in the same water, and showering or bathing in that water?

Some things to consider - in light of "there is no known safe level of lead exposure":

• This does NOT mean that every level of lead exposure is equally dangerous. In fact, we know that the lower the level of exposure, the lower the danger. If we can cut the exposure level by 90% that improves the situation a lot. If we can cut it by 99%, better yet. If we can cut it 99.9%, better yet.
• Just to give a sense of proportion: Some of the households in Flint MI had levels of 3000-4000 parts per billion. This is BAD. B-A-D bad. For a long time, the safe level for drinking water was thought to be about 40 ppb but then some measurable negative effects were found at that level vs lower levels (say, less than 10 ppb). Now 15 ppb is often considered an "action level" for water companies. They are thinking about lowering that to 10ppb or even 8ppb. Canada has set a level of 5ppb. The Environmental Defense Fund uses 3.8ppb. Some cities (Chicago?) are considering 2ppb. So part of the hesitation in setting a "safe" level is: They thought 40ppb was good, then they though 15ppb was good, now they have realized those are way better than 100ppb or 1000ppb or 10000 ppb but still there might be some lingering, smaller effects at those levels. So no one really has the data to say where there will be NO further lingering effects. Is it 10ppb or 5ppb or maybe 1ppb? Nevertheless: If you're at the equivalent of 1ppb drinking water exposure, or perhaps even way less than that - you can start to be pretty comfortable. And actual experience working to minimize lead levels in drinking water shows it's very, very hard to get lead levels below 1-3ppb. So if you have a source that is contributing only 1/10th or 1/100th of those levels - it's not going to be a significant contributor to a person's overall lead levels.
• This is a nice article giving an idea of what different levels of lead have been found or detected in different situations, and what problems have resulted from different levels of exposure. Just for example, they found that drinking water levels of 18 ppb lead to 50% of exposed children having levels of lead in the blood higher than the current CDC level of concern. That's the kind of thing that makes you think: Is 10 bbp low enough? Is even 5 ppb really low enough? But also it makes you think: OK, 0.5 pb is probably pretty OK, and definitely 0.05ppb is getting there for sure.
• For a quick reality check: All the things you are concerned about - showering, washing dishes, washing clothes in water with some lead content - amount to exposure levels equivalent to drinking water with lead levels far less than 1ppb. Even if you do these with hot water, which might have somewhat elevated lead levels. Even if you do them with water that has moderately elevated lead levels - like 20ppb or 50ppb. If you are concerned about reducing your lead exposure, these are simply not the things to be concerned about. Read on for details.
• Don't get mixed up by the idea that "there is no known safe level of lead exposure" and start thinking that 1ppb is just as dangerous as 40ppb or 100ppb or 3000ppb! Low levels like 1ppb are definitely many, many, MANY times safer than even 40ppb and definitely safer than 1000ppb or 4000ppb. It is just that no one can say for certain whether, even at a fairly low level like 5ppb or 1ppb, there is possibly some small remaining negative effect on the population at large, or, perhaps, a larger negative effect on some small percentage of the population. Whereas, say, 3000ppb would have a large negative effect on pretty much everyone.
• To give another sense of proportion: The usual method of detecting lead in water, which is a mass spectrometry method that is pretty darn sensitive, cannot even detect lead at levels less than 0.5ppb (See section 5.1.1 here)
• Another way of looking at it: Americans' exposure to lead is generally considered to be about 20% from drinking water. So think about this: If some other lead source (say: water from dishwashing, showering, clothes washing) is adding only 10% the amount of lead as we get from drinking water, that is only about 2% of a person's overall lead exposure. If the lead exposure source amounts to only 1% of the amount received in drinking water, that is only 0.2% of the average person's lead exposure. If it is only 0.1% of the amount received in drinking water, that is only 0.02% of a person's lead exposure. So you can spend your time freaking out and thinking IT'S STILL NOT ZERO!!!11!!!! but that fact is: If you've got source D down to 0.02% of the average person's lead exposure totals, you'd better stop obsessing about that one and spend a lot more time thinking about sources A, B, and C which make up the remaining 99.98% of the exposure.
• Yet another perspective: This study found measurable, fairly large, health impacts from cutting lead levels in the blood from roughly 3.6 μg/dL to 1.9 μg/dL. This gives us a fairly good idea that cutting lead exposure roughly in half will have a huge positive impact. The things we are talking about here - lead from washing dishes, washing clothes, showering - are more like 1/100th or even 1/1000th the lead exposure levels of the main contributors to human lead exposure. FYI, those main contributors are: Drinking water, air emissions (leaded gasoline prohibited from motor vehicles now but still allowed for aircraft, agriculture, racing cars, boats, and other specialized uses), house dust, old paint/paint flakes, soil, and consumer products. If you want to freak out about lead exposure, any one of those is probably a more productive direction. Just for example: Children living close to an airport have 21% higher lead levels. That is a really a YIKES!!!11!! - and something to be really concerned about. Something like washing dishes, washing clothes, and showering isn't going to amount to even 1% of that.
• Instead of thinking LEAD ZERO a more productive direction is something like that adopted by Canada: "As low as reasonably achievable." (See section 1.0 here.) This keeps us from freaking out if lead somehow manages to be somewhere above zero - which, since lead is pretty ubiquitous in our environment, it's going to be. Instead of thinking: "Is lead exposure ZERO" we can think: "Is lead exposure from this source a significant portion of my total lead exposure?" If it is, do something about it. If not, move on. "Is lead exposure from this source even measurable?" If not, move on. "Are there other sources providing orders of magnitude more lead exposure?" Concentrate on those - don't swallow a camel and strain at a gnat, as the saying goes. Look for the camels of lead exposure (large sources) and don't freak out about the gnats (sources that are, perhaps, above zero, but still pretty much immeasurably small).
• Regarding exposure during showering, through breathing (water vapor etc), and elsewise through the skin: "As lead compounds are not volatile, inhalation of lead is limited to particle-bound lead, an exposure scenario that is not applicable to drinking water. Furthermore, lead is predominantly found in its inorganic form in drinking water, and inorganic lead is not readily absorbed by the skin. As such, the dermal and inhalation routes of exposure to lead in drinking water were not considered significant in this assessment." (See section 5.7 here.) Again, that does not mean that absorption through these routes is literally zero. But the amounts you are getting here are so dramatically lower than those absorbed by drinking water that for practical purposes we can dismiss them. "But my little darlings always swallow water from the bathtub!" Again, think this through: Literally how much water do they drink out of the bathtub vs how much water they drink in a day? Even a 4-year old is supposed to drink 5 cups of water a day. In the bathtub they might get one swallow? Or a few drops? Even a good, solid swallow from the bathtub is something like 2% of their exposure from drinking water, at most. Obviously, encourage them to not drink from the shower or bathtub if you are worried about that lead exposure. Now that they are not purposefully gulping down water from the shower or tub, you are talking about a few drops at most that they swallow inadvertently, a no significant exposure through the skin, breathing, etc. Altogether, less than 1% the lead exposure from showering/bathing vs drinking. Again that is not actually right at ZERO but it is below our threshold of freakout. There are other lead sources FAR more significant to worry about. Spend your time and worry on those.
• Regarding dishwashing and "all my dishes and food are COVERED IN LEAD." Just for fun, I rinsed some of my dishes with water and measured how much of it drained off vs how much stayed on the dish to - presumably - evaporate and leave the lead behind. I poured 1/2 cup - about 110 grams - all over a plate then drained it off, not too thoroughly, to see how much remained on the plate. Every time I did this, I got better than 99% of the water back. Putting it another way, 1 gram or less of the water stayed on the plate. And if I actually let the plate sit and drain for a while - or used a dish towel to remove the remaining water - I could probably remove most of that remaining 1 gram as well. Again, let's put this in perspective: A 4 year old is supposed to drink 5 cups of water every day. That is 1200 grams. The amount of water that dries on a typical plate or bowl is 1 gram or (probably) much less. Looking at worst-case scenario here, the amount of lead you'll get from using cups and dishes in a day is equivalent to drinking 10-20 grams of that same water in a day. Your exposure here is AT VERY WORST around 1% that of your exposure to ordinary drinking of the same water. And that 1% level is an amount, as we've already discussed, that is well below the freakout threshold.
• Washing clothes: Again, as the document quoted above states, exposure through the skin or breathing just isn't a factor. Is it actually ZERO? Again, that is a question we can't really even answer. But regardless of whether it is actually zero, it is clearly way below measurable, and way below our freakout threshold. Other lead sources are going to be orders of magnitude higher than this source. If you want to worry about something, spend your time worrying about those other lead sources, which are far, far more significant (paint chips, dirt, your nearby airport or marina that sells leaded gas by the hundreds of gallons, contributions to lead in your drinking water that persist despite "low lead" regulations - and making sure to flush your fixtures for a few seconds before collecting drinking water, clean your aerator screens periodically, and replace older fixtures/taps with newer fixtures that have lower lead levels, etc).

So in practical terms, lead exposure from showering, lead exposure from washing dishes, lead exposure from washing clothes, and such things are simply not a practical concern - assuming the amount of lead in your water is anywhere near reasonable - say, 40 ppb or less, or even up to 100 ppb or 200 ppb.

Now even the CDC makes note that this situation could change if you live in one of those homes, as we saw in Flint, with like 4000 ppb lead in the water (polecat notes this, just above). 1% of 4000 is still 40ppb - a known problematic level - and even 0.1% is still 4ppb. Things like washing dishes or drinking a few drops while showering could - in that very extreme situation - still amount to a decent proportion of the average American's annual lead exposure, and more than you would really want anyone to take in. So in that extreme situation you would re-evaluate. But if you are in more the typical "high lead" situation where the water is at something like 10ppb, 15ppb, or even 40-50-100ppb, now the exposure from things like dishwashing or showering or clothes washing just doesn't amount to a significant percentage of the typical person's normal/ordinary/regular lead exposure. It's below the freakout level and even below the measurable level.

• To your other question: Interestingly enough, lead from hot water lines can, indeed, be elevated often enough to make it a very reasonable recommendation to never drink water from the hot water tap. I actually had a devil of a time finding actual definitive research on this. But one reason for the concern is that we know very well that lead dissolves from pipes, solder joints, and fixtures into water. And we know definitely that hot water dissolves things (anything - lead, calcium, copper, whatever) far more readily than cold water. Also, we know definitively that the corrosivity of water is a huge contributor to lead content. This has a lot to do with the minerals and such that deposit inside the pipes covering up sources of lead. If the water is corrosive, it can more easily dissolve those protective deposits, exposing the lead deposits, which can then far more easily dissolve into the water. This is exactly what happened in Flint - a change in water composition led to greater corrosion, exposing lead pipes and solder, and so lead content in water increased dramatically. Well, guess what? Hot water is massively more corrosive than cold water. (Two studies discussing changes in lead levels due to temperature.) Finally, hot water heaters can collect lead particles. These collect at the bottom of the water heater and/or near the element. The heated water can then dissolve and distribute this collected lead through the water. So those are all solid theoretical reasons to have some concern about the situation. And on top of that: Here is a study from the Environmental Defense Fund that examines this issue. Good news - only a few hot water taps, as actually measured in real life at real facilities, had actual elevated lead levels. In fact, the percentage of problems found there was actually lower than that found at cold water taps (!). Bad news: The hot water heaters were indeed massive lead reservoirs. Flushing them periodically seems to help a lot. Upshot is, this is a somewhat theoretical problem, but you could easily see situations where hot water could dispense double, triple, or quadruple the amount of lead you get from your cold water lines. And if you are drinking that hot water with lead levels elevated to that degree, that could be more than enough to raise the level of concern from low to moderate or moderately high. It could easily be enough of a change to raise lead levels from 2-3 ppb to 15-20 ppb. And keep in mind, 18 ppb is where 50% of kids show actual elevated levels of lead in the blood. So just drinking from the hot water tap is the type of thing that could, in theory at least, raise your drinking water lead levels from pretty damn good to pretty damn worrying. So maybe this doesn't happen all the time or every time. But why fool around, when you can just get water from the cold tap? And a typical example of a person getting most drinking water from the hot water tap is preparing formula for a baby. There is a situation where 90% or more of the baby's water consumption could be from the hot water tap. If the lead content is even 2X higher than the cold water tap, that is leading to unnecessary lead ingestion. So the hot water thing is something of an abundance of caution situation. But it is a situation where a lot of factors make you think the lead levels can be higher, and if they are higher, then the levels can easily be within the danger zone. On the other hand, as we've discussed, the danger level from washing dishes, washing clothes, showering, etc is orders of magnitude lower than this. You're (possibly/potentially) raising the lead level by (let's just say) 2X or 3X or 5X by using hot water rather than cold water. But now factor in that the exposure level from washing dishes, clothes, or showering is like 1/100th or even 1/1000th that of actually drinking that same water. So the advice to not drink from the hot water tap is reasonable, because even though the issue is somewhat theoretical, the very real possibility is there to raise lead ingestion levels from low-ish and acceptable (2-5ppb, say) to levels we know are concerning (15-25ppb, say). Whereas for washing dishes & clothes and showering with that same hot water, the exposure levels are so dramatically lower that the contribution to the average person (or even small child's) annual lead exposure is still going to be far below levels worth worrying about, even if it's done with water with a higher lead content.

In short, even after looking at it in some detail, I would rate the CDC's advice as pretty darn good. They are right to be wary of naming an exact safe level for lead in the water and environment - partly because they have set "safe" levels before and been burned at it, and partly because we really don't have good evidence of what an exact, completely safe level would look like.

They are right to be wary of drinking from hot water taps. The danger is somewhat theoretical, but the known danger is potentially enough to - sometimes! - push lead levels in drinking water from a pretty safe level at the cold tap into known dangerous levels from the adjacent hot tap. Why take that chance - even if it is a fairly small one, even if it happens only sometimes - when there is simply no reason to do so?

For the things you expressed concern about - washing dishes, showering, washing clothes in water with some lead content - their advice is on target as well. These activities have exposure levels so much lower than drinking water, that they pretty definitely fall into a that category of very low concern - even if done with the hot water at a moment when it is putting out lead at a few times the levels of the cold water tap. The amount of lead they contribute to a person's annual lead accumulation is probably not literally zero, but it almost certainly is low enough to amount at very worst to a percentage or two of a person's annual lead exposure. If you really want to minimize annual lead exposure your time and effort is far better spent on these other known lead exposure problem areas, and not worrying about dishwashing, showering, and clothes washing.

And if you want real action items to reduce lead in drinking water specifically, this report from the Environmental Defense Fund has some realistic, tested, and actionable steps:

• Avoid using hot water for cooking or drinking. These taps can have higher lead levels, and hot water heaters are proven lead reservoirs.
• Avoid using hose bibs (outdoor or at utility sinks) for drinking. These fixtures have higher lead levels, and the lead from those fixtures does leech into the water at measurable levels. Newer water taps and fixtures have lower lead levels, but brass and other constituents of these fixtures actually has some percentage of lead in pretty much all cases. For hose and utility fixtures, the attention to minimizing the lead content is noticeably lower. The higher lead levels from these utility and hose fixtures were obvious in the testing done in this study.
• Flush fixtures (minimum five seconds) before drinking to reduce lead levels. 30 seconds is even better than 5 seconds. After long periods of non-use (e.g., vacations), a longer flush may be needed.
• When cleaning aerators, soak in vinegar for several minutes to dissolve any lead present.
• Follow the manufacturer’s instructions for routine flushing of the hot water heater to reduce accumulated sediment (including lead), extend the life of the water heater, and improve its overall energy efficiency. Annual flushing would be reasonable.
• Taking it to the next level: Test each water fixture, with the samples analyzed by an approved laboratory method. Where lead levels from a fixture exceed 5 ppb, replace the fixture, then re-test. There currently is no effective method to predict which fixtures will have significant levels of lead without testing the water. Lead literally leeches from the fixture itself, because lead is present in brass and other constituents of the fixture. Older fixtures will have higher lead levels; newer fixtures are better (but still non-zero).

posted by flug at 3:21 AM on September 28, 2023 [10 favorites]

Response by poster: flug, you’re amazing. Thank you!!
posted by stoneandstar at 3:12 PM on October 14, 2023 [1 favorite]