Lead

Lead is a highly toxic heavy metal with no safe level of exposure in humans. Even the lowest detectable levels can cause serious, often irreversible health damage, particularly to the developing nervous system.

Lead has one of the longest histories of human use among metals, dating back over 8,000 years due to its low melting point, malleability, ease of extraction (often as a byproduct of silver mining), corrosion resistance, and density. Ancient peoples viewed it as a versatile "miracle metal" or "useful metal," despite early awareness of its toxicity in some cases. It used to be used for everything from water pipes to cookware to weapons to paint. They even used to use it as a sweetener. Today it is still very much in use, but with some more precautions as countries have put rules in place to remove it from where it was once common.

Disclaimer: My usual disclaimer is that I'm not going to have a bulletproof argument here, I'm going to lay out the facts as I understand them and my goal is to make you think and judge for yourself. I try to be thorough but also it'll take way too long to be perfect.

Prevention - The Good

I'll “start” with the good part, lead is very easy to protect yourself from. I'll explain the exposure routes and solutions in a lot more detail below but basically just remove them from your life in proportionate order and severity. Pregnant wife or kids around? No holds barred war on everything lead. An old man living alone? Not worth the hassle.

Additionally, the lead has been phased out a lot in recent decades, no more leaded gasoline or lead paint or new lead pipes. So there are some big wins.

Common Exposure Routes

Because lead was used for so long, the main exposure routes are legacy contamination in developed nations, but there are still ‘new’ exposure pathways.

Paint: Homes with paint older than ~70s, after which it was banned, almost all used lead containing paints. Why? Because it resulted in improved color, durability, corrosion protection, moisture resistance, and a couple other things. The fine dust that comes off of the walls lands on surfaces and can end up in your mouth but especially children who are basically always eating something dusty off the floor.
1. → Solution: Make sure there's no lead paint in your house or schools or areas you'd go. It's worth every penny for kids.
Drinking Water: Cities used lead pipes since even before the Romans because it made the pipes softer to work with and corrosion resistant. Water that sits in pipes for hours (overnight or during the day) can pick up lead. Here's an article from the EPA outlining how serious it is [Basic Information about Lead in Drinking Water]. Why don't they just replace the pipes? Municipalities don't have a comprehensive list of which pipes are lead and aren't economically incentivized to dig them up or probe to check. And it can come from safe pipes but that have lead solder. Public health agencies make the decision for you that it's an acceptable risk (even though again, there is no safe limit). Cities to enforce rules like how new pipes must not be lead and if there are any renovations that find lead the pipe must be replaced and some others here and there, but overall the issue is still very much present in many cities of developed nations. A couple more things to point out
1. Testing method: You might have heard that the city has tested your water and determined there is no lead. Their testing method is misleading at best because what they do is run the water for 5-10 minutes and then sample it. With the goal being to detect the level of lead that comes out of the city water. A true test with the goal of helping people would reflect real world use cases, most people don't let the tap run for 10 minutes before taking a drink. Actually you'll get the same cities telling you to save water by not letting it run too long.
2. Mitigation: Orthophosphate is a kind of chemical they often add to drinking water which reacts with lead in pipes to form a layer so it doesn't leach into the water. This chemical is definitely the lesser of two evils but it's not a permanent fix or healthy for you to drink.
3. Impact on Children: For reasons I'll get into in a following section, again children are impacted hardest by lead contamination in drinking water
4. →Solution: Any good water filter, ideally an RO system since it'll get rid of a bunch of other things you don't want as well, like atrazine and pharmaceuticals but that's a story for another post.
Environment: Soil and random items in the environment around us can and often are contaminated with lead from decades of industrial, paint, and gasoline use which spread lead throughout our environment. People today often overlook why the gas you fill your car with is “unleaded”, it's because back in the day they added lead (Tetraethyllead) to prevent engine knock and add lubrication. Although personally I don't worry too much about this exposure route since it's too hard to account for and not as big as the others.
1. → Solution: Mainly going to need to come from the top down. Your city / country needs to work this one.
Products: Lead compounds were (and sometimes still are) added to things like pottery glaze, glass, and even some cosmetics like eye makeup. Crystal glass is glass with some lead added to it which improves the clarity, shine, and workability but it can leach into the liquid holding it if the liquid sits for a long time and is acidic, like a wine bottle. Should be minimal in developed nations though.
1. →Solution: Be careful what you buy
Work / Hobby: Ammunition is still predominantly made of lead for it's density and workability. Lead is used in solder for electronics because of its low melting point. As well as in many other cases.
1. → Solution: I expect that if you're working with lead then you know the risks.Touching lead is fine, just don't eat it.

There are probably even more uses since it really is a miracle metal, too bad we can't use it much.

Health

Lead does two main things. Permanently reduce intelligence and damage organ systems.

The effects are not binary, it's not you're either sick or you're not. Low levels, even at the lowest measurable levels, still cause problems just proportionally less than high levels of lead. Even in low levels, common well documented effects are the following:

Permanent reduction in IQ: yes permanently reduced IQ. Along with reduced attention span and ADHD (sound familiar?)
Learning and Behavioral issues: Difficulty learning, developmental delays, and irritability.
Physical: Hearing issues, speech problems, anemia, headaches. Increased blood pressure, reproductive issues (like lower sperm count in men and miscarriage in women among other things). Nerve disorders, memory problems, muscle and joint pain.
Cardiovascular: It deserves its own category since an estimated millions of deaths result annually from related causes. Here is a direct quote from the WHO [Source]

Lead exposure was attributed to more than 1.5 million deaths globally in 2021, primarily due to cardiovascular effects.

5. Acute: If levels are high enough, and the threshold isn't that high, acute symptoms appear such as seizures, coma, convulsions and even death.

Overall terrible nasty effects. It should be one of the biggest health concerns of developed nations but it's not because of the spectrum of impacts. Public health agencies rarely if ever treat chronic non-acute issues, especially ones that don't pay.

To put some numbers behind it, he CDC uses a “blood lead reference value” of 3.5 micrograms per deciliter (µg/dL) to identify kids with levels higher than most children in the U.S., but harm can occur below that too. Globally, an estimated 1 in 3 children (up to ~800 million) have blood lead levels at or above 5 µg/dL. Very high levels are also an issue, but that's rare in developed nations. I don't like solid numbers because it's very hard to measure lead levels from a blood test since that's not really where most of the lead resides once in your body.

Here are some more references

Absorption & Accumulation

Lead enters the body mainly via the lungs and gastrointestinal tract. Adults adsorb ~10% of the lead they're exposed to, while it's closer to 50% in children. It doesn't really enter via the skin so touching lead is fine. It then enters the bloodstream as is, doesn't get metabolized or changed, like organic mercury sometimes does.

Note: I'm going to be using half life, that's not how long it takes to be eliminated, it's how long half the remaining lead takes to be eliminated. Big and important difference just in case that wasn't already clear.

Blood usually only contains 1-5% of the body's total lead at any given moment, so blood tests can actually severely underestimate lead. Technically there is a half life which is ~30 days but it's not what you'd think since it's not going away. It depends on the timing and frequency of exposure along with the constant absorption and release of lead from soft tissue and bones. Lead is excreted very slowly as it gets into the blood via the kidneys in urine.

Soft tissues contain another small fraction of total lead, maybe 2-8%. Half life here is around 40 days but creates ongoing toxicity. The brain is the big issue since lead readily crosses the blood brain barrier. Once in the central nervous system the half life becomes much longer, at least 2-3 years. That's why persistent cognitive, behavioral, and neurological effects remain even after blood levels normalize.

Bones (and teeth) contain ~95% of the total body lead in adults. Around 70% in children with the rest being in their brains because of the higher turnover of bone tissue. Here the half life is decades, 10-30+ years, basically forever. Bone lead is the best biomarker of cumulative exposure.

Links for some papers I only read the conclusions on but you might want to dig deeper:

Impact on Children

For some sick reason that I can't understand, children are both hit the hardest and most likely to come into contact with lead. Lead exposure is one of the most preventable yet persistent environmental health threats to children worldwide. Again, no known safe level of exposure in the blood—even very low levels can cause permanent harm, and the effects are often invisible at first. This is a quote that I got from Grok a while back that I saved.

Lead poisoning in children is a quiet, cumulative tragedy. Unlike an acute illness with obvious symptoms, low-level exposure often looks like “normal” developmental struggles—trouble focusing in school, lower test scores, or acting out—making it easy to miss. Yet the societal costs are enormous: lost IQ points, higher special-education needs, increased behavioral issues, and long-term health burdens that affect individuals, families, and economies.

Almost all of the brain related effects I mentioned in the Health section apply mainly to kids. Lead damages the brain and nervous system causing a loss of intelligence (and IQ), learning difficulties, developmental delays, reduced attention span, hyperactivity, irritability, behavioral problems, slowed growth (physical as well), speech and hearing problems too.

Why do kids get hit harder than adults?

Young children can absorb up to 4–5 times more lead from an ingested dose than adults do (about 40% absorption rate vs. 5–15% in adults). This is even worse if they’re iron- or calcium-deficient, which is common in some populations. I don't understand why that is, but it's true.
A significantly higher proportion of the absorbed lead ends up in children's brains than adults.
Their brains and nervous systems are still forming at a furious pace, making them very sensitive to toxins. The blood-brain barrier and liver detoxification systems are also immature, so lead crosses into the brain more easily.

Why are children more likely to be exposed? The primary exposure routes I mentioned above apply mostly the same to adults and kids except that kids put things in their mouths a lot. So it's everything the adults get + more like more lead dust, more soil contamination, more lead from objects that they put in their mouths.