Reid: Lead is a handy metal: it's soft, it's dense, and it's great at resisting corrosion. But it's also incredibly toxic, which is not so handy. And despite knowing that it's bad or us, humans have been using lead for thousands of years. By the 20th century, you could find it in everything from paints to pipes to petrol (a.k.a. gasoline).

But things are different now. There are all kinds of laws that keep lead out of everyday items, and we owe that improvement at least in part to a man who didn't set out wanting to ban lead for health reasons; he just wanted to know how old the Earth was.

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Clair Patterson wasn't the first person to try and figure out our planet's age. Back in 1778, Georges-Louis Leclerc got his estimate by measuring how long it took for iron balls to cool down after being taken out of a furnace. Extrapolating that to an Earth-sized ball of iron yielded an age around 75,000 years old, but he suspected the Earth was even older. Based on geological evidence, like the buildup of rock layers, he imagined an age somewhere around three million years. Unfortunately, rock layers can't tell you much just by looking at them; it's not like they have a date stamped on them.

And in fact, during the 19th century, many geologists assumed a kind of "steady state" Earth, which had always existed and would continue to exist as-is for all eternity. But the British physicist, William Thomson, who would later become Lord Kelvin, was outraged by this unscientific reasoning. So in 1860, he revisited Leclerc's ideas of a gigantic, slowly cooling ball of stuff. Thomson assumed that the Earth started out molten and cooled from the outside in, like a turkey you just pulled out of the oven. And the yet-to-be-lord's estimate was somewhere between 24 to 400 million years old. But by assuming the Earth was like a giant cooling turkey, he missed something -

- something important. The molten parts of the earth don't transfer heat the same way that the solid bits do. In fact, they keep the planet hotter for longer. So one of Thomson's former assistants adjusted the math to account for this, and he calculated that if the crust were 50 km thick, the Earth could be as much as 2-3 *billion* years old, which finally gets us into the right order of magnitude.

But in 1903, we also learned the Earth had an extra source of heat. It came courtesy of radioactive decay, which was this fancy physics phenomenon that had been discovered a few years earlier. And while it didn't adjust the Earth's age nearly as much as accounting for its molten bits did, radioactive decay would play a crucial role in pining down the Earth's age.

But instead of monitoring how much heat radioactive elements shed as they decay, scientists turned to a new technique called "radiometric dating." One of the most famous kinds of radiometric dating relies on an unstable version of carbon, carbon-14. And eventually, every carbon-14 atom will turn into stable nitrogen-14. Scientists call the starting radioactive atom the "parent isotope" and the thing it decays into the "daughter isotope." And all radioactive decay happens on a predictable timescale, known as the "half life." For carbon-14, it's around 5,700 years. So while each individual atom decays at a random moment in time, we know that half of any lump of carbon-14 atoms will have decayed when it hits 5,700 years old.

Now, the most common form of radiometric dating involves measuring both the amount of daughter isotope *and* the amount of parent isotope you've got in your sample. And then, you compare these counts against how much of each you started with. It can be a little tricky to figure out those starting values, but ultimately, the more daughter isotope and the less parent isotope you've got, the older your sample is.

Now, carbon dating is great for archaeology, but it's not so useful when it comes to dating the Earth.

- dating the Earth. But even if the Earth were as young as Leclerc's 75,000 year estimate, that would mean the amount of carbon-14 our planet started with was cut in half 13 times over. Modern levels would be way too low to measure them accurately. So, if the Earth really were billions of years old, scientists would need an isotope with a half-life on the scale of billions of years, too.

And that's where Clair Patterson comes in; born in 1922, Patterson's early passion for the natural world led to an innate scientific curiosity and a knack for chemistry. And right out of college, he was recruited to join the Manhattan Project. His job was to use mass spectrometry - basically a giant magnetic sorting bin for elements - to separate rare uranium-235 isotopes from their much more common and just a bit more heavy counterpart, uranium-238. The uranium-235 that Patterson isolated would eventually make its way into atomic bombs, and we all know how that story ended.

So, like any sensible person, Patterson was horrified by the devastation his work had caused. After the war, he returned to academic life at the University of Chicago, and when he arrived, his adviser challenged him to use radiometric dating or uranium to find the age of the earth, because unlike carbon-14, both uranium-238 and 235 have long enough half-lives to have plenty left over billions of years later. Uranium-238 decays with a half-life of roughly 4.5 billion years, and after a series of steps, ends up as lead-206. Uranium 235, meanwhile, has a much shorter but still decent half-life of 703 million years, and it winds up becoming lead-207.

So as the Earth ages, it will end up with less uranium and more lead. To date the planet, you just need to find a sample from its earliest days and measure the proportions of each. And that's where this -

-  uranium-lead dating is easier said than done, because almost every bit of Earth's crust has been recycled over its long history; it's almost impossible to find pieces that are still hanging around from day-one. But there is one very solid option; there are hearty little crystals called "zircons."

Zircons form inside igneous or metamorphic rocks, but they aren't easily eroded or changed over time, even when the rocks that contain them are reduced to sand. But they're not just a good target because they can watch the world change around them; they're also great for uranium-lead dating because they tend to form with lots of uranium inside them but hardly any lead. That means all of the lead you find inside zircons *should* come from radioactive decay - it makes the math pretty straight-forward.

So, the first step in Clair Patterson's quest to date the Earth was the study some zircons. Along with his lab partner, they needed to refine the uranium-lead dating technique so it would actually produce legitimate ages. But you can't refine a technique on something you don't actually know the age of, so they tested some zircon crystals from a rock they knew was about 1 billion years old.

But despite Patterson's skill and expertise, the numbers kept coming out weird. They knew how much lead there *should* be in the sample, but the amounts they were measuring were much, much higher. So Patterson suspected something as contaminating the samples. After a bunch of tests, he found that his lab was full of unwanted lead; in fact, it seemed to be coming from just about everywhere: the glassware, the tap water, the paint on the walls, dust, even his own body. Because remember, people had been putting lead in stuff for millennia. And in Patterson's mid-20th century lab, it came in toughened glass, lead pipes, lead wiring, and most recently, outside in the streets in leaded fuel.

So the solution was obvious, right? Create an environment where the only lead is the stuff in the rock you're trying to date. And Patterson went to great lengths to do just that: he mopped, vacuumed, installed -

- extra fans, he even dunked his glassware in potassium hydroxide. It took him five years to perfect his cleaning techniques, and finally - success.

He managed to get the right age of his billion year old zircon, earning himself a PhD. And that meant Patterson could finally move on to finding the age of the Earth, but he'd have to switch both target and technique, because as hearty as zircons are, it's hard to be sure where one formed relative to the whole planet. You might find one that's super old, but that doesn't mean it formed inside one of the very first rocks. So, scientists have another, more reliable source that falls from outer space.

Meteorites are effectively rock-based time capsules that were created and sealed back when the Earth and the Solar System's planets were forming. In other words, the age of a meteorite is the age of the Earth itself. But there's one important downside; unlike zircons, meteorites start off with lead inside them. That means you need a slightly more involved radiometric dating technique; one that can distinguish the lead that formed as a result of radioactive decay from the lead that's been hanging around the whole time. This technique is called "lead-lead dating," and it relies on precise measurements of three types of lead: the lead-206 and lead-207 created as uranium decays, and the slightly lighter version of lead that uranium will never decay into, lead-204.

If any of the lead in your space rock is lead-204, you know you can ignore it when trying to figure out how much radioactive decay has been going on. But to figure that out - to distinguish one version of an element from another - you need a mass spectroscope. And lucky for Clair Patterson, a mass spectroscope was something he had gotten very good at using.

So he left the Windy City for sunny Southern California, starting his post-doctoral work at Caltech, and it was there that he planned to figure out how old the Earth is by lead-lead dating a meteorite. But to accomplish that, he knew -

- he'd have to build the cleanest lead-free lab in the world. He tore out all the lead pipes and replaced all the electrical wiring that had lead solder. He had a special pressurized air system installed and separate rooms for each step of the lead isolation process. The refit was so extensive, and therefore expensive, that the geology department had to sell their fossil collection to fund it.

Patterson got his lab. Installing himself as the "kingpin of clean," he cleaned his meteorite sample, used his state-of-the-art mass spectroscope to count how much of each type of lead there was, and plugged those numbers into the equations that would tell him the age of the Earth. It was 4.5 billion years old. If that sounds familiar, it's because it's basically the number we still cite today. It's one of the greatest scientific accomplishments of the 20th century. But Clair Patterson wasn't happy.

It was now the 1960s, and in his decades-long quest to get the purest, most uncontaminated samples for his research, Patterson had realized just how much lead there was out there. And that's bad news, because - as mentioned - lead is super toxic; it causes hallucinations, compulsions, coma, and eventually death, among other [sarcastic tone] wonderful outcomes. People knew this to some extent as far back as Roman times more than 2,000 years ago, but we kept using it anyway. So in the 1960s, Clair Patterson became an anti-lead activist; he railed against the gasoline industry in particular, which had started putting lead into fuel back in the 1930s to help car engines run more smoothly. And according to the "kingpin of clean," that lead was coming out in the exhaust fumes and poisoning everyone.

Of course, the powerful and lucrative gasoline industry didn't want to hear these accusations, so they actively denied his claims. And unfortunately, other scientists didn't have the same super-clean practices in their labs, so they weren't able to -

- replicate his results. But Patterson was undeterred, and he went to the ends of the Earth to prove his point. Literally. 

He traveled to both the Actic and Antarctic to sample snow and ice cores, and revealed that modern lead levels in those extremely remote regions were 300x higher than they were in the 1700s. Patterson also braved seasickness to chart the oceans and fish for tuna so that he could compare their lead levels against the tinned tuna on grocery shelves. Those tins were sealed with lead solder, which could contaminate the fish, and he found that there was as much as 10,000 times more lead in the tinned tuna than wild tuna. Patterson also studied Egyptian mummy bones and found they had roughly 600 times less lead in them than the bones of a modern American. Ultimately, he showed beyond all reasonable doubt that lead levels had soared since the rise of cars and the introduction of leaded gasoline. 

But Patterson was not alone in this quest; they may not have travelled to Antarctica with him, but medical doctors were learning more and more about the biological side effects of lead contamination. For example, high lead levels were linked to children having lower IQ scores, and eventually, other scientists began to cite Patterson's work and implement his lab practices. With all that science on their side, they forced governments into finally taking action.

In 1990, the United States' Environmental Protection Agency amended the Clean Air Act; leaded gasoline would be completely banned after a five year grace period. But unfortunately, Patterson died just three weeks before that ban took full effect. He lived his entire life in a world polluted by lead and spent much of his adult life working to eliminate it from his lab, and then from the environment. But that work saved an untold number of lives. In the 4.5 billion year history of Earth, it might be the best use of lead ever.

Thanks for watching -

- this episode of SciShow. And if you're one of the... 3,751 subscribers to SciShow's patreon, you get more than our thanks; you get our undying adoration because we would not be here without your support. You may not be like Clair Patterson, spending decades of your life fighting to stop us from inhaling lead day in and day out, but you are just as important to us. You are awesome.

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