Hank Green:
    As elements on the periodic table go, lead is pretty handy. It's not exactly oxygen or nitrogen, but it has some admirable qualities. Like, it's heavy but soft and malleable, in the form of lead dioxide, it's highly conductive, which makes it useful in batteries, and it's so incredibly dense that it's great at blocking out things like ionizing radiation. Lead is good for all sorts of things, but it is also poisonous, really, very poisonous, and for the last 5000 years of so, humanity has been so enamored of all of its wonderful traits that we've been killing ourselves with it.
    When inhaled or ingested, lead is readily absorbed into the blood, where it can slow down the transport of oxygen and eventually build up in your kidneys, brain, liver, heart, and especially bones and teeth. Once it's in the body, lead not only displaces metals you actually need, like calcium, iron, and zinc, but it's excellent at blocking receptors for glutamate, the most important neurotransmitter for normal brain function and learning. Lead poisoning can affect nerve transmission to the brain and cause abdominal pain, anemia, and the loss of developmental skills in children. 

    So what makes this naturally occurring heavy metal so valuable that we overlooked its terrible side effects for so long? Well, because of its nice combination of softness, durability, and availability, the ancient Romans and Greeks used lead in practically everything, from their plumbing and eating utensils to their writing tablets. But the Romans in particular really liked to eat it. 
    Don't test this out, but lead actually has a naturally sweet taste, and Romans sprinkled lead acetate into their wine to sweeten it. They called it sapa, and it was consumed in massive quantities by the aristocracy. It was also found to be an effective preservative because, you know, it kills stuff. According to some historians, the common use of this lead additive caused widespread infertility and dementia in ancient Rome, especially among the ruling class. So think about that next time you're wondering whether aspartame is bad for you; the Romans wanted sweeter wine so bad they put lead in it.

    Lead has been used in paint for thousands of years as well, and it's so useful that we kept using it through the late twentieth century, well after we understood its dangers. In particular, lead carbonate, otherwise known as white lead, was widely used because it made paint so opaque that a small amount of lead-based paint could cover a large area. It's also highly insoluble, meaning it doesn't dissolve in water, making it great at resisting the moisture and mildew that can cause other paints to crack over time. Lead carbonate can prevent decomposition in the oils that make up paints, allowing them to look fresh longer, and lead-based paints dry faster than their counterparts. 
    There was so much to like about lead paint, except for the part where it killed a bunch of people -- because people did get poisoned from lead paint, especially children, either from inhaling dust with flakes of the paint in it or jut because, as previously mentioned, lead makes things sweet and kids like sweet things, even if the sweet things are paint chips. The federal government finally banned the use of lead in paints in the U.S. in 1978, but lead was still all over the place, particularly in the air, because of cars.

    In addition to its uses as a wine sweetener and a paint improver, lead offered even more benefits as a gasoline additive in the form of a compound known as tetraethyl lead. In the early 1920s, carmakers were looking for a solution to engine knock. The engines work best when the fuel in the cylinder burns all at once and at the exact moment the spark plug fires, but sometimes, especially in older cars or with cheaper fuel, the fuel would actually ignite before the spark plug fired, damaging the pistons and cylinders in the engine and making an audible pinging noise. The tetraethyl lead increased the ignition temperature of the gasoline, eliminating pre-ignition, but, of course, the lead was released in the car's exhaust, which was then inhaled by people and absorbed into their bloodstreams.
    As with paint, it took a while for the problems associated with leaded gas to show up, but some clues came early, like the fact that workers who handled tetraethyl lead often experienced hallucinations and many of them died at a young age. So in 1974, the Environmental Protection Agency developed guidelines for eventually eliminating leaded gasoline. It wasn't easy; other additives had to be developed for cars that had been designed to run on leaded gasoline, many of which had their own problems, but in 1996, the last leaded gasoline was sold in the United States, and most gas stations still use labels to assure everyone that even their cheapest fuel is unleaded. 

    So quick review: if you're using lead to shield yourself from X-rays or make batteries or something like that, you're probably doing it right, but if you're using lead in any way that winds up in your mouth or up your nose or in any of your face holes, then you're using it wrong.
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