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What is heavy water, and is it safe to drink? Heavy water, or deuterium oxide, has some pretty important scientific applications. But what would happen to your body if you actually drank it? Hosted by Hank Green.

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Mmmm, a refreshing glass of cold water. If you think I’m talking about H-2-O, then you’re right.

But I could be sipping on heavy water, or deuterium oxide, aka D-2-O, just as easily. Well, not just as easily… it’s super expensive, and you have to, like, buy it from lab supply stores, but you get my point. Deuterium is a form of hydrogen, or isotope, with a bonus neutron, which makes it twice as heavy as the regular hydrogen we all know and love.

That’s why when there’s deuterium in water molecules, it’s about 10% heavier, and we call it ‘heavy water.’ And one of our patrons on Patreon, who goes by Kutsop, wanted to know what would happen if you just drank a glass of heavy water. Well, in small amounts -- even a glass or two -- heavy water is fine. In fact, in combination with an oxygen isotope, it’s used in lots of nutrition experiments in humans because it’s a useful way to track how much energy you’re spending.

But don’t go filling up all your bottles with pure deuterium oxide, because too much will almost definitely kill you. Mice and rats die when about a third of the water in their bodies has deuterium in it. Plants also aren’t big fans, and will stop growing.

The toxicity is related to what’s called the kinetic isotope effect. Basically, the extra mass of the isotope slows chemical reactions down -- including all the super important reactions that keep you not dead. In the case of heavy water, the bonds between deuterium and oxygen are stronger than those between hydrogen and oxygen in normal water.

That means it takes more energy to break those bonds, which slows everything down. It’s not totally clear how much heavy water would do a person in, but experts estimate that anything more than 10% of your total water intake could start causing problems. So a sip is okay, but don’t push it.

Also it's not going to help you at all, and it's going to cost extra money. I don't really know why you would do this. Still, there is a form of heavy water you should avoid: tritium oxide, or super-heavy water.

Instead of hydrogen or deuterium, it contains tritium, an isotope of hydrogen that has two neutrons. And is radioactive. Now, as far as radioactive things go, tritiated water is pretty weak.

It emits low-energy , which are a type of electron that can’t penetrate skin. But drinking them is another story. There’s some evidence that once inside the body, beta particles might even be better at causing cancer than the higher-energy gamma rays that come from things like radium.

Scientists don’t actually have a great sense of how dangerous tritiated water is, they're not just putting it in cups and having people try it, but they assume exposure isn’t good. We all come into contact with a tiny amount of it naturally, which isn’t a big deal. But because tritiated water is used in nuclear reactors, the real concern is when those reactors leak, or if there’s an accident.

So, I’m not recommending you do this, but if you really want to test out some heavy water, make sure it’s deuterium oxide. Me however, I'll just stick with the stuff that comes out of the water cooler at work, because it's nice and cool and made up of normal things that I understand. If you liked learning about the weird chemistry of heavy water, you probably like getting smart and staying smart. is a problem-solving website that teaches you how to think like a scientist. Brilliant presents short, conceptual quizzes that supplement what you have seen on SciShow. A great way to retain that information is by actively solving problems on Brilliant.

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By breaking the problem into bite-sized concepts, thinking clearly through each part, and building up to the stunning conclusion, You better understand the process behind Dyson spheres. If humans keep a keen focus on advancing science and math, anything is possible. 4 million people are already using Brilliant, so join them in sharpening your STEM skills. To support SciShow and learn more about Brilliant, go to and sign of for free.