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When you hear the words “carbon monoxide,” nothing good probably comes to mind. And that's… pretty reasonable.

I mean, carbon monoxide can be a deadly gas:. It binds to the hemoglobin in your blood and stops it from moving oxygen around your body. That's one reason smoking cigarettes is so harmful:.

It causes this stuff to build up in the bloodstream. In fact, people who consume two or more packs a day can have at least 6 percent of their hemoglobin bound up by carbon monoxide. And it only takes reaching 20 percent to see serious symptoms kick in, like headaches, dizziness, and shortness of breath.

But then, there are elephant seals, who are out here challenging our ideas. They naturally have elevated levels of carbon monoxide in their blood sometimes as much as 10 percent. And the gas is thought to protect them.

And if we can understand more about how, we might be able to use carbon monoxide as an effective therapy for heart attacks and stroke. The first thing you notice about elephant seals might be the males' noses. And well… there is a reason they're called elephant seals.

But these animals are also some of the most extreme divers in the animal kingdom. While at sea, they spend 90 percent of their time underwater, often diving for 20 to 30 minutes at a time, to depths of 500 meters. Then, when they surface, they'll only take a two- or three-minute breather before going down again.

On top of that, these creatures also have bad cases of sleep apnea:. While asleep on land, they'll spontaneously hold their breath for 20 minutes or more. When they're holding their breath this long, their bodies reduce the blood flow to non-essential tissues and organs, to keep oxygen flowing to the more important bits — like the brain and heart.

It's a smart move, but when oxygen comes flooding back to these tissues, it still has the potential to cause a lot of harm, thanks to reoxygenation injuries. Which, like the name suggests, is an injury caused by blood returning to tissues. In a sort of complex cascade of events, reoxygenation can lead to inflammation, problems with regulating blood pressure, and blood clots.

It can also cause cells to die before they should, leading to dead tissue inside the body. Except… even though the seals are constantly holding their breath, they don't suffer from these injuries. And evidence suggests it has to do with the carbon monoxide in their blood.

Now, the gas doesn't come from their secret smoking habit:. Instead, they make it themselves. And actually, all mammals do.

We all have low levels of carbon monoxide in our bodies, made as a byproduct when red blood cells break down. For instance, in our blood, around one percent of hemoglobin is bound to carbon monoxide all the time. And we keep the number from getting higher by breathing out the excess gas.

But elephant seals are a special case. They have one of the highest concentrations of red blood cells and hemoglobin of any mammal. So, with more cells to break down, they also make more carbon monoxide.

And that leads to up to 10 percent of their hemoglobin being bound up. Also, because they spend so much time holding their breath, they don't release the excess gas like other mammals do. But that might be great for them, because researchers believe these higher levels are most likely protective.

It seems counterintuitive, but having some levels of carbon monoxide in your body is actually good! We're still figuring out the exact mechanisms behind how it works, but we know carbon monoxide kicks off changes from the cellular to the whole-body level. It's been shown to be anti-inflammatory, lower blood pressure, and prevent dead tissue from building up, by stopping cells from dying when they shouldn't.

And if that sounds familiar, there's a good reason:. A lot of the damage caused by reoxygenation injuries is something carbon monoxide protects against. So, researchers believe that, because elephant seals have such extreme diving habits, they've evolved high levels of carbon monoxide in their blood to protect them from these injuries.

And there might be something we can learn from this, too. If we can understand how these higher levels of this gas help the seals, we could learn to confidently use carbon monoxide as a therapy for humans. After all, reoxygenation injuries are often seen during heart attacks, strokes, and organ transplants cases where tissues are deprived of oxygen for a long time.

Studies have found that exposing patients to low levels of carbon monoxide decreases the risk of these injuries, and it's already being investigated as a therapy for organ transplants. But elephant seals could also help us better understand the mechanisms of how carbon monoxide works in the first place. So… while you should absolutely avoid large amounts of carbon monoxide in everyday life, it turns out that this gas may not be all bad.

In the right amounts, under controlled conditions, it could be extremely helpful. We just have to learn about how. This episode of SciShow is brought to you by our patrons on Patreon.

Patrons make SciShow happen, and they're also a community of smart, thoughtful, curious people. We love hanging out with them in the Discord, and we couldn't ask for a better team. If you want to support free science education on the internet, you can learn more at Patreon.com/SciShow. [♩OUTRO].