Why does fire spread when we blow into it?


Fire needs oxygen to burn, so why does it spread when we blow carbon dioxide into it?

In: Biology

You breathe out a good amount of oxygen, too. Your lungs don’t absorb all of it. So blowing gently both introduces a bit of fresh oxygen and displaces the heat enough to spread the fire a little.

You’re not releasing so much carbon dioxide from your one breath that it will be enough to extinguish a fire (unless it’s a simple candle). On the other hand, your air *is* helping push the flame and heat faster than it might naturally have grown otherwise.

When you breathe out you’re not just breathing out carbon dioxide. You’re lungs are pretty good at getting co2 out and o2 in, but they do not pull 100% of the oxygen out of the air you breathe in in one go. So when you breathe out you’re still breathing out, blowing, a bunch of oxygen.

Also, that stream of air you’re blowing out is going to interact with the air outside and push Some of it along too.

Your breath still contains plenty of oxygen. Plus, it’s moving. The CO2 doesn’t really do anything once it’s made – it just sits there, not doing anything.

When a fire burns, the air next to it tends to lose its oxygen fairly quickly, meaning that the fire needs new air.

When you blow on a large enough fire, you’re adding new air to it. Sure, there’s less O2 in breath than atmosphere, but it’s still plenty for a fire.

(When you blow out something like a candle, the CO2 doesn’t really matter – it’s just that your breath is moving fast enough, to push away the hot flame and replace it with cold air. Do this enough, and the wick cools down and stops burning.)

You breath in 78% Nitrogen, 21% Oxygen, and 1% other stuff and gasses. You breath out 78% Nitrogen, 15% to 18% Oxygen, 4% CO2, and the remaining percent of other stuff and gasses. So most of what you breath out is Nitrogen and Oxygen. This is one reason why the “masks are bad for you” myth doesn’t hold up.