Because it lets you store a huge volume of gas as a liquid, which you can’t do with air.
The chemical in air dusters can be easily compressed into a liquid with only a reasonably small amount of pressure. It’s like how lighters are filled with liquid butane. [In the clear ones you can even see the liquid inside](https://www.qualityimprint.com/cdn/shop/products/Q177711-transparent-green-lighters-with-logo-3_grande.jpg?v=1547584685). When you open the nozzle some gas comes out, and a tiny bit of the liquid boils until the pressure is restored.
The advantage is that liquids are hundreds of times denser than gases, can be even 1000x or more. That means the air duster can hold like 1000x more volume of gas this way vs if it was pressurized air. With pressurized air it would have to be a heavy thick steel tank at hundreds of PSI and you still wouldn’t get anywhere near as much gas vs the chemical they use where a whole room volume of gas can be easy compressed into a small canister of liquid at less than 50 PSI.
**TLDR:** The chemical isn’t that dangerous, compared to the downsides of a compressed air can that would be heavy, a potential pressure bomb at hundreds of PSI, *and* also run out in about 30 seconds of spraying.
Bonus: The propane in BBQ tanks is pressurized into a liquid too. Once again, by using propane as the fuel this lets you store a gigantic volume of gas in a small volume and at a much safer lower pressure.
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