<rant>
I searched for why people do this 1337-substitution [1], and apparently the justification's that "C02" can be quicker to type on some mobile keyboards since "0" is a single press while "O" requires hitting Shift too.
But in chemistry, we'd often write chemical formulae in linear format. For example, "H2O" is water while "C6H12O6" refers to some simple sugars. The basic rule is that digits after any letters are in a subscript position, while digits before any letters are a scalar multiplicand.
So, when I see "C02", I immediately parse it to "{C}_{02}" rather than "{C0}_{2}". This reduces to "{C}_{2}", which obviously doesn't make sense, so then I have to figure out what's going on. Obviously it's not too tough to figure out from context that "CO2" was meant, but since I have to infer that from context rather than from direct parsing, it comes off as an obnoxious typo. For example, if someone wrote "C2O", I could figure out what they meant from context too, but it's still annoying to have to be like, "What?".
Then I don't even know why it exists. I mean, who talks about CO2 so much more than other chemical species to memorize a 1337-alteration that saves them having to hit a Shift key? It seems like a rather niche micro-optimization that certainly can't be worthwhile to too many typists, even if we ignore the obnoxiousness it imposes on readers.
Which, tl;dr, my point's just that "C02" might be a cute 1337 substitution if you don't usually read a lot of chemical formulae, but if you do, it invokes a parsing error that you either have to consciously resolve or else memorize a new slang term to avoid.
</rant>
I didn't mean "why don't you just open a window."