system
Given the following post, respond as if you're commenting on Reddit:
human
[batman] Why doesn't Batman use fear gas?

You'd think if he can cure scarecrow's gas, he'd be able to synthesize it. Why doesn't he just get a gas mask and use fear gas against his enemies?

Side note: why doesn't he just wear a gas mask to avoid getting gassed in the first place?
gpt
Some versions of Scarecrow's fear gas can be absorbed through the skin, and most versions are too fine to be filtered out by a gas mask.  The only way to protect oneself from those versions of the gas is a hermetically sealed suit with its own air supply.  While such a suit is feasible for Batman, it's not practical since O2 tanks are heavy and the suit's seals can still be ruptured by damage, turning it into a significant hindrance.  While Scarecrow does use a gas mask to limit his exposure, he's usually immune to his own gas anyway.  It's likely that any functional mask scarecrow wears is actually using an antidote to neutralize the toxin rather than attempting to filter it out.  

Scarecrow also frequently uses other delivery methods for his toxin such as pills, darts, or liquids.  A gas mask doesn't help when he can get it directly into your bloodstream.

Batman usually doesn't need fear toxin to incapacitate his enemies, and it's not really the wisest thing to use liberally since a squad of joker goons suddenly exposed to it will act unpredictably and are just as likely to shoot each other or civilians as the are to cower.  In the Frank Miller book Dark Knight Returns, Batman does use something with a similar effect to Fear Toxin at one point but it doesn't actually work the same way.