The Alan Dean Foster TFA novelization addresses that: it was because toxin filtration wasn't **standard** in First Order suits, but needed special filters - and in this tactical situation, he didn't expect stormtroopers to consider using them by default, or even to check if they are needed.

> Next to him, Rey was working hard to undo the results of her earlier repair. {{ to get the poison gases }}
“**This’ll work on stormtroopers?**” she wondered as she manipulated the tools she had used earlier and left behind.  
“**Standard issue helmets are designed to filter out smoke, not toxins. To cope with the latter, a trooper needs to engage one of several special filters, depending on the specific contaminant**. Identification is the province of one or two squad leaders. Having brought this ship on board theirs, I doubt anyone will think to check for airborne pollutants. It’s not like leading a ground assault, or forcing entry to an enemy warship. This is just an old freighter. **Any kind of internal defense, much less something as nebulous as a gas counterattack, would be the last thing a squad sent to take its crew into custody would expect**.”

To address the seeming contradiction, there are two possible answers:

- Either the air filters mentioned in the answer's source were just as optional as First Order ones (the wording is brief enough to be unsure either way)

- Or, the New Disney Canon simply disregarded the fact from EU/Legends, which the asnwer's source (*The New Essential Guide to Weapons and Technology*) is