"thats just over the top silly, i wonder if they really think they need them? "
I thought it was ridiculous too, but apparently its to avoid poisoning from CO2 intense grows, where the levels can fuck you up...
Here in south uk the new police thing seems to be that they reckon vietnamese gangs booby trap and mine [!] their grow houses [EDIT: oops I mean 'cannabis factories'] in case the fuzz show up, so when they raid they sometimes do it real sneaky and careful, like HAZMAT + bomb squad combined into the ultimate paranoid bastard team