While its instructions state that “When ten or more heaters are used inside a vehicle or shelter, ensure the ventilation system is operating or a top hatch or door is open,” it gives me peace of mind to let my flameless ration heater (a water-activated magnesium, iron, and salt chemical heating device for MREs) do its thing on my supper next to a cracked window when I’m indoors as part of its reaction produces hydrogen. Also, it pays to remember that these things get HOT!
