@rockandroll626 - The temperature is below freezing, so the metal canister for their oxygen is growing colder and frosting over - just like a can would if you put it in the freezer. The danger is when the valve on the BA freezes, and doesn't let any air through. It's not as dangerous to use the BA in freezing conditions, as gas contracts in the canister and doesn't expand. If you try and freeze a can of cola, for instance, you'd see the liquid expands - and breaks the can!
@rockandroll626 - The place that they're rescuing the boy from is a 'cold storage unit' - where frozen foods are stored in a warehouse. When supermarkets order food, they will load up their refrigerated lorries from these warehouses, which are kept way below freezing to preserve the food.
It's a warehouse that's has it's temperature set to below freezing to preserve the food. Since it's -26° Centigrade, it's lethal if you stay in there without warmth for more than a extended period of time. So they had to rescue him because the forklift was on top of him.
The valve that allows air from the tank into the mask is meant to open when you breathe in an close when you breathe out (so the exhaled breath goes through the vents on the mask) if yhe valve freezes and is unable to open the air supply is cut off and you can't breathe.
