Think a mask a good idea if the whole lot's mouldy - farmer's lung. This snow is a good opportunity to find your rats' nests - they have a long, slender foot and will quickly run a channel through the snow about 10-12cm wide, generally starting at the edge of a barn or under a pallet. That's the place to put your rat bait trap or, if it's a hole, just pour a teacupful down it, covering with a bit of wood or tile to stop hens or wild birds eating it. The rats may take a while to eat it but they will, especially in this weather.