ALTERNATIVELY you can use wet wipes, whether it be baby, cleaning or flushable, to wipe the dust off the blades and fan case. DO Not wipe circuit boards with them. Because they hold the dust to them they work well for cleaning fans, internal case surfaces, PSU cases, optical drive cases, SSD's and HDD's tops.
Due to me having a child still at home I keep the flushable Equate wipes around (they cost $3.25 for 3 packs). I use these things for cleaning all the time whether it is in the bathroom or the inside of my case. Like I mentioned before do not use them on circuit boards themselves, this is where you use compressed air, because they can leave some lint behind when they get caught on soldier joints.
EDIT: MarkW is correct with...