Good point on the explosion proof fan. The motor should definitely be explosion proof, and there are three different classes of spark resistant fan construction. Class A is everything is made of non-sparking material (usually aluminum or possibly plastic) and constructed so no rotating part can touch a non-rotating part. Class B requires that the wheel be non-ferrous and constructed to minimize contact with not-rotating ferrous metals (usually an aluminum rub ring is installed.) Class C is constructed to minimize contact between rotating and non-rotating ferrous materials (again an aluminum rub ring is typically installed.) These ratings assume that something goes wrong like the wheel comes loose from the shaft. For short term service, Class C is least expensive and probably fine for what you're doing.
