Can anyone tell me what the requirements are for explosionproof equipment located outside. I am installing a circuit to a propane transfer pump. The owner want the switch mounted on the outside of an adjacent building located about 18 feet from the tank and pump. Would that switch need to be hazardous rated?

If you're mounting the switch more than 18" above the ground, you should be okay at that distance. Don't forget to use IMC or GRC and the appropriate sealing fittings for the conduit run.
And I would suggest you run it past your local AHJ just to avoid having to change it after you're all finished with the installation.