Instead, fans are generally classified by "spark resistance". AMCA has created a standard that defines three different levels of spark resistance, classes A, B and C.
These classes, listed in decreasing order of assurance, are generally concerned with the prevention of sparks caused by the rubbing together of spark-producing metallic components (generally ferrous materials). These classes only address spark risks due to an explosive airstream, and do not address explosive conditions outside the fan. A summary of the different levels of protection is found in this helpful engineering paper from Twin City Fans.
Type C: The fan is designed so that if the impeller or shaft comes loose and shifts during operation, two ferrous parts will not come into contact.
Type B: In general, this requires a nonferrous impeller and a nonferrous rubbing ring around the shaft hole. Also, extra locking systems are required to prevent the fan impeller, shaft, and bearings from shifting.
Type A: This requires a nonferrous airstream. Also, the extra locking systems are required as in Type B.
As with any engineering decision, the correct level of spark resistance to specify depends strongly upon the particulars of the project: The gasses or substances expected, the concentration of these contaminants, the location of the air-moving device, etc.