As has already been mentioned, you need to have a recognized certifying authority test your device to ensure that it meets the requirements for Intrinsic Safety.
Your power level is fine but you have to ensure that the total amount of stored energy is below the limit for your voltage rating. That means that capacitors can't be larger than some value; same for inductors.
Another option that you can explore is to use a certified Explosion-Proof enclosure. Again, there are rules that specify how the connections are made to such an enclosure (epoxy-sealed conduit, etc). However, if you are building only a few devices, this can be a less expensive method than going for full Intrinsic Safety certification.
FWIW - it costs us anywhere between Can $5,000 to $10,000 for Intrinsic Safety certification for each of the hazardous-location devices that we manufacture. There are also quarterly inspections that each cost several hundred dollars for devices that are type-approved.