An ISM band (Industrial, Scientific, Medical) transmitter or transceiver can be used. Or you can use a 802.15.4 (Zigbee), Bluetooth or other wireless type modules. Even cellular modem modules and satellite modules have been used in explosive atmosphere uses. A lot of this depends upon how you package and/or power the system. The type of radio used depends upon range, power consumption, data rate, and battery life you need. Premade radio modules that carry FCC (US) CSA (Canada) or other countries certifications as 'intentional radiators' can be bought off-the-shelf. Certification for the whole system will have to go through ATEX, IECEx, or other testing for hazardous environments.
There are complete systems out there ready to lease or buy that carry all the certifications. For LPG or CNG there are systems available for everything from residential to industrial tank applications.
