If you follow the manufacturer's directions for your fire pit & fuel components...

...propane is a very safe option. This fuel is heavier than natural gas and will sink to the bottom of your fire pit. That being said, it needs oxygen from both the surrounding area and an air mixer (which should be installed along with your gas line configurations) to help dissipate and lighten the gas. This is precisely why fire pits have vents built into them.

Additionally, the type of media that you use can also be of concern with regards to your fuel type. Ceramic river rock and pieces of fire glass, for example, will not allow the appropriate amount of air circulation for propane to disperse. It will trap the gas at the bottom of your fire pit, and this pressure can cause an explosion. Therefore, we highly insist on using lava rock with your propane feature. It is porous and lightweight and has proven to be the safest choice for this fuel type. Fire glass can be used with natural gas burning features ONLY. Fireside Expressions is adamant about this for the safety of our valued customers. 

Vented gas log sets can also be used with propane fire pits, as long as you do not cover the burner with lava rock. In this case, it is recommended that you add volcanic ash or cinders from about 1 inch below the bottom of the burner to 1-2 inches above the top. The entire setup can rest over lava rock.)

Having a certified gas technician install your propane fire pit is a very wise and preferred course of action. He or she can ensure that all of your connections and fittings are secure and in good working order. If however, you should hear a whistling noise once your fire is lit, this may be a good indication that there is a leak in your gas line. Shut off your feature and contact your gas technician to check it as soon as possible!