Approximately 50 percent of tobacco produced in the U.S. is flue-cured, and propane is the primary fuel used in the process. Almost all bulk-curing barns built since the early 1970s are equipped with propane gas burners.

Combustion gases have been found to produce tobacco-specific nitrosamines, which are deemed to be carcinogenic. Producers are now required to retrofit, or change all flue-curing barns to operate only with indirect-fired curing systems. In these systems, direct mixing of flue gases with curing combustion gases is prevented by passing combustion gases through heat exchangers or by allowing combustion to take place outside of the barn with the resulting heat being conducted into the barn via hot water or steam.

Since some heat loss is unavoidable with indirect curing systems, it is very important for growers to gain as much heating system efficiency as possible in order to control fuel costs. The single greatest reason for burner inefficiency is too little or too much air. Many growers have switched from fuel oil to propane because propane systems are more efficient - propane gas and air mix better during combustion than fuel oil and air. This allows for a lower excess air setting on the gas burner and improves the efficiency of the system. On average, a well-maintained and operated barn will cure approximately 9 pounds of leaf per gallon of propane.