The new US Tobacco GAP Program Flue-Cured Tobacco Barn Testing Protocol adopted by all the major tobacco purchasing companies requires that all barns have their heat exchangers checked and pass every three years by a certified operator using a calibrated CO2 meter. The results are to be maintained by the grower for review by his contracting companies and possible auditors. 

Some portions of this new Protocol may vary slightly from the information below which was developed at the time heat exchangers were installed in tobacco curing barns.

Recent research has shown that a class of carcinogenic (cancer-causing) compounds known as tobacco specific nitrosamines (TSNAs) may be formed in flue-cured tobacco leaves during the curing process. These compounds are not found in green (uncured) tobacco. Research suggests that TSNAs are formed through a chemical reaction between nicotine and other compounds contained in the uncured leaf and various oxides of nitrogen (NOx) found in all combustion gases, regardless of the fuel used. Eliminating NOx compounds in the curing air by using a heat exchanger system has been shown capable of reducing TSNAs to undetectable levels in cured tobacco. The direct-fire curing systems used in curing barns are considered to be the major factor contributing to elevated levels of TSNAs in flue-cured tobacco. Further, there is no known fuel treatment or burner design that can eliminate these nitrogen compounds from combustion gases without the use of a heat exchanger (found in all indirect-fired systems). It is believed that reducing the levels of TSNAs in tobacco products would reduce some of the health concerns associated with tobacco use.

To market tobacco in the U.S., producers must retrofit, or change, all barns used to cure the crop to operate with indirect-fired curing systems. An indirect-fired system passes the combustion gases through a heat exchanger and out of the barn, thereby preventing the mixing of flue gases with curing air. Systems with the combustion entirely outside the barn and that conduct the heat to the barn with hot water or steam have proven entirely satisfactory for reducing TSNAs and are acceptable.