Research established the most significant mechanism associated with Tobacco Specific Nitrosamines (TSNAs) formation in flue-cured tobacco was a result of the exposure to nitrogen oxides (NOx) that subsequently reacted with the naturally occurring alkaloids in the green leaf. Curing with a direct-fired heating system was determined to be a significant source of TSNA formation in flue-cured tobacco and as a result the tobacco industry mandated a conversion to indirect-fired heating systems. This work began with the random monitoring of direct- and indirect-fired bulk curing barns to better understand the NOx concentrations that the green leaf is exposed to during the curing process. The predominate NOx species measured for a direct-fired system was nitrogen dioxide (NO2), but for an indirect-fired system was nitric oxide (NO). The combustion system modifications associated with a heat exchanger resulted in changing the primary NOx nitrosation agent from predominately NO2 to NO. Controlled exposure tests were conducted during the 2004 and 2005 seasons to evaluate differences in the cured leaf TSNA response as a result of exposure to NO or NO2 during curing. Two electric heated curing barns were utilized to perform the exposure study. The treated barn was dosed with a NOx species and the control was untreated. A total of six cures were conducted during the 2005 season of which three were dosed with NO and three with NO2. Over the two year study, a total of 6 cures were treated with NO2 and 5 cures were treated with NO. The TSNA levels in the cured leaf, averaged across both seasons, were approximately 1.891 ppm and 0.505 ppm for the cures treated with NO2 and NO respectively. The two year average TSNA level in the control barn was approximately 0.255 ppm. The adsorption or diffusion of NO2 in the green leaf and the consequent reaction with the alkaloids resulted in higher TSNA levels in the cured leaf as compared to NO.