This study characterized the effects of smoldering incenses and combustion conditions on gaseous pollutant yields. Incense comes in three types: non-smoke (A), reduced-smoke (B) and traditional-smoke incense (C and D). Each incense type was burned in a test chamber with various combustion conditions (airflow rate and relative humidity). An extractive Fourier transform infrared (FTIR) was used to measure gas pollutants from smoldering incense in real time. Concentrations of methane, ethylene, methanol, formaldehyde and ammonia were measured using the IR spectra of smoldering incense samples. The resulting order of total emission factors of the identified gas pollutants (sum of methane, ethylene, methanol, formaldehyde and ammonia) were non-smoke < reduced-smoke < traditional smoke incenses. Total gas-pollutant emission rates and factors increased logarithmically as the airflow rate increased (2–28 L/min). Finally, the emission rates and factors of ethylene and methane decreased linearly as relative humidity increased (18–97%), while those for ammonia, methanol and formaldehyde increased. Results can be utilized to solve indoor air pollution problems caused by burning incense. Assuming that incense will continue to be burned when paying respect to ancestors, using incense made of low-volatility materials, with high carbon levels, low airflow rates and high environmental relative humidity can minimize gas-pollutant production.