Burning characteristics and emission products related to metallic content in incense

This study mainly compared particulate emission factors of nine popular incense. The influence of metallic contents upon the burning rate, ash and particulate emission factors of combustion was discussed. The contents of Ca and K were the highest among all metallic elements in the raw materials of the incense (the geometric mean contents 8.7 and 2.5 mg g−1, respectively), followed by Al, Mg, Fe, Na (0.1-1.0 mg g−1) and Ba, then Sr, Mn, Cu and Zn (<0.01-0.1 mg g−1). Most calcium existed as inorganic salts, such as CaCO3. Under the same burning conditions, the particulate generation rates are similar (0.50 mg min−1 ± 9%) among different types of incense-the shorter the combustion duration of a stick, the lower the total suspended particulate emission. Additionally, with the same incense weight burned, the greater the emission of ash is, the lower the emission of suspended particulate. It is recommended that consumers select incense which produces more ash, hence less particulate, to minimize the threat to their' health. Additionally, when the total metallic content (with Ca as the major component) was lower than 2% of the raw materials, properly increasing the total metallic content (from 0.5 to 2.0%) can effectively reduce the level of particulates (about 40%) during the combustion.