The objective of this study is to determine the concentration of reactive oxygen species (ROS) generated from burning incense in an experimental chamber with a volume of 288 liters and a continuous supply of filtered air at 15 1/min. A Micro-Environmental Monitor (MEM) collected PM1 and PM2.5 from the incense smoke on 37-mm polycarbonate membrane filters. The ROS in particles were extracted using dichiorofluorescin-horseradish peroxidase (DCFH2-HRP) reagent. Additionally, two parallel sampling trains, each consisting of a filter cassette and 3 impingers connected in series, were employed to simultaneously collect ROS in particles and in gas phase. Each impinger contained 10 ml of the DCFH2-HRP reagent for absorbing ROS in gas phase. The samples obtained by filter cassette were treated as the MEM samples. Subsequently, the extracts and impinger samples were incubated at 37°C for 15 min and the fluorescence intensity of resulting dichlorofluorescein (DCF) was determined. Fluorescence intensity data were converted to equivalent H2O2 concentrations using calibration curves obtained from fluorescence measurement of standard DCF solutions. For samples collected 1 hr after one joss stick of black lignaloes was lit in the experimental chamber, the mean ROS concentrations in PM1 and PM2.5 were 15.60 ± 1.00 and 13.50 ± 1.30 nmol H2O2/mg of particles, respectively. In contrast, the mean ROS concentrations in PM1 and PM2.5 sampled from an apartment (free of incense smoke, cooking fumes, and cigarette smoke) were 6.54 ± 1.00 and 4.06 ± 2.04 nmol H2O2/mg, respectively. When one joss stick of black lignaloes was burned for 1 hr in the experimental chamber, the mean ROS concentration was 0.94 ± 0.06 nmol H2O2/l of air in gas phase and 1.06 ± 0.05 nmol H2O2/I in particles. The finding could have important health implications because particulate ROS in inhaled air could easily reach the alveolar region.