Hazardous gases and oxygen depletion in a wet paddy pile: an experimental study in a simulating underground rice mill pit, Thailand.

During the rice harvesting season in Thailand, large amounts of fresh paddy are sent to rice mills immediately after harvesting due to a lack of proper farm storage space. At certain levels of moisture content, rice grains may generate hazardous gases, which can replace oxygen (O(2)) in the confined spaces of underground rice mill pits. This phenomenon has been observed in a fatal accident in Thailand. Our study aimed to investigate the type of gases and their air concentrations emitted from the paddy piles at different levels of moisture content and duration of piling time. Four levels of moisture content in the paddy piles were investigated, including dry paddy group (< 14% wet basis (wb)), wet paddy groups (22-24, 25-27 and 28-30%wb). Our measurements were conducted in 16 experimental concrete pits 80 × 80 cm wide by 60 cm high. Gases emitted were measured with an infrared spectrophotometer and a multi-gas detector every 12 h for 5 days throughout the experiment. The results revealed high levels of carbon dioxide (CO(2)) (range 5,864-8,419 ppm) in all wet paddy groups, which gradually increased over time. The concentration of carbon monoxide (CO), methane (CH(4)), nitromethane (CH(3)NO(2)) and nitrous oxide (N(2)O) in all wet paddy groups increased with piling time and with moisture content, with ranges of 11-289; 2-8; 36-374; and 4-26 ppm, respectively. The highest levels of moisture content in the paddy piles were in the range 28-30%wb. Nitrogen dioxide (NO(2)) concentrations were low in all paddy groups. The percentage of O(2) in the wet paddy groups decreased with piling time and moisture content (from 18.7% to 4.1%). This study suggested that hazardous gases could be emitted in moist paddy piles, and their concentrations could increase with increasing moisture content and piling time period.