Original article. Hazardous atmosphere in the underground pits of rice mills in Thailand

Background: Asphyxiation is the most dangerous hazard in confined spaces. In Thailand, several large rice mills have underground pits in order to transport moist paddy to baking machines. Prolonged unbaked paddy piles can produce several gases that can displace oxygen inside the pits.

Objective: This study describes concentrations of hazardous gases and oxygen content inside the underground pits of rice mills in Thailand.

Methods: At six randomly selected large rice mills in this study, fresh paddy piles were divided into four groups, based on moisture content (<14%, 15-20%, 21-30%, and >30% wet basis (w.b.)). Gas measurement was conducted with direct reading instruments including infrared spectrophotometer (Miran 1BX for CO₂, N₂O) and toxic gas detector (QRAE plus for CO, NO₂, % O₂ and flammable gas) at three different depth levels (top, middle, bottom) inside the pits.

Results: The concentrations of CO₂, CO, N₂O, NO₂, and % O₂ were in the ranges of 1.7-5438 ppm, 0-61 ppm, 0-4.9 ppm, 0-0.8 ppm, and 19.9-21.6%, respectively whereas concentrations of flammable gas (% lower explosive limit; [%LEL]) were not detected. The highest concentrations of gases were at the middle depth pit level, inside the paddy piles with the highest moisture contents (>30% w.b.). The detected concentrations of gases were higher than limit values based on time-weighted average but below limits immediately dangerous to life.

Conclusion: Although oxygen-deficient atmosphere was not detected under the working conditions of the survey. The findings demonstrated that concentrations of hazardous gases were dependent on both storage time and the moisture content of paddy piles. Measures should be developed to ensure that entry into these pits strictly complies with safety laws