Introduction Paraquat is hazardous chemical, widely used as herbicide, and was imported for 31 552 tons to Thailand in 2016. However, there was almost no report of inhalation risk assessment in Thailand. This cross-sectional analytic study was designed to quantify the inhalation exposure to airborne paraquat during spray operation of knapsack sprayers.

Methods The study was conducted in 30 voluntary herbicide knapsack sprayers in a District of Khon Kaen province, Thailand. The airborne paraquat concentration, working and personal characteristics of sprayers were used for inhalation intake calculation following U.S. EPA (1991) equation. The selected concentration of airborne paraquat was from monitoring with active personal sampling using PTFE filter membrane and analysed with HPLC.

Results The paraquat knapsack sprayers were farmers in sugarcane, cassava, rice, and corn field. Paraquat dichloride was used at 0.1–2400 litres/year. Adverse symptoms related to respiratory system were throat/upper airway irritation, runny nose (not from flu), wheezing, and difficulty breathing. The inhalation intake of paraquat exposure in short term effect, long term effect, and specific effect of lung (chronic pneumonitis) were calculated by using paraquat concentration at 125.49 µg/m³. The intake estimations were between 0.00011 to 0.04610 mg/kg/day. The health risk was presented by hazard quotient (HQ>1). HQ_{long term} was 0.263–115.25 when compared to recommended AOEL_{long term} (0.0004 mg/kg/day). HQ_{short term} was 0.211–92.202 when compared to recommended AOEL_{short term} (0.0005 mg/kg/day). HQ_{chronic pneumonitis} was 0.023–10.245 when compared to the reference dose (0.0045 mg/kg/day).

Conclusion It can be summarised that at the selected concentration and without using respirator of sprayers, this study found that 66.67%, 63.33% and 13.33% of Thai knapsack sprayers were under unacceptable risk of long term exposure, short term and chronic pneumonitis, respectively. This information should be communicated to the public health related institutes and farmers for seriously preventive regulation on inhalation exposure to paraquat.