Inhalation exposure and non-carcinogenic risk assessment of bacterial and fungal bioaerosols among wastewater treatment plants workers.

Workers in wastewater treatment plants (WWTPs) are routinely exposed to airborne bacterial and fungal bioaerosols generated during wastewater processing. Although bioaerosol exposure has been widely reported, integrated assessments that capture spatial variability across treatment units, diurnal work shifts, and combined bacterial-fungal exposure remain limited, particularly in developing-country WWTPs. This study aimed to evaluate occupational inhalation exposure to culturable bacterial and fungal bioaerosols in municipal WWTPs and to estimate potential non-carcinogenic health risks using hazard-based metrics. A cross-sectional study was conducted in August 2024 at the Sanandaj WWTP, Kurdistan Province, Iran. Airborne bioaerosols were sampled across ten operational units, including influent wastewater, screening, grit chamber, aeration tank, settling units, anaerobic digester, and office areas. Sampling was performed over four consecutive working days during morning and evening shifts using a BioStage single-stage impactor connected to a calibrated Quick Take 30 pump. A total of 320 air samples (160 bacterial and 160 fungal) were collected at breathing-zone height (1.5 m). Non-carcinogenic inhalation risks were estimated using hazard quotient (HQ) and hazard index (HI) approaches based on standard exposure assumptions. Culturable bacterial bioaerosols were dominated by Gram-positive bacilli (~ 41%), followed by Gram-negative bacilli (30-36%), Gram-positive cocci (17-23%), and Gram-negative cocci (4-8%). Mean bacterial concentrations were highest in the aeration tank (921.67 CFU m⁻³) and grit chamber (743.52 CFU m⁻³), while the activated sludge bed and office areas showed minimal concentrations (~ 20 CFU m⁻³). Nineteen fungal species were identified, with Aspergillus niger, A. flavus, Mucor, and A. fumigatus being most prevalent. Comparisons between high-exposure units (aeration tank and grit chamber) and low-exposure areas (office building and activated sludge bed) showed a very large contrast in mean bioaerosol concentrations (Cohen's d = 2.14; 95% CI: 1.61-2.66; p < 0.001). HQ values were highest in the aeration tank (0.60) and grit chamber (0.49). The cumulative HI for the facility was 1.9, indicating exceedance of the selected reference concentration. The findings revealed marked spatial and temporal variability in bacterial and fungal bioaerosol exposure within WWTPs operations. While the estimated HI suggests potential concern, results should be interpreted cautiously due to methodological uncertainties. Targeted exposure control measures, routine monitoring, and future studies incorporating molecular techniques are recommended to better characterize occupational bioaerosol risks.