Objectives: The inhaled endotoxin is considered as a causative factor in the process of acute bronchial obstruction, which can be measured by a decrease in forced expiratory volume in 1 s (FEV₁). The aim of this study was to assess endotoxin exposure among sewage treatment plant workers (STPW) and its effect on across-shift changes in respiratory airflow. Material and Methods: A group of 78 STPW from a large sewage treatment plant was studied. Inhalable dust for endotoxin assessment was collected using personal aerosol samplers. Endotoxin was assayed with the kinetic, chromogenic Limulus amebocyte lysate test. Across-shift spirometric measurements were performed on Mondays, after 2-days absence from work, with the use of portable spirometer. The forced vital capacity (FVC), and FEV₁ parameters were analyzed. Multifactor regression modeling was performed to determine parameters significantly associated with endotoxin exposure. Results: The concentration of inhalable dust and endotoxin ranged from 0.01–1.38 mg/m³ and 0.68–214 endotoxin units per cubic meter of air (EU/m³), respectively. Endotoxins were characterized with the skewed distribution (arithmetic mean (AM) = 38.8 EU/m³, geometric mean (GM) = 15.4 EU/m³, geometric standard deviation (GSD) = 4.21). Through the use of multifactor analysis, which excluded the main confounders (inhalable dust and smoking habit) it was found that, despite low levels of endotoxin, it had significant impact on the observed across-shift decline in FEV₁ (p = 0.044). For this parameter, the regression slope was additionally calculated (r = –0.017, p = 0.071). Conclusions: Relatively low levels of endotoxin among sewage treatment plant workers may cause small, but significant across-shift declines in FEV₁. The observed relationship was independent of organic dust concentrations and smoking habit. The respiratory protection should be provided for STPW.