The aim of the study was to determine bacterial endotoxin concentration in the water flowing from a high-speed handpiece of a dental unit and in the air contained in the bioaerosol formed during dental conservative treatment. The air was collected in the space between the patient and dentist. The study was conducted on 25 operative sites (units) and had two stages: before application of a dental unit waterline (DUWL) disinfectant and after a 2-week application of disinfection procedure. The research showed that the mean concentration of bacterial endotoxin in the water flowing from high-speed handpieces was significantly reduced after the use of a disinfectant. The mean concentration of bacterial endotoxin in the air was similar at both stages - before and after application of waterline decontamination procedure. The study showed that in dental air-water aerosol, water is the main source of bacterial endotoxin contaminating the aerosol during the work with dental handpieces. Application of a user-friendly water disinfectant to significantly decrease endotoxin concentration in the DUWL water and in the aerosol, is one of recommended methods to reduce health risk.