High-speed surgical instruments and electric coagulator devices produce contaminated particles and aerosols. This study aimed to investigate the presence of blood-contaminated aerosol in a class 6 and 7 (ISO14644-1) operating room environment. Test filters were fitted to the extract ducts of a standard central air conditioning system, and were left in place for 1, 2, or 4 weeks. In a separate experiment to analyze contributing factors, the test filters were changed following every operation. Green dots showing a reaction with leucomalachite green method were counted. In the accumulation experiment, the number of positive dots tended to increase over time (26, 60, and 143 dots after 1,2, and 4 weeks, respectively). The distance between the test filters and surgical field ranged from 3.8 to 4.6 m. In our analysis of contributing factors, positive filters were presenting of 10 surgeries that used high-speed instruments and in 16 of 17 using electric coagulator devices. Surgical procedures performed with these devices produce aerosolized blood, which can float in the operating room. Thus, caution must be taken by perioperative staff, such as anaesthetists and surgical assistants, as well as by surgeons. (C) 2010 Elsevier B.V. All rights reserved.