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Indoor air quality is an important issue in health care facilities such as hospital operating theatres. The outbreak of avian influenza and SARS has given much warning to the community about the importance of minimizing the infection risk through airborne and/or direct-contact routes. In hospitals, the specialized ventilation systems for operating theatres and isolation wards are to provide comfortable and healthy environment for the patient and the surgical team. In Hong Kong, their design and construction are mostly based on the UK Health Technical Memoranda (HTM), the US Centres for Diseases Control and Prevention requirements, as well as the supplementary guideline of the Architectural Services Department of the Hong Kong Government.

Dr. Chow Tin Tai and his research team member Dr. Lin Zhang studied the ventilation provisions at the operating theatres in a sample of hospitals in Hong Kong. It was observed that the ventilation designs in some hospitals were not fully complied with the recommended specifications. "In most cases, these were because of the continuing revisions and tightening of the required standards, and the physical constraints of the building itself in the older hospitals," said Dr. Chow.

Numerical simulations were used to examine the importance of full compliance with the above mentioned technical specifications. The outcomes help to understand the limitations and the proper applications of the spec-ialized ventilation systems, and give a detailed evaluation of its health effects on people. For example, in some ultraclean ventilation systems, the velocity of the supply air at the perforated ceiling diffuser was found lower than the design specification. Comparative air flow studies were performed to examine the effects of changed supply airflow rate as well as the main and auxiliary medical lamp positions. Through computational fluid dynamics (CFD) analysis, the dispersions of infectious particles from both the surgical team and the patient were examined. The variations in supply velocity and medical lamp positions in practice were found to affect very little the thermal comfort environment. But on occasions, these may have severe effects on the movement of infectious particles, and hence on the cross infection risk.

It was also reckoned that full compliance with engineering standards not necessary implies a risk-free environment, as in the case of unpredictable SARS cross-infection. When treating a SARS patient, the protection of the surgical staffs and others outside the operating theatre is equally important. In 2004 a negative pressure operating theatre was made ready in one hospital of Hong Kong for the treatment of suspected or confirmed airborne infection cases. This was converted from a conventional operating theatre designed for positive pressure originally for stopping the particulate ingression from the outside. In the absence of relevant design guide, site measurements and numerical airflow study were performed. The simulation results show that the physical environment and the bacteria dispersion in this negative pressure operating theatre are as good as those in the one originally designed for positive pressure. The research outputs provide useful references for related installations in other hospitals.

Dr Chow Tin-tai

Division of Building Science and
City University of Hong Kong