Microbial Decontamination of Ventilation Systems

Abstract

The American Conference of Governmental Industrial Hygienists (ACGIH) has issued a number of recommendations aimed at preventing abnormally high levels of bioaerosols. These include the elimination of stagnant water, microbial substrates, and microbial growth centres in ventilation systems, and the implementation of effective maintenance programmes. Disinfection tech niques (e.g., HEPA vacuuming, hypochlorite disinfection, and steam cleaning) are capable of eliminating microbial substrates and growth centres from ducts and other contaminated surfaces, but there is no information in the scientific literature on the optimum frequency of these operations. This study evaluated a number of corrective measures aimed at reducing aeromicrobial flora (bacte ria and saprophytic moulds). Samples of inside and outside air were taken at a laundry serving 30 hospitals. The outside air and that from 3 ventilation sys tems was sampled using a Burkard (Jet Spore) sampler, while an Andersen N-6 sampler was used to gather air samples at 6 workstations. Four 1-day sampling surveys were performed; in each case, samples were taken at least twice a day. In the first survey, carried out in June 1988, the contamination of outside air was found to differ significantly (p ≤ 0.05) from that of the air at some of the workstations, both qualitatively and quantitatively. The same workstations were re-evaluated in December 1988, 2 months after the ventilation system had been disinfected with hypochlorites and the operating conditions modi fied. No significant differences, either qualitative or quantitative, were observed between inside and outside air. The third sampling survey was undertaken in September 1989, 2 weeks after a specialized firm had per formed a complete disinfection of the system and ducts (HEPA vacuuming, steam cleaning, and disinfection with dilute hypochlorites). Visual inspection of the workplace and the ventilation system, now operating under normal con ditions, was also performed, in keeping with ACGIH recommendations. No significant difference was observed between inside and outside air. A 4th sur vey was conducted 8 months later, in May 1990, to verify the frequency of maintenance operations. Again, no significant differences were observed between inside and outside air. Ventilation systems should be disinfected before use, i.e., in the spring and autumn for air-conditioning units and heat ing units, respectively. The frequency of maintenance operations should be established on the basis of visual inspection.