Prevention of nosocomial aspergillosis

Unfiltered outside air averages 1-15 pathogenic Aspergillus sp. colony forming units (cfu) m^-3 although short-term fluctuations are substantial. Seasonal variation reflects increased spore prevalence during periods of greater availability of non-viable matter. In hospital, airborne spores reflect incomplete filtration, infiltration of outside air and shedding of adherent spores from introduced objects. In highly protected hospital areas supplied with air filtered at high efficiency, where aspergillus cfus may be as low as 0·01 cfu m^-3, infiltration and shedding contribute a high fraction of ambient spores. Nosocomial aspergillosis occurs in linear proportion to the mean ambient hospital airborne spore content. An analysis presuming a steady-state dynamic equilibrium is imperfect because repeated sampling produces occasional high counts which violate a Poisson distribution. 'Mini-bursts' arise from disturbance of settled spores in dust, shedding spores from clothes or other subtle sources. These sources are best mitigated by increasing the air change rate. It is most important to protect bone marrow transplant patients, leukaemia and lymphoma patients undergoing intensive, potentially curative therapy. The optimal protective environments include high filtration efficiency, point-of-use filters, protection against infiltration and filter bypass, elimination of in-hospital sources, and high air change rates.