Evaluation of Air Disinfectants

EVALUATION OF AIR DISINFECTANTS

The decontamination and disinfection of air is an important consideration for both infection and contamination control. A large number of important infectious diseases are spread via microbial contamination of the air. This cross-infection can occur in a variety of situations (hospitals and care facilities, airplanes, public and institutional buildings), while stringent control of air quality with respect to airborne contaminants and particulates is critical for contamination control in many aseptic procedures. With the increasing public concern regarding the perceived heightened threat of bioterrorism, effective air disinfection procedures have been reviewed as a potential counter-measure. The microorganisms themselves may be contained in aerosols, or may occur as airborne particles liberated from some environmental source, e.g. agitation of sporeladen bed linen, decaying vegetation, etc. Disinfection of air can be carried out by increased ventila-tion, filtration of air through high-efficiency particulate air (HEPA) filters, chemical aerosol/vapour/fumigation or by ultraviolet germicidal irradiation (UVGI). Although UVGI disinfectant approaches have demonstrated efficacy against a range of airborne pathogens and contaminating organisms, it is often more practical to use some form of chemical vapour or aerosol to kill them. The use of formaldehyde vapour is the most commonly employed agent for fumigation procedures (not strictly air disinfection), although vaporized hydrogen peroxide may be used as an alternative agent. Due to the potential for formation of carcinogenic bis(chloromethyl) ether when used with hydrochloric acid and chlorine containing disinfectants, formaldehyde should not be used with hypochlorites.

The work of Robert Koch in the late 1880s demonstrated that the numbers of viable bacteria present in air can be assessed by simply exposing plates of solid nutrient media to the air. Indeed, this same process is still exploited in environmental monitoring in the form of settle plates. Any bacteria that fall on to the plates after a suitable exposure time can then be detected following an appropriate period of incubation. These gravitational methods are obviously applicable to many microorganisms, but are unsuitable for viruses. However, more meaningful data can be obtained if force rather than gravity is used to collect airborne particles. A stream of air can be directed on to the surface of a nutrient agar plate (impaction; slit sampler) or bubbled through an appropriate buffer or culture medium (liquid impingement). Various commercial impactor samplers are available. Filtration sampling, where the air is passed through a porous membrane, which is then cultured, can also be used. For experimental evaluation of potential air disinfectants, bacterial or fungal airborne ‘suspensions’ can be created in a closed chamber, and then exposed to the disinfectant, which may be in the form of radiation, chemical vapour or aerosol. The airborne microbial population is then sampled at regular intervals using an appropriate forced-air apparatus such as the slit sampler. With viruses, the air can be bubbled through a suitable liquid medium, which is then subjected to some appropriate virological assay system. In all cases, problems arise in producing a suitable airborne microbial ‘suspension’ and in neutralizing residual disinfectant, which may remain in the air.