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.
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.
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