Area UV disinfection units have proliferated as of late due to their effectiveness and ease of use in decontaminating hospital rooms and surfaces of nosocomial pathogens of HAIs (Hospital Acquired Infections). However, the levels of UV irradiance and UV dose produced can vary widely from unit to unit. It is routinely assumed that the lower irradiance levels produced by some of these area disinfection units can be compensated for by increasing the time length of the exposure, a principle that is known as reciprocity. Reciprocity, however, may not apply to very short exposure times or to very low levels of irradiance due to the fact that some microorganisms can resist and cope with low UV doses and even recover via repair mechanisms (IUVA 2005). Herein we examine the effects of UV dosages produced by typical area disinfection units including the IPT™ whole room UV light disinfection systems.
Longer Exposure Times Ineffectual for Lower Dose UV Units
Some lower power UV area disinfection units are required to operate for longer time periods to achieve the desired level of disinfection. The problem with extending the exposure time is that it assumes reciprocity, in which the irradiance multiplied by the exposure time is assumed to have the same impact on UV dose regardless of the level of irradiance. This is not always true and systems with low irradiance levels may not be equated with systems that produce high irradiance levels due to the fact that the response of microorganisms is not strictly a function of UV dose when the irradiance level is too low. Many microbes, especially spores like Aspergillus and Clostridium, can have a shoulder in their decay curves that delays the response to UV exposure. The lower the irradiance, the longer the shoulder. This is illustrated in Figure1 for Aspergillus niger where the shoulder is manifest for several seconds before the first stage of decay develops. Aspergillus is a common contaminant in hospital environments and is often found in operating rooms. It tends to settle on floors and horizontal surfaces and is re-aerosolized by activity.