UV Dosage Critical to Decontaminating Hospital Rooms of Pathogens

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.

Figure 1: Survival of Aspergillus niger under an irradiance of 17.8 W/m2. Based on data from Kowalski (2001) with a two-stage curve and a resistant fraction of 0.1.

It is important that the irradiance level be sufficient and the exposure time be long enough for the first stage of decay to significantly reduce the population. The data for Aspergillus suggests that an irradiance level of at least approximately 5 W/m2 is necessary for any significant reduction to occur after a minute or two of exposure (Kowalski 2001). The D90 value (90% inactivation dose) for Aspergillus niger is about 1250 J/m2.

Figure 2 illustrates the amount of time required by each of several popular area disinfection units to reach the D90 dose for Aspergillus niger. The IPT 3200 unit would develop this dose on a surface at one meter distance in about 18 seconds. The IPT 2280 unit would develop this dose about 36 seconds. The IPR 1140 unit (one single unit) would develop this dose in about 72 seconds.

Figure 2: Required Exposure Time to achieve a D90 (90% reduction) of Aspergillus niger at 1 meter distance from indicated units.

In Figure 2, the exposure times for the Tru-D, Clorox, and Xenex UV light disinfection units were estimated based on the best available information provided by the manufacturers. The Tru-D unit has a total UVC power output specified as 504 W. The Clorox unit includes four G643T5L UVDI lamps that are rated at 25 UV W each for a total of 100 W UV output. The Xenex UV disinfection unit produces approximately 68 W of UV output. All of these power output levels are illustrated in Figure 3.

Figure 3: UV power output of the leading area disinfection units. From left to right the power outputs are 1488 W, 942 W, 471 W, 504 W, 100 W, and 68 W.

Lower Dose UV Units Lack Reflective Power

A second problem with lower power UV area disinfection units is that the reflected levels of UV may be so low as to be useless in disinfecting surfaces within the required exposure time. The UV reflected off of walls and surfaces contributes to diffusing the irradiance in the exposed room and will expose shadowed areas. The walls and floors in healthcare environments are typically not very reflective for UV radiation and may have a reflectance of about 25% (white paint or enamel). The result will be an approximate reduction of 75% of the irradiance after each bounce. Clearly after two or three reflections there will be little irradiance left to diffuse into the room. One practical option is to coat the walls and surfaces with a UV-reflective coating. Another option is to use the highest levels of UV irradiance, as the IPT units do, so that the reflected irradiance will still have the capacity to eradicate pathogens hiding in shadowed zones. Simply extending the exposure time for lower power UV disinfection units like the Xenex and Clorox units, may be inadequate to eliminate shadowing effects, and as discussed earlier, may fail to overcome the inherent resistance of those microbes that have a shoulder response in their decay curves.

Higher Dosage UV Disinfection Units Most Reliable

In conclusion, the safest and most reliable approach to eradicating hardy spores like Clostridium and Aspergillus, and UV-resistant bacteria like Acinetobacter is to maximize the dose by maximizing the irradiance levels. The IPT line of area disinfection units, which clearly produce the highest UV dosages and levels of irradiance of all the area disinfection units available today are the best choice for hospital personnel serious about reducing the risk of healthcare acquired infections.

  

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