QUV/uvc Part III: What are key guidelines for developing UVC exposure tests?

In the last post on the QUV/uvc, we discussed two published standards with UVC exposure protocols, covering quite different applications. In this post, we’ll discuss designing QUV/uvc exposure methods for materials that encounter UVGI protocols in use, based on pandemic-era thinking regarding those UVGI protocols and other considerations.

Scientific literature on germicidal UVC exposures includes references to a wide range of doses for effective disinfection, from as low as about 2 mJ/cm2 up to 1 J/cm2, almost three orders of magnitude variation. One reason for the range is that some microorganisms are more resistant to the effects of UVC radiation than others, but significant differences are also linked to the “log reduction” of disinfection achieved or targeted in different studies and the large uncertainties surrounding many of the measurements. As a reminder, a 4-log reduction in a pathogen refers to a 99.99% kill rate, while a 1-log reduction refers to a 90% kill rate. 1-log reduction is also often called D90 disinfection and is a common threshold used in biological studies. Due to these differences, and to add a safety margin to the exposure, 1 J/cm2 has been proposed as a typical UVGI cycle. It should be noted that due to the non-uniform UVC irradiance in a real-life room exposed to a mobile germicidal UVC device, some materials may be exposed to much higher doses during a typical cycle.

Given this, what are some guidelines for running UVC exposure tests?

First, the total UVC dose in the test should be commensurate with the dose expected over the service lifetime of the material. In the case of materials exposed to UVGI cycles, as in the BIFMA standard, the expected number of cycles should be estimated and multiplied by the estimated UVC dose per cycle. Adding additional dosage may be prudent due to the non-uniform exposure issue noted above. For UVGI applications, the UVC dose required to achieve a given log-reduction of pathogens is shown to be independent of the irradiance, within the limits of lamp output capability and measurement accuracy. This implies that higher irradiance produces faster log reduction of pathogens. It is unknown whether material degradation due to UVC exposure proceeds similarly, and it’s likely that this characteristic is material specific, so reciprocity studies are strongly recommended to determine the linearity between results of low and high irradiance tests.  The QUV/uvc chamber can control the irradiance at any value between 1 and 13 mW/cm2 at 254 nm for such studies.

To simulate most UVGI applications, test temperatures should be limited to 30 °C because higher temperatures may promote thermal degradation that dominates overall material performance, thereby masking the effects of the UVC exposure. Higher-temperature UVGI exposures can be problematic in applications such as personal protective equipment (PPE) where items such as masks and gowns can degrade significantly at temperatures of 50 °C or higher. There may be cases, however, where heat and UVC exposure are combined for more effective disinfection, so test temperatures of 40 °C to 50 °C may be considered. For non-UVGI UVC applications, exceeding the maximum in-service temperature carries the same risks discussed above.

In the final post in this series, we’ll propose general QUV/uvc cycles and give ideas for new standardization as well as potential modifications to the existing methods.

A Series: Testing Durability of Materials Exposed to UVC Light
Read Part I.
Read Part II.
You are reading Part III.
Read Part IV.