Learn about UV Light
PURITY LIGHT IN FIELD TESTING RESULTS
UV light is a reliable, well-studied antimicrobial technology. It works primarily by destroying the DNA inside bacteria, viruses and fungi. The high-energy portion of the UV spectrum called UV-C is most effective. UV-C light has been used for decades to disinfect industrial surfaces and sanitize drinking water. It is especially advantageous for use in hospitals because it kills the spore-forming bacteriumClostridium difficile, which is a major source of hospital-acquired infections.
Whole-room UV disinfection systems were first introduced to US hospitals around 2007. Since then, popularity has surged because they sanitize practically all of the surfaces in a room at once, with minimal labor and without hazardous chemicals.
All of the devices produce UV light using either mercury-UV bulbs that run continously or xenon UV bulbs that pulse. Mercury UV bulbs primarily emit light at 254 nanomers, while pulsed xenon UV bulbs emit UV light at several different germicidal wavelengths.
Shapes, sizes, and features of UV room disinfection devices vary. Most are the size of a small refrigerator or office water cooler. Some run for short periods of time while others run longer. Certain devices run until UV sensors placed in the room measure a particular UV dose. Some have mirrors that focus the UV light as the beam rotates around the room. Some are controlled digitally by touch-screens, while others are more simple analog devices. Many have motion sensors which shut the device off automatically if a person enters the room during treatment.
All UV-C devices kill microorganisms to some extent, but with so many different configurations, features, run times, and UV wavelengths it can be difficult for purchasers to determine effectiveness. The goal of this article is to arm users and prospective purchasers of UV devices with the information they need to properly evaluate effectiveness.
HOW ARE UV ROOM DISINFECTION DEVICES REGULATED?
The United States Environmental Protection Agency (EPA) is the primary regulator of chemical pesticides and pesticidal devices, though FDA and various US States also take part. EPA defines microorganisms as pests, disinfectants as pesticides, and disinfecting devices as pesticidal devices. Pesticidal devices are not subject to pre-market approval by EPA, though EPA does require data supporting efficacy to be held on file. Companies that make UV devices must register with the Agency, then report how many units are sold each year thereafter.
EPA does not generally review or approve data supporting performance of UV devices before they are sold, so the onus is on infection control practitioners and hospital buyers to ensure the machines are killing microorganisms as promised. Careful evaluation of manufacturer claims is necessary to ensure the UV devices deliver the real benefit: reduction of hospital-acquired infections.
HOW IS THE PERFORMANCE OF A UV DISINFECTION DEVICE DETERMINED?
The main ways UV device companies to substantiate performance are listed below:
- Dose-response models, where UV-dose is measured, then used to estimate device effectiveness in hospitals.
- Tests conducted in microbiology labs, where rate-of-kill is measured for various pathogens under tightly controlled conditions.
- Environmental effectiveness tests, where hospital rooms are swabbed before and after UV treatment.
- Clinical outcome studies, where reduction in infection rates resulting from UV device usage is calculated.
Not all effectiveness data is equally reliable. The remainder of the article describes each category in detail, as it relates to marketing and use of UV room disinfection devices.
MATHEMATICAL DOSE-RESPONSE MODELS AND UV DOSE-METERS (DOSIMETERS)
Generally speaking, UV disinfection is a function of UV dose. The correlation is "log-linear," meaning a line is formed when microbial populations are plotted on a logarithmic scale at various treatment intervals. For instance, if a study were to begin with one million microorganisms on a test surface, it might show 100,000, then 10,000, then 1,000 viable cells after being treated with UV light for 10, 20, and 30 minutes.
The straightforward relationship between UV dose and disinfection is a blessing and a curse: It enables smart UV companies to build accurate dose-response models for their machines, but fools less sophisticated UV companies into thinking that no laboratory testing is necessary so long as they have a way to measure or estimate UV dose.
UV dosimeters have found a variety of uses in UV room disinfection. Some companies use UV dosimeters to "prove" their device has disinfected a room. Other companies use UV dosimeters to tell the device when to turn off.
UV dosimeters are most accurate when used to measure narrow-spectrum UV light, the kind of UV light that mercury bulbs produce. Dosimeters are not useful to measure high-intensity broad-spectrum UV light, since the brief pulses of broad-spectrum light exceed the measurement capacity of most dosimeters.
Predictions based on UV dose measurements are only as accurate as the dose-response model used to make the prediction. The use of data from even slightly dissimilar studies (different device, different bulb, different surface type, etc) can render predictions unreliable. Therefore, extra scrutiny should be applied to claims of effectiveness based solely on dose-response modeling, especially if the source data that serves as the basis for the model was taken from previous, unrelated studies.
ENVIRONMENTAL EFFECTIVENESS DATA
An obvious way to test the effectiveness of a room UV system is to swab surfaces in a room before treatment, then swab them after treatment and compare results. Such studies have the advantages of being relatively easy to conduct and measuring performance in the actual environment where the device is used.
Unfortunately, environmental swab studies are confounded by several problematic technical factors, described in detail below. Taken together, these factors make environmental swab studies some of the least reliable means of testing UV effectiveness.
The first major confounding factor of environmental swab studies on UV efficacy determinations is mathematical in nature. Initial microbial populations in indoor or hospital environments are often low. There are are frequently only about 100 total bacteria per 10 square centimeters of surface. That is not much of a challenge for many UV systems, meaning the extent of the UV effects may not be fully measurable. On top of that, lab techniques used to enumerate microorganisms on the swabs often result in a poor limit of detection, meaning that viable cells on the surface may not be detected if they are present in low numbers.
The second problematic aspect of environmental swab studies is related to microbiological technique. Populations of microorganisms often vary widely from one spot to the next, even on the same surface. If the same exact location were swabbed before and after treatment this would not be a problem, but the act of swabbing a surface or sampling it with a press-plate effectively cleans the surface, removing microorganisms in the process. Swabbing pressure and surface area are also variable. Even the best researchers find it challenging to swab different surfaces, yet maintain the same pressure and cover the same surface area. Doorknobs and sink handles, for example, are more challenging than a table section.
The third major issue with environmental studies is the impact of spore-forming, non-pathogenic bacteria on total bacteria counts. Approximately 50% of bacteria present on a hospital surface at any given time are spore-forming organisms such as species of the genus Bacillus. These types of bacteria are almost never pathogenic so they are largely irrelevant. However, they show up on virtually every total bacteria count agar plate in great numbers. These non-pathogenic endospores make it challenging for researchers to separate disinfection trends in studies from background microbial "noise."
CLINICAL OUTCOME DATA
As described above, dosimetry can substantiate claims if used carefully with dose-response data generated for the particular device under realistic conditions, in vitro laboratory studies are an excellent means of claim substantiation, and environmental effectiveness data is generally poor because of the technical problems that come with it.
Studies designed to assess reductions in infection rates in actual use are called clinical outcome studies, and are the last type of data UV room disinfection device makers use to substantiate claims. These studies are challenging to conduct because they require a great deal of time, planning and resources. When done correctly, they provide an excellent indication of device effectiveness.
UV device companies have reported several instances of infection rate reductions that correlate well with implementation of UV room disinfection. Several of these studies have been peer-reviewed. Prospective purchasers of UV devices should focus on peer-reviewed scientific studies because it is easy for companies to cite anecdote, but usually only "real" outcomes withstand critical review by a panel of technical experts.
The impact a UV room disinfection device will has on infection rates depends on several factors:
- Nature of the infections in the hospital (person-to-person or surface-mediated).
- Frequency of room disinfection.
- Actual effeciveness of the device (a function of contact time, technology, placement, etc).
Introduction of UV room disinfection to a hospital will not prevent all hospital acquired infections, but a good system introduced to solve a problem related to environmental contamination can reduce infection rates up to 50%. The infections that are not prevented are likely spread in ways that do not involve environmental surfaces as a pathogen reservoir. For instance, by doctors who forget to don gloves or wash their hands between patients,
The UV room disinfection device category is booming. Many different companies now make and sell UV devices, which have various levels of effectiveness. The different devices are based on one of two main UV technologies, mercury UV or pulsed xenon UV. Each device is designed to be used in a different fashion from the next.
Prospective purchasers of UV devices will benefit from learning the four types of data companies use to substantiate UV efficacy claims: dosimetry, in vitro studies, environmental studies, and clinical outcome data. Dosimetry is acceptable if used carefully, lab studies are excellent, environmental studies are riddled with technical problems, and peer-reviewed clinical outcome studies are fantastic, though costly and relatively rare.