More people than ever are utilizing ultraviolet UVC irradiation in their homes and businesses. This increase in popularity is a direct result of the response to COVID-19 over the last eight months.
One of the most common questions we receive is how to measure UVC dose or intensity at a certain distance. This is an important question, and mathematically modeling UVC intensity at certain distances can be complicated or expensive with a UVC light meter.
Thankfully, UVC dosimeters have been available and affordable for years. They can be used for single measurement applications like prototyping and testing, as well as on a continual operation basis.
Organisms and Regulations
There are two important reasons to consider UVC dosimeters.
First, many organisms have been tested and a certain UVC dose has been determined for deactivating or killing 99% or more of a population of that microorganism.
Second, recommendations have been published by the CDC for using ultraviolet germicidal light to sterilize N95 masks for reuse. While the shortage of masks is mostly over, there is little drawback to extending the life of a mask.
According to Signify and Boston University, the UVC dose required to deactivate 99.9999% of COVID-19 is 22mJ/cm2. Using a 25 or 50mJ/cm2 dosimeter card at the same distance as the surface being sanitized with UVC should guarantee similar deactivation rates.
MRSA and C-diff
Two common bacteria that cause serious conditions are Methicillin-resistant Staphylococcus aureus (MRSA) and C. difficile (C-diff). MRSA has a deactivation rate around 50mJ/cm2, while C-diff is 100mJ/cm2. For medical settings, using dosimeters that measure up to 100mJ/cm2 can be vital for reducing the risk of infection.
CDC N95 Mask Guidelines
For reusing N95 masks, the CDC examined various methods of disinfection, including ultraviolet light. The recommendation was for 1J/cm2 of UV dose. In response, new dosimeters were developed by Intellego Technologies to measure 500mJ/cm2 and 1,000mJ/cm2. These dosimeters were designed to be placed in areas where masks are being sanitized to show that the CDC recommended threshold had been reached.
So the importance of UVC dosimeters has been well established, and the cost for dosimeter cards, dots, and squares is very affordable. Using them can help ultraviolet system operators avoid using too much or too little UV dose in a given situation.
Too Much Ultraviolet Dosage
In terms of pathogen survival rates, using too much ultraviolet dosage is little concern. Once a population of microbes are damaged beyond their ability to repair or survive, more UV intensity or time will have a marginal effect. However, there are good reasons to limit UV exposure to the amount required for deactivating target pathogens.
Possible health concerns/UV leakage
While most ultraviolet rays are absorbed or blocked quickly, some small percentage can be reflected or transmitted to the point of damaging human skin or eyes. This is more a concern for devices with exposed UVC bulbs, but limiting UV operation to thresholds necessary for microbe deactivation can reduce the risk of stray light rays damaging humans.
More important to consider is material degradation. Ultraviolet light can compromise the life of plastics, resins, rubber, and other materials. Turning off UVC air/surface systems once a dosage threshold is reached can ensure pathogens are deactivated while not significantly damaging sensitive materials.
There is a balance to be struck between disinfection of a surface and preserving the life of materials, and UV dosimeters can indicate when a system could be shut off.
Ultraviolet light bulbs and UV ballasts have expected lifetimes. Bulbs may last 8,000-11,000 hours and ballasts 3-5 times as long. Depending on the bulb's power, distance from a surface, and time operated, UV bulb and device life can be shortened by keeping them on too long.
Once the UV dosimeter indicates a threshold has been reached for ultraviolet dose, there may be little reason to keep the device operating, and further operation may only reduce the bulb's life rather than provide extra disinfection.
Too Little Ultraviolet Dosage
While too much UV dosage can lead to shortened device life or compromised material integrity, too little is a greater concern for health.
As more ultraviolet dosage is applied to a population of microbes receive, fewer are likely to survive. Too little dosage can lead to faster rates of the surviving microbes reproducing, raising health risks.
As mentioned above, the ultraviolet dose required to deactivate 99.9999% of COVID-19 has been estimated at 22mJ/cm2. Applying smaller doses could result in higher virus survival rates, which could infect people in an area.
Some pathogens have the ability to repair themselves after exposure to ultraviolet germicidal light.
One way to combat this ability is utilizing UVB wavelengths in addition to UVC. UVB light can damage the enzymes microbes use to repair themselves.
However, the easiest way to prevent photoreactivation is simply to increase the UV intensity or dosage. The more a pathogen's DNA is damaged by UVC, the less likely it is to photorepair.
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