Ultraviolet germicidial irradiation (UVGI) is a sterilization method that uses short wavelength light in the ultraviolet spectrum to inactivate microbes. It was discovered in the late 1800’s when sunlight was seen to hinder microbial growth. It was later found that the energy from the ultraviolet spectrum of sunlight was the cause of this phenomenon.
How Does UVGI Work?
Today we know that the specific frequency most efficient at stopping microbial growth is at a wavelength of 260 nm. DNA has its absorption peak at 260 nm and very efficiently absorbs energy from light at this wavelength. The high energy photons disrupt and damage the DNA, leaving the microbes unable to reproduce as well as perform vital cellular functions. Since the microbes are unable to reproduce, they are considered dead.
Low pressure mercury fluorescent bulbs have a spectral peak at 254 nm and emit more than 95% of their ultraviolet radiation at this peak. These bulbs are close enough to the peak absorbance of DNA that they have a high efficiency at damaging microbial DNA. Thus they are commonly used as germicidal UV light sources, radiating in the UV-C spectrum of light.
What Parameters Are Important?
There are two parameters of interest for UVGI sterilization: intensity and dosage. Intensity is based on the power output of the light in the UV spectrum. However the intensity is not a very important parameter in itself. It’s mainly used to calculate dosage. Dosage is the amount of UV light the microbes absorb and will determine whether sterilization occurs. Stronger, higher intensity lights will sterilize faster, but lower intensity lights will also perform the same sterilization. It just takes longer.
UV light dosage is defined as:
Dosage = Intensity * Time
Where intensity is power per unit area and commonly written in units of mW/cm^2. A UV lamp with an intensity of 10 mW/cm^2 shone on a surface for 1 second will provide the same dosage as a lamp with an intensity of 1 mW/cm^2 shone on a surface for 10 seconds.
In this project, we’re able to exploit this relationship by using lower power, lower cost, off-the-shelf germicidal bulbs rather than expensive, high power bulbs. Off-the-shelf 40W germicidal bulbs cost around $20 USD whereas specialty high output germicidal bulbs cost thousands of dollars.
How Do I Measure Disinfection?
To quantitatively evaluate disinfection, it’s important to measure the intensity and dosage. Measuring UV-C intensity is commonly done with a UV sensor of some type. This is often a photodiode sensitive to UV light.
Positioning the photodiode in the location of interest will provide a quantitative measurement of the ultraviolet light at that position. Multiplying the measured intensity value with the elapsed time from the last measurement will give the dosage. This dosage is accumulated, meaning it’s added to the sum of all the previous values, giving a total dosage. This dosage value can be used to determine the likelihood that a specific microbe has been inactivated.
For example, an influenza virus requires 10.2 mW*s/cm^2 of dosage to be 99.9% inactivated. Often a large margin of error, equivalent to a 5x to 10x increase in dosage is also applied to ensure the microbe is inactivated to a very high degree of certainty.
How Come Hospitals Aren’t Using It to Decontaminate Equipment?
UV germicidal irradiation is usually done on hard, non-porous surfaces and is well characterized for that. Although there was some research into using it for protective equipment reuse, there’s no definitive research to state it’s efficacy for something like N95 reuse. Basically, it’s uncharted waters at the moment which is why many hospitals don’t want to take any chances. At the time of this writing, vaporized hydrogen peroxide (VHP) is the technique that hospitals have adopted.
In any case, we feel it’s important to have an open UVGI system available, which can start the conversation and hopefully start the research on it’s efficacy for protective equipment reuse. This method is low cost, uses standard components, and is already known to be useful for room and space decontamination.