Germicidal action of UV radiation
When designing, building or installing a UV light two key questions must be answered first:
"How much irradiance is needed?"
"What exposure time?"
The answers can be found in the many studies that show the effectiveness of UV light in disinfection or sterilization. While there is no shortage of studies, there is a very high variance of the results, which presents a challenge.
We will present our recommendations by analyzing the results of 413 reasearch papers, as found in the compilation "Fluence (UV Dose) Required for up to 99% disinfection from Viruses, Bacteria, Protozoa and Algae" that can be downloaded at the links below:
The research studies present the fluence required to achieve a log reduction from 1 to 5, for different types of UV sources.
The effectiveness of sterilization or disinfection with UV light depends on the exposure, time, wavelength and irradiance.
- Exposure or fluence (sometimes called dose) is measured in mJ/cm2 (where 1 mJ/cm2 = 10 J/m2)
- Exposure time is measured in seconds (s), minutes (m) or hours (h)
- Irradiance is the flux of radiant energy per unit area, in other words how much of the UV radiation power (measured in W = 1000 “miliwatts” mW = 1.000.000,00 “microwatts” μW ) reaches the surface. Irradiance is measured in mW/cm2 or W/m2 (1 mW/cm2 = 10 W/m2) and is dependent on the radiant power, distance and dispersion of the radiation emitted by the lamp source.
Log reduction explained
"Log reduction" is a mathematical term (as is "log increase") used to show the relative number of live microbes eliminated from a surface by disinfecting. For example, a "5-log reduction" means lowering the number of microorganisms by 100,000-fold, that is, if a surface has 100,000 pathogenic microbes on it, a 5-log reduction would reduce the number of microorganisms to one, equal to 99.999% kill rate.
- 1 log reduction means the number of germs is 10 times smaller (101)
- 2 log reduction means the number of germs is 100 times smaller (102)
- 3 log reduction means the number of germs is 1000 times smaller(103)
- 4 log reduction means the number of germs is 10,000 times smaller(104)
- 5 log reduction means the number of germs is 100,000 times smaller(105)
|Log Reduction||Kill rate of microorganisms|
Compilation of results from more than 400 disinfection experiments with UVC radiation
The tables are a compilation of Log reduction from 431 experiments about the effects of disinfection with UV radiation on Bacteria, Protozoa, Viruses and Algae. These studies and others like them, should be the science behind the design and deployment of a UV lamps system for sterilization or disinfection.
For each pathogen the fluence (dose) required to achieve the given log reduction is written in mJ/cm2 when exposed to the UV radiation of the test lamp. Lamps used for the tests, as detailed for each result are:
- LP: low-pressure (LP) monochromatic mercury arc lamp or filtered polychromatic UV light is used to achieve a narrow band of irradiation around 254 nm
- MP: polychromatic medium pressure (MP) mercury arc lamps
- UVC LEDs
Tables 1-5 present a summary of published data on the ultra- violet (UV) fluence-response data for various microorganisms that are pathogens, indicators or organisms encountered in the application, testing of performance, and validation of UV disinfection technologies. The tables reflect the state of knowledge but include the variation in technique and biological response that currently exists in the absence of standardized protocols. Users of the data for their own purposes are advised to exercise critical judgment in how they use the data.
Maximum fluence (dose) for 90% disinfection rate (log 1 reduction) can be less than 20 mJ/cm2
In 81.90% of the 431 studies in the compilation a fluence of less than 20 mj/cm2 was enough to achieve a 90% kill rate (log 1 reduction) of the analyzed viruses, bacteria, protozoa or algae. In 8.82% of the studies the dose had to be increased to 30 mJ/cm2 while in the rest 9.28% a dose of 30 to 50+ mJ/cm2 was needed.
|Maximum Fluence (dose) for 90% kill rate (log 1 reduction)||Number of studies||% of total studies|
For a fluence of 20 mJ/cm2 with an irradiance of 10W/m2 (1mW/cm2) an exposure time of 20 seconds is required, 66 seconds at 3W/m2 (0.33mW/cm2) and 200 seconds at 1W/m2(0.1mW/cm2 = 100 µW/cm2).
Very interesting is that 6 mJ/cm2 was found to be enough in 51% of the studies for a 90% kill rate. This will require an exposure of only 60 seconds at 1W/m2.
Viruses are much more resistant to UV than bacteria
All the research made on bacteria has found that a fluence of less than 12 mJ/cm2 will achive 90% inactivation, from 164 studies. In 82% of cases, as low as 5 mJ/cm2 is required. These results show that disinfection from bacteria can be much easier to achive with UV light, very good news for hospitals fighting with drug-resistant strains.
Viruses are significantly more resistant, requiring a fluence of up to 20 mJ/cm2 in 75% of the studies and up to 80 mJ/cm2 in 22%.
|Pathogen||Number of studies||Maximum Fluence (dose) for 90% kill rate (log 1 reduction)||% of total studies|
90% disinfection rate should be the UV system design goal
When designing a UV disinfection system it can be very useful to consider the fluence increase required to achieve microorganisms reduction of more than 90%, especially if the target value is 99.9% (log 3). From the compilation the average increase in fluence required to obtain 99.9% disinfection instead of 90% is >200% on average while 99% kill rate can be possible with 95% increase.
In practical terms an increase on average of 95-200% for exposure time or irradiance per m2 , therefore installation and operational costs, can improve the result by maximum 10%.
It is our opinion that a 99.9% disinfection rate while ideal, cannot be achieved with UV technology in the practical, economical way required to make it widespread enough to fight a pandemic.
If 99.9% is not possible the 90% disinfection should be the goal, especially considering this technology can be augmented buy other means, such as cleaning or personal hygiene.
For a fixed budget, the 90% disinfection design goal could mean three times more health care facilities are equipped with a continuous disinfection system.
As our analysis of the compilation shows, the UVC disinfection system is recommended to achieve a fluence of 20 mJ/cm2 in the time planned for it to function. For tight budgets or bacteria only it 5 mJ/cm2 can be recommeded.