UVC Fluence, Dose recommended for disinfection from Viruses, Bacteria, Protozoa and Algae

UV Fluence (Dose) recommended for 90% or 99% disinfection from Viruses, Bacteria, Protozoa and Algae

 When designing, building or installing a UVC light, these key questions must be answered first:

"What irradiance level is effective?"

"What is the required exposure time?"

Fluence (UV Dose) Required to Achieve Incremental Log Inactivation of Bacteria, Protozoa, Viruses and Algae

While there are many studies that show the effectiveness of UV light in disinfection or sterilization, a high variance of the results exists, which presents a challenge to find an answer to these questions. 

We will present our recommendations by analyzing the results of 413 research papers, as found in the compilation "Fluence (UVC Dose) Required for up to 99% disinfection from Viruses, Bacteria, Protozoa and Algae"  that can be downloaded at the links below:

PDF: Fluence (UV Dose) Required to Achieve Incremental Log Inactivation of Bacteria, Protozoa, Viruses and Algae

The research studies present the fluence required to achieve a log reduction from 1 to 5, for different types of UVC sources.

The effectiveness of sterilization or disinfection with UV light depends on the exposuretimewavelength 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/cmor 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") that shows the relative number of live pathogens 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.

"Log Reductions" from 1 to 5 are the most common in research papers about UV light disinfection. Their meaning:
  • "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
1 90%
2 99%
3 99.9%
4 99.99%
5 99.999%
 

 

Compilation of results from 431 disinfection experiments with UVC radiation

The tables in the PDF are a compilation of Log reduction from 431 experiments on the effects of UV disinfection on Bacteria, Protozoa, Viruses and Algae. These studies and similar others, must 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 ultraviolet (UV) fluence-response data for various microorganisms that are pathogens, indicators or organisms. 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

A fluence of 20 mj/cm2 or less was found to be enough for a 90% kill rate in 81.90% of the 431 studies in the compilation, equal with " 1 log 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 required.

 

 

Maximum Fluence (dose) for 90% kill rate (log 1 reduction) Number of studies % of total studies
0.1-5 202 46.67%
5-10 76 17.83%
10-20 75 17.40%
20-30 38 8.82%
30-40 17 3.94%
40-50 9 2.09%
>50 14 3.25%

A fluence of 20 mJ/cmcan be reached with an irradiance of 10W/m2 (1mW/cm2) in an exposure time of 20 seconds, in 66 seconds at 3W/m2 (0.33mW/cm2) and in 200 seconds at 1W/m2(0.1mW/cm2 = 100 µW/cm2).

6 mJ/cmwas 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 achieve 90% inactivation, from 164 studies. In 82% of cases, as low as 5 mJ/cmis required. These results show that disinfection from bacteria can be much easier to achieve 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/cmin 75% of the studies and up to 80mJ/cm2 in 22%.

 

Pathogen Number of studies Maximum Fluence (dose) for 90% kill rate (log 1 reduction) % of total studies
Viruses 149 20 mJ/cm2 75%
Bacteria 112 5 mJ/cm2 82%

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 higher than 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 m, 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 3 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.

Tables with Compilation of results in PDF

For table in excel format please contact us.
 


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