When designing disinfection systems, one of the core starting requirements is selecting the targeted reduction level of a specific microbe or, more specifically, reduction of colony forming units (CFU) of the targeted microbe.
Colony Forming Unit: For practical purposes, it would be too time consuming, complex and expensive to use a microscope and count every individual microbial cell of a sample. Instead, by diluting a sample and spreading this across a petri plate, microbiologists can count groups of microbes, called colonies. Each colony is assumed to have grown from a single CFU.
Similarly, when calculating and reporting the changes in CFUs after disinfection, rather than state the magnitude of change in individual CFUs, microbiologists express the performance as a percentage reduction in terms of a reduction factor and, for convenience, typically in factors of 10 using a logarithmic (log) reduction scale – a log reduction factor (LRV).
Log reduction is a mathematical term that is used to express the relative number of living microbes that are eliminated by disinfection.
Log reduction = log10 (N0 /N)
N0 = colony forming units of the microorganisms before exposure to UV light
N = colony forming units of the microorganisms after exposure to UV light
For example, a 1 log reduction corresponds to inactivating 90 percent of a target microbe with the microbe count being reduced by a factor of 10. Thus, a 2 log reduction will see a 99 percent reduction, or microbe reduction by a factor of 100, and so on. Table 1 (below) shows the chart of log reduction.
UV Dose Response: Every pathogen, based on its biological make-up, has a unique spectral sensitivity “fingerprint.” Depending on the UVC wavelength range being used, each microbe will require a different amount of energy to be inactivated. By selecting UVC wavelengths and greater doses of energy, the amount of disinfection (i.e. LRV of the pathogen) can be improved. Dosage is determined based on the intensity of the UVC energy and the exposure time at a specific wavelength.
The table below shows the UV dose required for common target microbes for varied log reductions. Whereas Pseudomonas aeruginosa requires more energy to achieve inactivation, microbes like E. coli and staph become inactivated with a relatively low dose of radiation.