The Healthy Facilities Institute (HFI) with support from Advanced Vapor Technologies, has produced the following Commentary. HFI encourages the infection-prevention community to consider the following perspectives as an aid to prevention of illness, since marketing abuses regarding 99.9 percent or 99.99 percent germ-kill claims are legion, and the three perspectives below provide a well-rounded discussion.
Some germicidal products tout 99.99 percent germ-kill-strength, a semi-meaningless claim when “post-sanitizing” viable microbe populations remain in the hundreds, thousands or more.  This Infectious Dose can still make you very sick, or worse.
Like its evil twin, “greenwashing” – egregious  “germwashing” may sicken or kill as noted by microbiologist, Dr. Benjamin Tanner, regarding Ebola: “… every last viral particle must be killed because the infectious dose is very low.”
Preventing Infectious Dose is Key
Product A claiming 99.99 percent kill applied to a surface hosting 1,000,000 pathogenic cells with a dwell time of 5 minutes, will leave 100 viable cells.
Product B claiming 99.99 percent kill applied to a surface hosting 1,000,000,000 pathogenic cells with a dwell time of 5 minutes, will leave 10,000 viable cells.
And this assumes proper pre-cleaning to remove organic soil that deactivates most chemical disinfectants and proper dwell or “wet-soak” time, an often-neglected process error given busy schedules, hasty applications and inadequate training.
Conversely, as an example, the TANCS steam vapor system eradicates 99.9999 to 99.99999 percent pathogens in 2-5 seconds, a better way to prevent leaving behind an Infectious Dose without lengthy dwell time or toxic chemicals.
Germs, Rabbits and Chess
Remember, live microbes reproduce quickly – much faster than rabbits – many having a doubling time of just 20 minutes or less, or 3x or more per hour. In 8-24 hours, bacterial growth is exponential.
Think of it this way: If you placed one pathogenic microbe on the first square of a chessboard, 2 microbes on the second square, four on the third, and so on, then after 63 reproductions (there are 64 squares on a chessboard) or doublings you would have 18,446,744,073,709,551,615 germs – a horrific, potential Infectious Dose to be sure.  (Note – bacterial cells reproduce asexually by binary fission.)
Zero Tolerance for Germs
So practicing Zero Tolerance for pathogens is better than just reducing them by 99.9+ percent, as it helps prevent exposure to a microscopic but often-deadly Infectious Dose.
Eliminating live microbes better serves the public interest, although not the marketing interests of chemical purveyors who seem to have mastered the art of “lying with statistics”, and reduces the likelihood that today’s pathogen lineup will become deadly.
According to Dr. Charles P. Gerba, Professor, Dept. of Soil, Water and Environmental Science, University of Arizona, Tucson:
“We have been looking at the needed reductions on hard surfaces using Quantitative Microbial Risk Assessment (QMRA) and data on the actual concentrations of bacteria and viruses reported on surfaces. This approach suggests that in most cases only a 99 percent reduction is needed to reduce the daily risk of infection to 1:1,000,000. We have a number of articles in press and submitted where we release bacterial viruses in homes, offices and health care environments and it tends to support a 99 percent level in terms of needed reduction in exposure.”
According to Benjamin Tanner, PhD, President and CEO, Antimicrobial Test Laboratories:
“It is also important to understand EPA-registered antimicrobial pesticide (disinfectant/sanitizer) claims.  The reason many products say ‘kills 99.9 percent’ of bacteria on the label is because that is the performance threshold for the sanitizer test EPA requires (ASTM E1153) if people want to market products as sanitizers. In other words, a 99.9 percent reduction is EPA's arbitrary cutoff for sanitizer performance.  The initial viable microbial population in these ASTM ‘sanitizer’ tests is about one million cells, so products that make that claim may be leaving behind almost a thousand viable cells after treatment.  Many consumers interpret that particular label statement to mean that a product kills 99.9 percent of microbial species or types. Really it just means that when used as directed on the bottle, it kills 99.9 percent of the types of microbes listed on the label.  There is one catch though:  If a product kills 99.9 percent of both Staph (Gram positive) and Enterococcus (Gram negative), then EPA will permit the label to say ‘kills 99.9 percent of bacteria.’  EPA typically frowns on sanitizer or 99.9 percent claims for molds and viruses, so seeing those in the market is rare.”

Other points:
• Performance threshold for non-food-contact sanitizers is 99.9 percent (test uses films of bacteria dried onto glass slides).
• Performance threshold for food contact sanitizers is 99.999 percent (but this test is suspension-based, so easier to pass).
• Performance for "disinfection" is more or less total kill from a starting population of a million cells or more.
• In cases where pathogenic microorganisms both have a low infectious dose (<10) and may be present on surfaces in populations greater than 10,000, non-food-contact sanitizers don't cut it.  Some pathogens like Salmonella have infectious doses closer to 10,000 cells, so in that case sanitizers do the job nicely.
• Note that non-pesticides, like UV lights, can claim whatever reduction the data supports, such as 90.5 percent if they want and it's legal.  Whether or not it's a good thing to do is up for discussion.