What happens if you use 99.9% effective sanitizer twice?

We put this question to Jane Greatorex from the Health Protection Agency in Cambridge:

Jane - Well, it's not a straightforward answer. The 99.9% claim that manufacturers put on their products is true based upon the tests that they carry out, but these are carried out under laboratory conditions in test tubes and on plates. It's a little bit more complicated than that in real life and it really depends on what we're talking about here. If we're talking about a cough droplet, a cough droplet might contain about 200 million 'flu germs in the winter season and about a million or so bacteria; and if you use that hand gel, then you will indeed get the numbers of viruses and bacteria down probably to levels below that which is infectious. However, you will not remove all of them and that's because of two things. One, your hands are very good at retaining bacteria and viruses, and two, the numbers of bacteria and viruses that you start with are such that even 0.1% is still a big number of bacteria and viruses. If you kill even that 0.1% with another hand wash, you'll still be left with some remaining.

Now if, for instance, we were talking about faecal contamination - not a nice subject, but then we're talking about much larger numbers of viruses and bacteria, and the big complication there is, you need fewer of those to cause an infection. So, the straight answer is, you won't remove all the bacteria and viruses, and that's in some situations, it's really not very good news.

Diana - Two scrubbings will remove more germs, but 99.9% of a lot is still going to be a lot. It all depends on what the infectious doses of a specific pathogen. If you only need one bacterium to infect you and sometimes even the most thorough washing isn't enough.