Why does bleach claim to kill 99.9% of bacteria?

Chris Smith answers listener science questions. They include, is it worth getting the flu vaccine? Is COVID still around? Is it possible to manufacture a car that runs on air? Why does the hair on our head keep growing, but not on our arms and legs? Is it good to finish a shower with a cold burst? Why are marathon runners thin? Like this podcast? Please help us by supporting the Naked Scientists (thenakedscientists.com/donate)