Used to be 3 microns was adequate, then 2, then 1. The most recent literature I've seen claims that a filter needs to be .1 microns to be effective...though I've never seen a filter better than 1 micron.

I too prefer my MIOX pen but I'm currently looking for an inline filter for my drinking bladder that will filter Cryptosporidium as Crypto is the only thing that MIOX doesn't treat instantly (15 to 30 minutes). Unfortunately I can't find consistent information on how big Crypto oocysts are. I've seen documentation putting them as small at 1 micron and as big as 9.