Rewording that, westwind .... Beta6=2 means that 50% of the particles of 6um or bigger are filtered. Beta19=20 means that 95% of the particles 19um and larger.
To fill in the unspoken part of it (that most people miss) ...
50% of the particles 6um and various levels of LARGER PARTICLES get through
5% of 19um and various lower levels of larger particles get through.
That is, if the beta ratio for 10um particles was listed ..it would be somewhat higher then Beta2 ..if the beta ratio was listed for 30um particles it would be higher then beta20
A minor distinction in view..but one that's kinda skipped over. It's not like ONLY 6um particles are getting through ...they're just the ones getting through at the 5% rate. Larger particles are getting through at 4%, 3%, 2%, 1%, and fractional percentages.