How to manually gas with C02? - (Dec/09/2009 )

Hello, I was reading a previous forum archive on here and I came across a thread discussing manual gassing with C02. The sigma protocol for this says to gas the flasks with a 0.25 m filter. What unit does 0.25 m refer to, and where can these filters be obtained?
Any input or protocols for this would be much appreciated.

Thanks,
arty

Millipore makes 0.2 micron filters; these are to remove any bacteria that may be in the CO2 line, before they get into the flask.