1.5 micron

1.5 micron filter paper for the DrainDroyd Filtration System. 2′ diameter. 100 sheets per package.

1 Micron = one-millionth of a meter or one-thousandth of a millimeter or thirty-nine millionths of an inch (0.000039)
This filter paper is 1.5 Micron making it suitable for filtering pretty much everything, including the majority of mold spores, bacteria, and viruses. However, it is not suitable to filtering smoke or some bacteria and viruses.

The smaller the micron rating, the more can be filtered out ? but this also means the more your filter will need to be changed. As the filter fills up, the filtering process will slow down.