Synthetic filter discs are made of heavy Polyethylene will filter down to .3 microns, allowing for air exchange while excluding airborne bacteria from your sterilized culture. Polyethylene does not absorb moisture making it especially suited for mushroom cultivation operations. At nearly 1/16 inch thick these discs will withstand repeated handling and sterilization. Available in 70m or 90mm to fit Narrow Mouth or Wide Mouth Mason Jars. Filter discs can be easily trimmed with scissors for a custom fit