The Filter Cap on Nunc Flasks effectively excludes micro-organisms larger than 0.22m while allowing optimal gas exchange. Maximum protection against contamination is provided by a unique hydrophobic filter. Bacterial and fungal spores can neither pass through nor grow through the matrix and contaminate the culture. The hydrophobic filter won't clog in a humid atmosphere, or if it accidentally comes in contact with the medium. The filter surface area in each size cap is proportional to the flask volume, assuring adequate gas exchange. Automatic gas exchange promotes quick stable buffering in a CO2 incubator.