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.