A liner must be designed specifically to fit the teatcup shell, which should ensure that it is of sufficient length to collapse fully around the teat base and allow proper circulation in order to avoid teat damage. Liners are the sole part of the milking plant which actually comes into contact with the cow, their condition and maintenance is critical in mastitis control and to make milking efficient. The lifetime of liners is limited as they are rubber-based and used every day so they should generally be replaced after 2,500 milkings. Liners are also damaged by chlorine and other chemicals used in milk production. This weakens the rubber leading to roughness, perishing and splitting. When the interior of the liner becomes rough, it can become more difficult to clean leading to an increase of cross-contamination between cows.