A filtering face mask that covers at least the nose and mouth of a wearer and that includes an exhalation valve. The exhalation valve opens in response to increased pressure when the wearer exhales to allow the exhaled air to be rapidly purged from the mask interior. An exhale filter element may be placed...http://www.google.de/patents/US6805124?utm_source=gb-gplus-sharePatent US6805124 - Face mask that has a filtered exhalation valve
