http://multimedia.3m.com/mws/media/604799O/ohes-h1n1-influenza-a-faq.pdf
Can a valved respirator be used for protection from the virus that causes H1N1?
A valved respirator is designed to allow for easy exhalation through a one-way
exhalation valve.
If a person is wearing a respirator to help reduce his or her exposure to airborne viruses, a respirator with an exhalation valve would be acceptable. It would not be acceptable for someone to wear a valved respirator if they have a suspected/probable/confirmed case of H1N1 as they would be exhaling into the environment.
Where
healthcare workers are required to wear a respirator the use of a valved
respirator must be in accordance with national and employer guidelines. For example, in some regions of the world such as Canada and the US, it is not acceptable for a healthcare worker to wear a valved respirator in a situation requiring a sterile environment, such as the operating room.