All masks allow your exhaled air to escape from under the mask either through a one-way valve or out the sides of the mask. If they didn't, you wouldn't be able to exhale. The back pressure would prevent it. Full face masks that are used to administer anesthesia do not allow exhaled air to escape. They are connected to a vacuum source that takes the patient's exhaled breath out of the building and releases it into the outside air. Without the vacuum, one of two things would happen: either the patient would be unable to exhale, or the operating room staff would be exposed to the anesthetic gases.
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Humility is not thinking less of yourself, its thinking of yourself less.
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