A safety system comprising an air bag configured to have at least one inflatable volume or chamber, an inlet through which inflation gas enters the inflatable volume, the air bag including at least one exit port or vent which when in an open state permits inflation gas to exit the volume or chamber, the exit port or vent located in a selected region of the air bag, a vent closure mechanism secured to the air bag and movable to close the exit port or vent by tension created in a tether which is part of the vent closure mechanism; as the air bag inflates, the tether has one end operatively secured to the air bag, a second end operatively secured to the vent closure mechanism, the tether including a medial portion slidably guided across a portion of the air bag.