Abstract: The present invention relates to a ventilation system for a protective suit for use in hazardous environments. In a further aspect it concerns the protective suit itself. An air purifying respirator draws air from outside the protective suit through a filter, supplies filtered breathing air via a breathing hose to a space within the face piece, and supplies filtered ventilating air via a ventilating hose to the interior of the protective suit. A ventilation valve in the ventilating hose automatically closes only during periods of high breathing demand to counter a pressure drop inside the face piece.