Abstract: An adaptor for controlling, in a fail safe manner, creation of a negative pressure in glove bags and similar types of collapsible protective environmental enclosures, has a make up air chamber having a flow control valve on the input side and a vacuum source connected to the output side, a HEPA filter positioned in an aperture in said chamber wall permits drawing of the required make-up air from ambient surroundings to the vacuum source when input from the glove bag is restricted. The HEPA filter also filters any discharge of air from the glove bag through the adaptor to ambient when the vacuum source fails.