A portable breathing air supply apparatus uses a membrane separation module to obtain oxygen enriched air from ambient air which is blown into the module by an electrically powered fan. Oxygen enriched air is withdrawn from the permeate side of the membrane by a vacuum pump and is stored in a reservoir while the user exhales. In a preferred mode, a conserver valve in a tube leading from the reservoir to the user's mouth or nose is triggered to feed the enriched air for a preselected duration after a sensor in the tube detects onset of inhalation. Power for the electrical components can be supplied by batteries. The portable apparatus is sufficiently compact and light to be transported by persons weakened by certain chronic breathing disorders, such as chronic obstructive pulmonary disease and emphysema, and thus frees the user to roam for long periods away from a primary source of oxygen.