CRJ200 flight crew report loud pop and loss of cabin pressurization at FL280. Emergency descent is initiated as Flight Attendant called to advise of an issue with the service door. Post flight reveals that service door seal has failed; possibly due to age.

Narrative

While enroute to the destination airport in cruise flight; the Captain and I heard a 'pop' and noted rising cabin pressure on the EICAS screen. Immediately; the Captain and I donned our oxygen masks; established crew communication and he continued with the pilot flying duties. The Captain announced he had the controls and the radios and requested that I complete the QRH for emergency descent. We advised ATC that we were descending to 10;000 FT immediately. ATC understood and cleared us. Center asked if we would continue to our destination airport and we elected to divert to the nearest airport; ZZZ. We spoke with the Flight Attendant and the passengers to advise of the situation and ensure that everything was OK in the cabin. We landed safely at ZZZ. The Flight Attendant used the Emergency Call button on our intercommunication system and we were able to confirm with her that a problem existed. She noticed an issue with the service door and said we needed to land immediately. Once we recognized that the cabin pressure was rising abnormally; the Captain and I executed our immediate action items for an emergency descent. Once at a safe altitude; we prepared for an unscheduled landing at ZZZ. We coordinated with the Flight Attendant and notified the passengers of the situation. According to the Mechanic; the seal around the service door showed signs of age/wear and tear. This allowed for an insufficient seal and leak between the pressurized cabin and the outside unpressurized air. The seals on doors; windows; or any compartment that might age/wear and effect cabin pressurization should be inspected regularly and/or replaced on a standard use of life schedule.