Air carrier First Officer reported a fume event during cruise was identified by the CO2 alarm activating. The flight crew diverted to an alternate airport where a safe landing was completed.

Narrative

Our flight was originally scheduled ZZZ1-ZZZ2-ZZZ3. All pre-flight activities; the ZZZ1-ZZZ2 leg and ZZZ2 turn were normal. Our ZZZ2-ZZZ3 leg was a dry-ice exemption flight and we followed all pre-flight procedures.Approximately 1+00 into the flight; approximately 250 NM East of ZZZ; FL370; my Dragger Alarm activated. After discussing the indications with the First Officer; and the fact that her Dragger remained silent; we considered deploying the 3rd Dragger Monitor from the Overhead Cabinet. Shortly after leaving her seat; the FO (First Officer) reported an odor in the aft cabin/galley area and feeling hypoxia-like symptoms. I also recognized symptoms similar to hypoxia. She returned to her seat and we immediately donned our O2 Masks. I remember feeling extremely lightheaded and dizzy as I donned my mask and we both agreed we had been feeling unrecognized physiological symptoms prior to the Alarm.Once we established communications; the FO and I quickly discussed our Physiological States; the Dragger Alarm and the presence of an odor in the cockpit. We quickly proceeded to the Smoke/Fumes/Fire Elimination checklists in the QRH. As I was already the Pilot Flying; I elected to have the FO remain the PM (Pilot Flying) and run the QRH while I communicated with ATC. We both agreed to coordinate a diversion with ATC. We elected a 180 turn to ZZZ for a straight in approach to Runway XXR. I coordinated an expedited descent; ATC clearing us to minimum step-down altitudes available due to the terrain below us. The FO quickly but methodically worked through the Smoke-Fumes-CO2 QRH checklists; keeping me apprised as she moved through the steps. However; the Dragger Alarm continued despite QRH actions so we elected to focus on preparing for an immediate landing in ZZZ. We continued with our diversion procedures and initially intended to Auto-Land Runway XXR. However the ILS ultimately wasn't available so we elected for a visual approach rather than change runways in close. We also discussed our post landing plan. Not knowing the true nature of our situation (toxic fumes; excessive CO2 or possibly both) we elected to stop straight ahead on the runway and prepare for a possible evacuation.The approach; landing and roll out were expeditious but standard and uneventful. Once setting the parking brake and running the applicable portions of the Evacuation Checklist; we elected to open our windows before removing our masks. That way if there was a toxic fume event/inability to breathe normally we would have the windows available for evacuation. We did not consider the Crew Entrance Door an immediate option as we were unsure of the atmospheric conditions behind us. As soon as we opened our windows the Dragger alarm ceased and we did not smell or note any abnormal fumes or other issues. We felt it more prudent to continue to assess the situation rather than immediately evacuate. We worked with CFR (Crash Fire Rescue) for a cabin inspection and egress via normal means. CFR reported high levels of CO2 but no toxic fumes. Once cleared to the crew entrance door and aided by CFR; we exited the aircraft.Post-flight; we were both evaluated on scene and then ultimately elected to go to a local area hospital. Diagnostic tests did not indicate any exposure to toxic substances or other lasting physiological effects. However; after describing the event/physiological symptoms to the on-call physician; she believed that we were likely exposed to a significant amount of Carbon Dioxide. Unfortunately there was no medical test available with which to confirm her diagnoses.