A320 Captain reported experiencing physiological symptoms a little while after the flight crew turned on the APU bleed that worsened a noxious odor that was previously faintly present in the cabin.

Narrative

Yesterday while operating flight ZZZ - ZZZ1; Aircraft X; we experienced a fumes event. When I arrived at the plane; it was hot and musty; and there was a very faint chemical odor in the cabin. Since the air wasn't hooked up and the temperature was nearing 80; I started the APU; and turned on the APU bleed. Once on; the noxious odor got worse; and the flight attendants came into the cockpit to let me know about it. I went out to the cabin and walked the length of the plane to confirm; then called Maintenance Control to come inspect. While waiting I turned off the APU bleed. Once Maintenance Control got there I troubleshooted with him; and then made a logbook entry; gathered my things and exited the plane. All things considered; I probably spent about 30 - 40 minutes in the aircraft. They decided to take that aircraft out of service; but about 30 minutes later while the new plane was being towed to our gate; I felt lightheaded; as well as a moderate headache coming on. I was also starting to perspire; so that's when I called crew services; medical advisory service; and the Duty Pilot to start the fumes event process. Medical advisory service deemed me and my First Officer unfit to operate; and at around XA:00; still feeling sick; I decided to check myself into the hospital. I was discharged at XG:00 after 6 hours of observation.