Air Taxi single pilot requested priority handling to an enroute airport due to experiencing physiological symptoms. Hospital doctor reported symptoms were possibly caused by carbon monoxide exposure which the pilot attributes to an improperly configured Hazmat box on board.

Narrative

Started the morning with a cargo delay departing almost 2-hrs later than scheduled. While loading cargo I noticed a dry ice box was leaking. I requested the loading crew to rewrap the box which they did thoroughly enough to allow the box to be loaded on the plane. I departed ZZZ and half way through my flight which was about 20mins in I started to feel tingling and numbness in my fingertips. Knowing that there was a leaking box of dry ice that needed to be rewrapped my thought was I could be experiencing carbon monoxide poisoning (actually carbon dioxide but I kept stating monoxide). Considering it is the silent killer I decided the safest course of action was to notify ATC of my suspicion and don my oxygen mask. As I was putting on the mask I felt momentary lightheadedness. At this point I completed donning my oxygen mask and connecting my comms and [requested priority handling]. I was already cleared for the ILS XXR into ZZZ1 just prior to the event so I clarified with approach that I was having a hard time hearing over the oxygen mask and to verify I was cleared for the ILS XXR. After confirming I was cleared I began to focus on the approach as I was fairly high and needed to descend to fly a stabilized approach. I notified approach I was switching to ZZZ1 tower and called for clearance to land. I did not hear any communications with the tower; but I continued the approach as I [requested priority handling] and knew approach had relayed the situation. When I broke out of the clouds I saw from the corner of my vision the flashing white light coming from the tower. I ignored this so I could remain focused on getting the aircraft on the ground safely (it is possible when I was setting myself up for the approach I had the wrong frequency). Upon landing I continued to broadcast my intentions of turning off the runway at taxiway 1 and continuing straight forward to parking to shut-down the aircraft. I parked the aircraft successfully and fully shutdown the engine and depowered. After the aircraft was safe I opened the door and removed the oxygen mask. I initially could still feel tingling in my fingertips and my hands began to shake. The fire department met me first and helped me out of the aircraft and had me sit on the left main tire while they questioned me and ran some tests. Afterwards the EMTs arrived and ran more tests. To my knowledge they ran a blood pressure test; blood sugar test (for diabetes); and checked my pulse and oxygen levels. Aside from slightly high pulse and blood pressure; which they stated was natural given the response to a [request priority handling] situation; they found no unusual readings that warranted concern and the fire department did not detect any CO in the aircraft (again I was constantly referencing CO rather than CO2 which means the CO2 was never actually tested for I may have been cognitively impaired per the ER doctor). They used a Draeger gas detector which included CO detection which read 0% (other gases were CH4 was 0%; O2 was 20.9%; and H2S was 0ppm). I was questioned if I had taken any medications before I told them no; but that I did throw up the night before after dinner but that I didn't consciously detect any irregularities from my episode that would be deemed a safety risk. I would also like to note this did wake me up and interrupted my rest; however I did not consciously feel tired and again did not deem any risk to safety. They mentioned it was possible that could have played a part in my initial feelings paired with the knowledge of the dry ice boxes in the back. They told me that because I didn't have any abnormalities that I was either free to go or could opt to take the ambulance for further testing. I got up and walked around with no difficulties or symptoms of any kind; so I opted to not take the ambulance and would call my company to report the event. I eventually went to the ER and with the how busy the ER was I was not actually seen and tested in until much later Ifelt tingling in my finger tips initially in the air. After being checked by the Emergency Room doctor; which was done over 8-hrs after the event; blood test showed my carbon monoxide levels were above normal range. Given the readings the doctor said I was definitely exposed to carbon monoxide at some point and that this would have been the reason for the symptoms I described feeling. She mentioned carbon dioxide could still have been an issue; however because the fire department did not test for the gas and the late testing there will be no way to tell if carbon dioxide played any role in my symptoms. To be clear all day I had kept referring to carbon monoxide rather than carbon dioxide which is was I initially concerned with. Luckily the doctor understood this and decided to test for both as a precaution; which she mentioned was a good idea since it revealed I had in fact been exposed to carbon monoxide. She also mentioned I was probably cognitively impaired due to the exposure so that was likely why I was so fixated on the wrong gas all day; though I did mention I didn't consciously feel like I was impaired.As I was flying single pilot and feeling what I believed to be the effect of a gas exposure I exercised my pilot in command authority to don an oxygen mask and advise ATC as I deemed that the safest course of action. Given the nature of the incident being an exposure to a colorless; odorless; tasteless; and non-irritating gas I do not believe there was any way to avoid this emergency as there is no way to 100% know where I was exposed to the gas. However; I do believe that having some sort of detection system for CO and CO2 would be beneficial and could have helped determine if exposure was coming from the aircraft or not. Though I must also mention that if a detector showed no signs yet I was feeling weird may have prolonged my response time and I the situation could have gotten worse due to that.