Air carrier First Officer reported the flight crew were exposed to carbon monoxide that may have been caused by a faulty ground heating cart.

Narrative

I awoke at the hotel between approximately XA:00 - XB:00 local time; having received a fair to good night's sleep. I worked out in my room and ate a substantial breakfast at the hotel; including coffee. Prior to departing the hotel at approximately XD:30 local time; I obtained a latte; placed it into my travel mug; and intended to drink it during preflight and/or the initial portion of the flight; ZZZ - ZZZZ.The Captain and I arrived at ZZZ Operations prior to the relief pilot. After approximately 10 - 15 minutes of conversation and reviewing the flight plan; the relief pilot arrived and we proceeded to Aircraft X. The relief pilot began the exterior walkaround while the Captain and I boarded the aircraft.Upon boarding; catering was actively servicing the aircraft; and we were using the R1 door for entry. With the outside air temperature around 0°F; my initial focus was configuring the aircraft to warm the cabin using the ground heating cart connected to the aircraft. To reduce significant cold air infiltration - likely originating from the main cargo deck due to the main cargo door being open for loading - I slid the rigid cargo barrier doors to a closed or near-closed position without locking them. This was done to reduce the noticeable draft flowing from the cargo area into the supernumerary area. Concurrently; I partially closed the R1 door by rotating the handle approximately a quarter turn; both to prevent the door from swinging due to wind and to further assist in warming the aircraft.I completed the limited pre-flight tasks that were possible while catering finalized their service and then proceeded to the cockpit. Upon entering the cockpit; the Captain made an off-hand comment regarding dirt he had cleaned near the L2 window. I independently observed what I would describe as black particulate around the R2 window. Upon further observation; it became apparent that black particulate was present throughout the cockpit; including on some vertical or near-vertical surfaces such as the EFB screens.I informed the Captain that the condition was unacceptable and; using the right radio; contacted Operations to request Maintenance or aircraft cleaners to address the contamination. I then informed the Captain that I would complete my remaining supernumerary preflight tasks - now that catering had concluded - while Maintenance cleaned the cockpit.Around the time Maintenance arrived at the aircraft; the relief pilot had completed his exterior inspection and was stowing his bags. Maintenance personnel began cleaning the particulate using galley napkins and water. During this process; the Captain observed that his hands were covered in black residue best described as soot; consistent with having handled a greasy or ashy substance.When I returned to the cockpit; I observed black particulate on the white lid of the Captain's coffee cup. This was the first moment I recognized that the particulate may have been airborne. Shortly thereafter; the relief pilot commented that I had grease-like streaks across my bald head; which further indicated airborne contamination. At that point; the Captain returned to the supernumerary area and solicited our input. We discussed the need to identify the source of the particulate; fully clean the aircraft; and the likelihood that we had been inhaling the substance. Based on these factors; we collectively decided to deplane and return to Operations. I had already begun making an entry due to Maintenance involvement. We gathered our belongings and deplaned.Upon regrouping in Operations; we interacted with another flight crew who noted visible particulate on our clothing and bodies. During this time; the Captain was on the phone with the Duty Officer. Members of the other crew commented that we would likely be sent for medical evaluation. I asked one of them to take photographs of areas of my body I could not easily photograph myself. It was during this time that I realized my travel mug - which has an opening so small that even a penny would have difficulty passing through - may have contained particulate. I emptied the mug and observed black particulate residue in the sink and remaining inside the mug. I had been actively drinking from this mug; leading me to conclude that I had not only inhaled but also ingested some of the particulate. I immediately informed the Captain; who relayed this information to the Duty Officer.After the Captain completed his call; he informed me the plan was to return to the hotel and then proceed to urgent care. We remained in Operations briefly to gather ourselves; discuss reporting requirements; and hydrate. At one point; I walked approximately 100 yards to a restroom. On the return walk; I noticed I was becoming light-headed and nauseous. Upon returning to Operations; I informed the Captain that I was not feeling well and immediately arranged a rideshare to take us directly to urgent care rather than returning to the hotel first. I proceeded to the rideshare and texted the Captain that it had arrived; indicating I was departing with or without him.The first urgent care facility we attempted was closed. The second facility advised they were unable to treat us and strongly recommended evaluation at an emergency room. I arranged a third rideshare to another hospital's emergency department.Sometime between leaving the aircraft and arriving at the emergency room; the Captain informed me that Maintenance believed the ground air cart used to heat the aircraft was problematic and that it had been disconnected during our time on board. Based on this information; we began operating under the assumption that a malfunctioning ground air cart may have been the source of the particulate.Upon arrival at the emergency room; we informed the triage nurse of this suspected source while also making clear that the exact cause was unknown. I stated that we had been exposed to an airborne particulate that smeared upon contact and that I was experiencing symptoms consistent with that exposure. During intake; the nurses observed something during the Captain's evaluation and moved him to a separate room. I was not privy to that evaluation and will not speculate.My initial evaluation lasted approximately five minutes. Afterward; we waited in a crowded emergency room waiting room for approximately five minutes before being called back ahead of several other patients. The Captain and I were placed in adjoining bays separated by curtains that remained open. Two nurses conducted intake processing; and a physician promptly evaluated us. The physician immediately ordered blood testing; noting results could take up to an hour. My blood was drawn first; approximately three hours after the exposure event. Approximately ten minutes after my blood draw; the physician returned and informed me that my results showed slightly elevated carbon monoxide levels - even accounting for the time elapsed since exposure - and that these levels met criteria requiring treatment under their protocol.At that point; my bay became active with medical interventions. I was placed on 100% oxygen; connected to an electrocardiogram (EKG); and placed on continuous vital-sign monitoring. The Captain; who had not yet had blood drawn to my knowledge; received similar treatment. After remaining on 100% oxygen for an extended period consistent with medical treatment protocols; the physician returned to reassess my symptoms. We were subsequently discharged and transported by rideshare back to the hotel. My discharge paperwork listed 'Diagnosis: Carbon Monoxide Exposure' and stated; 'Based on your slightly elevated carbon monoxide levels we believe you were likely exposed.'Upon returning to the hotel; I showered immediately and observed black discoloration on the shower floor and washcloth. After exiting the shower; I cleaned my ears with Q-tips; the first was heavily blackened (no photo taken); and the second showed residual black contamination; which I photographed. My clothing was visibly dirty; and when I cleaned my iPad screen and case with a screen wipe; the wipe showed a significant amount of black residue.I ate a small meal around XK:00 local time and then slept for nearly 11 hours. I felt exhausted and markedly lethargic. I was instructed to follow up with physicians upon returning home and intend to do so to ensure I am fully fit for duty prior to returning to a well status. For additional context; yesterday I awoke in my own bed after a full night's sleep and had a restful morning with appropriate nutrition. Due to the departure times in both ZZZ1 and ZZZ; my circadian rhythm was minimally disrupted; and nothing leading up to reporting for duty stands out as a plausible alternative cause of the symptoms I experienced.In hindsight; the onset of lightheadedness following minimal physical activity was likely related to carbon monoxide exposure; and seeking emergency medical assistance at that moment would have been appropriate. Having since reflected on this event; I believe I was experiencing a degradation in critical thinking and decision-making capacity at the time; consistent with the known effects of carbon monoxide exposure. As a result of this experience; I intend to establish more stringent personal 'red lines' moving forward; such that if specific physiological or cognitive thresholds are crossed; immediate action is triggered without reliance on impaired self-assessment.Cause: Likely a malfunctioning ground heating cart; but that is an assumption.Suggestions: To reduce the risk of similar events in the future; several systemic improvements should be considered. When ground heating equipment is attached to an aircraft; active monitoring within the cockpit - such as carbon monoxide or air-quality monitoring - should be required to provide immediate indication of hazardous conditions. Early detection would allow crews to take timely corrective action before exposure occurs.Clear; standardized protocols should also be established for suspected airborne contamination or exposure events. These protocols should define specific triggers for aircraft evacuation; maintenance involvement; and medical evaluation; removing ambiguity and minimizing reliance on individual judgment during potentially impaired conditions.Additionally; the Company should arrange and coordinate medical evaluation and transportation following exposure events; rather than placing that responsibility on affected crewmembers. Individuals experiencing potential toxic exposure may have degraded cognitive function; nausea; or impaired decision-making capacity; making self-coordination unsafe and inappropriate.More broadly; this event highlights a systemic issue in which the current framework places excessive onus on individuals to recognize; assess; and act during circumstances where mental acuity may already be compromised. This concern closely parallels fatigue risk management; where decision-making is similarly degraded; yet real-time forecasting and objective safeguards remain limited. Implementing better real-time monitoring; clearer decision gates; and company-driven responses would reduce reliance on impaired individual judgment and improve overall safety outcomes.