On April 2, 2024, about 1056 central daylight time, a Mooney M20E airplane, N86UM, sustained substantial damage when it was involved in an accident near Alpine, Arkansas. The pilot sustained minor injuries. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. According to the pilot, while in cruise flight in a gradual descent, the airplane’s two CO detectors began alerting. The pilot closed the cabin panel vent and opened a small cockpit window and 4 overhead vents. The pilot’s concern “turned to intense worry and fear as [he] noticed a feeling of lethargy and confusion, sort of slow motion coupled with graying peripheral vision.” He attempted to open the cabin door but was unsuccessful. The pilot decided not to continue the flight to the nearest airport, which was about 30 miles from his position; he shut down the engine and performed a forced landing to a field. The pilot reported that after he shut down the engine, his symptoms gradually dissipated, and he did not lose consciousness. During the forced landing, the airplane impacted unsuitable terrain, nosed over, and sustained substantial damage to the forward fuselage. After the accident, the pilot took a photograph of his iPad, which displayed, “Caution, Sentry CO level is 79 ppm.” Postaccident examination of the airplane revealed an existing repair weld on the exhaust pipe. The weld contained multiple cracks and holes, with engine oil and exhaust sooting present in the cracks. A cabin air scat tube displayed residual engine oil, black soot, and missing material in an area next to the exhaust pipe repair weld. The muffler contained a pinhole along the edge of a weld. The cabin air system contained a junction box, and two of three outlet ducts were covered with sections from an aluminum beer can and hose clamps. A review of maintenance records showed that a cracked exhaust pipe was replaced on August 16, 2019. An additional maintenance record, dated November 18, 2021, showed that the exhaust pipe was removed, repaired, and reinstalled, at a tachometer time of 5,447.56 hours. The tachometer time at the most recent annual inspection, which occurred just over 9 months before the accident, was 5,478.49 hours. The pilot provided his postaccident medical records for the NTSB Chief Medical Officer to review. According to the records, the pilot was evaluated in a hospital emergency department following the accident. The pilot sustained minor injuries not requiring hospitalization, and no CO poisoning was identified. The carboxyhemoglobin level in blood collected at 1248 on the accident date was 1% (normal), as measured by arterial blood gas testing with co-oximetry. No oxygen administration was documented. Carbon monoxide is an odorless, tasteless, colorless, nonirritating gas that can be produced during hydrocarbon combustion. Exposure to carbon monoxide usually occurs by inhalation of smoke or exhaust fumes. When CO binds to hemoglobin in blood, carboxyhemoglobin is formed, impairing the blood’s ability to deliver oxygen to body tissues. Nonsmokers normally have carboxyhemoglobin levels of less than 1 to 3% while heavy smokers may have levels as high as 10 to 15%. The pilot was a nonsmoker. Symptoms of low-level carbon monoxide exposure are nonspecific and variable, and may include headache, nausea, and tiredness. Increasing levels of exposure may become impairing or incapacitating, resulting in more serious neurocognitive, cardiac, and/or vision problems, and progressing to death above carboxyhemoglobin levels of about 50% (or lower if other serious medical conditions coexist). However, symptoms are not simply predictable from carboxyhemoglobin levels. After carbon monoxide exposure stops, the carboxyhemoglobin level typically takes about 4 to 5 hours to decrease by half; this time may be shortened to about 1 to 1.5 hours with medical administration of high-flow oxygen via mask. Carbon monoxide poisoning may be insidious and difficult for an exposed person to recognize; there is no reliable physical sign of exposure. The Occupational Safety and Health Administration’s 8-hour time-weighted average occupational CO exposure limit is 50 parts per million (ppm). Acute exposure guideline levels specify that the general population could experience irreversible or other serious, long-lasting adverse health effects or an impaired ability to escape when exposed to a CO concentration of 420 ppm for 10 minutes, 150 ppm for 30 minutes, or 83 ppm for 60 minutes.