Occult carbon monoxide poisoning: a cause of winter headache.

Headache is an early symptom of carbon monoxide (CO) poisoning, occurring at carboxyhemoglobin (COHb) levels of greater than 10%. We investigated 37 patients presenting to an emergency department during the winter-heating season with a complaint of headache for evidence of CO exposure. Seven of the 37 patients (18.9%) had COHb levels in excess of 10%, with a mean of 14.0%. Three of these seven patients (42.9%) used gas space heaters or kitchen stoves to heat their homes, and three (42.9%) had cohabitants at home with headache. Of the 30 patients with COHb levels of less than 10% (mean level, 3.2%), only four (13.3%) used gas space heaters and kitchen stoves for heat (P = NS), and none (0%) had cohabitants with headache (P = .0045). In six of the seven patients with elevated COHb levels (85.7%), gas company investigation or historical information obtained retrospectively revealed a definite or probably toxic CO exposure. In none of these patients had the diagnosis of CO poisoning been suspected by the emergency department physician or the patient prior to obtaining the result of the COHb level. Occult CO poisoning may be an important cause of winter headache.