T he 37-year-old systems analyst who came to see me had been experiencing daily headaches for several months—ever since he’d taken on a higher-pressure job that involved a nearly 4-hour daily commute. He was doing the drive in a 20-year-old jalopy.

One doctor had already put him through a workup that included MRIs, CT scans, and numerous laboratory tests. He was then referred to a neurologist and told he had tension headaches.

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Despite using over-the-counter pain medications and prescribed muscle relaxants and tranquilizers, his headaches persisted. Friends suggested that the stress of the new job and commuting might be the cause.

When he arrived at my office, I assigned him to the medical student working with me. She took a detailed medical history, as medical students are taught to do, which disclosed that his headaches occurred only when he was driving to and from work.

Armed with that clue, I asked him to leave work early the next day and stop by the office. He arrived with a splitting headache. A blood test showed that he had an elevated level of carboxyhemoglobin, indicative of carbon monoxide (CO) poisoning.

He junked that car so fast we never had time to define the source of the probable exhaust leak that could have cost him his life. His headaches disappeared just as quickly.

Here, what you need to know to protect yourself from carbon monoxide poisoning:

How This Gas Does Harm

CO, one of the most deadly gases known, is colorless, odorless, and tasteless—­undetectable by any of our natural senses. It binds to hemoglobin (the oxygen-­carrying protein in blood) with a tenacity that is 240 times as strong as the oxygen whose place it usurps.

It also interferes with unloading whatever oxygen remains attached to the hemoglobin molecule. As a result, the body is robbed of oxygen. The ultimate result—death due to lack of oxygen—is contingent on the duration of exposure as well as the concentration of CO in the inspired air.

Nonfire-related carbon monoxide poisoning is responsible for almost 50,000 emergency-room visits and 1,200 deaths annually. Smoke from fires is the most common source of carbon monoxide poisoning (many people trapped in burning buildings are asphyxiated). Other sources include gas stoves, hot-water heaters, furnaces, snow blowers, or any malfunctioning or poorly ventilated fuel-burning system.

What probably helped save my patient’s life was his habit of driving with his window open a bit, even in cold weather.

Symptoms Can Be Confusing

The earliest symptoms of carbon monoxide poisoning—headaches, dizziness, nausea, vomiting, and lethargy—resemble those of a variety of conditions. This can easily lead both victims and professionals to attribute them to a virus or food poisoning.

More recently recognized is a delayed neuropsychiatric syndrome (DNS), characterized by a decline in cognition, muscle weakness, and gait abnormalities, that occurs anytime up to eight months following a seemingly full recovery from carbon monoxide poisoning.

DNS can affect up to 40 percent of those exposed to CO and, curiously, has been observed to bear little or no relationship to the actual levels of carboxyhemoglobin in the blood. Because it has now been almost two decades since my patient’s harrowing experience, he seems to have escaped this devastating complication.

Protect Yourself

Not everyone has a curious and thoughtful medical student at their beck and call, so the best means of detecting an impending disaster is a CO detector.

Every house or apartment should have at least one of these small, battery-run, relatively inexpensive alarms, and ideally, such a device should be in each room with a potential source of CO fumes. Batteries should be replaced once a year. (See Consumer Reports’ reporting on CO detectors.)

If an alarm goes off, don’t waste time attempting to find the source. Open the windows, leave the house, and call 911. Most victims of acute poisoning can be handled in the nearest emergency room with oxygen given by mask.

Those who are unconscious or have very high levels of carboxyhemoglobin may benefit from oxygen administered under high environmental pressure (hyperbaric oxygen).

And, of course, keep your car’s inspection sticker up to date.

Editor’s Note: This article also appeared in the December 2017 issue of Consumer Reports on Health