ABOUT A quarter of U.S. homes now have a carbon monoxide (CO) detector, and the U.S. Consumer Product Safety Commission recommends that everyone get this safety device to warn against the invisible, noxious gas. But what about when you’re traveling? As CO detectors are not widespread, you could be putting yourself at risk if a furnace or other fuel-burning appliance has not been properly maintained.

That’s why Eunice Wlcek, 61, who offers travel tips on her Theory of Simple YouTube channel, brings a portable CO detector when traveling (they cost as little as $25). “You want to make sure, especially in the winter, that there’s not an issue.”

According to the Consumer Product Safety Commission, CO levels of up to 70 parts per million will usually not cause symptoms. But levels above 70 ppm can cause headaches, nausea and fatigue. Concentrations above 150 ppm can lead to brain damage or death. Older people, young children and people with lung or heart disease are most at risk.

Here’s what to look for in a portable CO monitor, according to Charon McNabb of the National Carbon Monoxide Awareness Association and John D’Alessandro of the Firefighters Association of the State of New York:

• Battery power, in case there’s no convenient outlet.

• Meets specifications of a reputable testing laboratory such as Underwriter Laboratories Inc.

• A display that’s always on. Readings should be visible without your having to “wake” the device.

• A readout that displays parts per million in real time, meaning it shows current levels and not an average over time.

• A “max button,” which you can press to display the highest reading since you last checked. In the morning, you could see the highest amount you were exposed to overnight. —Jenna Gyimesi