Staying snug within a watertight tent as a storm rages around you is one of the joys of modern camping and modern tents.

But if the weather suddenly turns nasty on your next camping trip, or nights are just colder than you expected, don't be tempted to bring your cook stove inside. Levels of poisonous carbon monoxide (CO) from the burning stove can build up fast.

That's the warning from several Michigan emergency room doctors who wanted to know if varying the type of stove fuel might make a difference in such situations. They also wondered if the more rugged, four-season tents favored by those who like to be ready for anything would raise the concentration of carbon monoxide more than airier three-season versions. Quick answers from their small, first-pass test: Yes and yes.

"Four-season tents are built really well to keep out the elements," says Dr. David Betten, a medical toxicologist in the Department of Emergency Medicine at Sparrow Hospital in Lansing, Mich. "Unfortunately, that same tight construction means they're not all that well ventilated."

In several 20-minute burns inside the two types of tents, Betten and his colleagues tested the output of a popular backpacking stove as it heated a pan of water. In each trial, described online in the journal Wilderness & Environmental Medicine, they either burned unleaded gasoline or used "white gas," the cleaner-burning petroleum fuel that is commonly sold with camp stoves and lanterns.

Burning unleaded gas inside the four-season tent led to far-and-away the most carbon monoxide. Burning white gas inside the three-season shelter produced the least.

But in just 20 minutes, even the "cleaner" white gas raised levels of carbon monoxide inside the four-season tent above the 150 parts-per-million limit that occupational safety groups say workers shouldn't be exposed to for more than 15 minutes.

Inside the breezier three-season tents, CO levels stayed under 100 ppm with both types of fuel. "But that was just a 20-minute test," Betten says. "If you fired it up for an hour or two — or fell asleep — it's tough to say what would happen."

The concern is not just hypothetical. It's true that most of the 400 or more Americans each year who die from carbon monoxide poisoning, and the 4,000 additional people hospitalized, are poisoned inside their homes, garages, or cars. A study published in the Journal of American Medicine just this week even suggests that a large buildup of CO gas from fuel-burning appliances in one apartment can spill into the next.

The CDC has a whole bunch of recommendations for protecting you and your family indoors.

We hear less about CO risks away from home. But the Centers for Disease Control and Prevention notes that some campers have fallen asleep and never awakened after pulling propane stoves or charcoal grills into their tents to cook or keep warm.

The headaches, dizziness, and queasiness that can be symptoms of such poisoning may be discounted as food poisoning or altitude sickness — or go completely unnoticed during sleep — until it's too late. Keeping a tent-flap open isn't always enough.

Modern tents and stoves come with big warnings regarding carbon monoxide, says Betten, who once camped for more than five months as he hiked the Appalachian Trail from Georgia to Maine.

"Still," he says, "when the temperature drops at night, or the weather turns bad, it can be really tempting to bring the stove inside the tent to cook dinner." Betten's advice: "Don't."

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