Every winter, news reports in Japan and Korea describe campers and drivers found unconscious or dead in tents and parked cars. The cause is usually the same: carbon monoxide (CO) from a heater, stove, charcoal grill or idling engine. Most of the people involved were not reckless. They thought a gap in the door, a "camping" heater or a cracked window was enough. This guide explains why CO is so dangerous in small spaces, why those assumptions fail, and how to stay warm without running combustion where you sleep.

The short version is simple: do not run any flame-based heat source inside a closed tent or vehicle, and never sleep with one running. The rest of this article explains why, so that the rule sticks when you are cold and tired.

What carbon monoxide does to the body

Carbon monoxide is a gas produced whenever fuel burns with too little oxygen. Kerosene, propane and butane, white gas, wood, charcoal and petrol all produce it. It has no colour, no smell and no taste, and it does not irritate the eyes or throat. The smell you notice from a kerosene heater or a campfire comes from other combustion products, not from CO, so "it doesn't smell bad" tells you nothing.

Once inhaled, CO binds to haemoglobin in red blood cells far more strongly than oxygen does; the figure usually quoted is roughly 200 to 250 times more strongly. The resulting carboxyhaemoglobin cannot carry oxygen, and CO also makes the remaining haemoglobin release oxygen to tissues less readily. The brain and heart, which need the most oxygen, are affected first. Because CO leaves the blood slowly, exposure accumulates: a moderate concentration over several hours can be as dangerous as a high concentration for a short time.

Survivors of serious poisoning can have lasting problems with memory, concentration and movement, sometimes appearing days or weeks later. CO poisoning is not just "a bad headache"; treat any suspected case as a medical emergency.

Why the symptoms are so easy to miss

Early symptoms are vague: headache, dizziness, nausea, tiredness, shortness of breath on exertion and confusion. On a winter trip, each of these has an innocent explanation. A headache is blamed on a long drive. Nausea is blamed on dinner. Drowsiness is blamed on the cold, a beer, or a day of skiing. At a high campsite, people blame the altitude.

The most dangerous feature is that CO affects judgement before it incapacitates. A person who is becoming poisoned tends to want to lie down, which is exactly the wrong response. And once asleep, nobody notices symptoms at all. Many fatal cases involve people who simply went to sleep with a heater running and did not wake up.

Warning signs that should make you act immediately:

  • More than one person in the tent or car feels unwell at the same time
  • Symptoms improve when you step outside and return when you go back in
  • Symptoms began after a heater, stove, grill or engine was started
  • A pet or child is unusually lethargic or vomiting

If you suspect CO: turn off the source if you can do so in a moment, get everyone into fresh air, and call 119 in Japan (fire and ambulance). Do not go back inside to "check", and do not let someone with symptoms drive.

Concentrations and effects: commonly cited ranges

CO is measured in parts per million (ppm). The figures below are general ranges that appear widely in safety literature for healthy adults. Individual responses vary greatly: children, older people, pregnant women, people with heart or lung disease, and anyone who has been drinking are more vulnerable. Use these as a sense of scale, not as a safe limit.

  • Outdoor air: usually well under 1 ppm away from traffic.
  • Single-digit ppm: the range used by many long-term indoor air guidelines. Japan's building hygiene standard for indoor air sits in the single digits, and World Health Organization guidelines for exposure over several hours are of a similar order.
  • Around 35 to 50 ppm: levels often used as occupational limits for an eight-hour working day. Not a level to sleep in.
  • Around 200 ppm: headache, fatigue and nausea can appear within a few hours.
  • Around 400 ppm: strong headache within one to two hours; prolonged exposure can become life-threatening.
  • Around 800 ppm: dizziness, nausea and convulsions within about an hour; loss of consciousness possible within two hours.
  • Above roughly 1,600 ppm: severe effects within tens of minutes, with death possible within an hour or less as concentration rises further.

A small tent has a volume of only a few cubic metres, and even a large car interior is similar. A combustion heater running badly in that space, or charcoal smouldering in it, can push concentrations through these bands far faster than in a room. That is why cases happen even with heaters that seem to work perfectly well indoors.

Why "a little ventilation" is not a safe plan

The most common belief behind tent accidents is "it's fine, I left the vent open". Ventilation in a tent is not a fixed quantity you can rely on:

  • It depends on the weather. A tent breathes mainly because wind pushes air through vents and gaps. On a calm, cold night, which is exactly when people want heat, air exchange can drop sharply.
  • Winter seals tents. Snow piled on skirts and around the base, frozen condensation on mesh panels and fly sheets pegged low against wind all reduce airflow, often without you noticing.
  • Combustion changes over time. A heater that burns cleanly at first can produce much more CO as oxygen in the space falls, as the wick or burner gets dirty, or as it is turned down to a low, smoky setting.
  • CO does not stay in one place. It mixes with the surrounding air rather than collecting neatly at the top or bottom, so a high vent does not "let it out" in a predictable way.
  • Sleep removes the safety check. Even someone who would notice a headache while awake will not notice it asleep.

Oxygen-depletion and incomplete-combustion shut-off devices fitted to household heaters are designed and tested for rooms, not for tents or cars. They are not a substitute for keeping combustion out of small sleeping spaces. Vestibules and tarp-enclosed areas attached to the tent count as part of the same air space.

Choosing and placing a CO alarm

A CO alarm is a backup that can wake you if something goes wrong. It is not permission to run a heater in a tent. With that understood, carry one on any cold-weather trip, including car stays.

What to look for:

  • A dedicated, battery-powered CO alarm, not a combined gadget whose CO function is an afterthought. Many inexpensive "CO checkers" sold online give no information about calibration or testing. Prefer a unit that states compliance with a recognized alarm standard (for example EN 50291 in Europe or UL 2034 in North America) and has a loud audible alarm, not just a display.
  • An operating temperature range that matches your trip. Sensors and batteries have lower limits, commonly around freezing or a little below. If the night will be colder than the rated range, keep the alarm inside the sleeping space where it stays warmer, and treat its readings with caution.
  • A test button and an expiry date. Electrochemical sensors wear out, typically after several years. Check the end-of-life date and replace the unit when it is due.

Where to put it:

  • Near your head while sleeping, at roughly the height of your face when lying down, so it measures the air you are actually breathing.
  • Not right next to a stove or heater, where a brief burst on lighting can cause nuisance alarms, and not in a direct draught from a vent, where readings will be unrepresentative.
  • In a car, in the cabin near the sleeping position, not in the boot under gear.

Before each trip, press the test button and fit fresh batteries. Understand that alarms are deliberately designed not to sound instantly at low concentrations, to avoid false alarms: at moderate levels they may take tens of minutes or more before alarming. A silent alarm does not mean the air is clean. If it does sound, get out first and investigate later.

Kerosene heaters, gas heaters and stoves in tents

Kerosene heaters have become popular for winter camping, and many tents are photographed with one glowing inside. It is worth being clear about what manufacturers say. The instruction manuals of mainstream kerosene and cassette gas heaters generally prohibit use inside tents and vehicles, and many cassette gas heaters are labelled for outdoor or well-ventilated use only. Read the manual of your own heater; if it says not to use it in a tent, that is the answer.

Many experienced campers still use heaters in large tents while awake, with heavy ventilation and an alarm. If you choose to accept that risk, these are the minimum rules, not optional extras:

  • Never sleep with any combustion heater running. Turn it off and let it cool outside the sleeping area before you lie down. This single rule would prevent most fatal tent cases.
  • Never use a heater or stove in a small tent, a sleeping inner tent or a car. The volume is too small and the margin is gone.
  • Keep it lit only while everyone is awake and alert, with large vents open on both sides and a CO alarm running.
  • Maintain it. A worn wick or dirty burner produces more CO. Use the correct fuel; old or contaminated kerosene burns poorly.
  • Watch for fire. Tent fabrics burn fast. Keep clear space, stable flat ground and nothing hanging above.

Cooking stoves follow the same logic. Cooking in a vestibule in bad weather is common, but a stove left running to warm the tent is a heater, and should be treated as one. Cook with the door open, turn the stove off as soon as you are done, and never leave a pot simmering while you rest.

Charcoal: the worst option

Charcoal deserves its own warning because it causes a disproportionate share of fatal cases. A charcoal fire produces large amounts of CO, especially once the flames die down and the coals are glowing. Coals that look almost finished can keep producing CO for hours, and they give off little visible smoke or smell to warn you.

The typical accident goes like this: a group cooks on a charcoal grill, the night turns cold, and someone brings the still-warm grill into the tent, the vestibule or the car to take the chill off. Do not do this, in any amount, for any length of time. Let charcoal burn out completely outside, away from the tent, and extinguish it properly according to the campsite's rules (many sites have a dedicated ash disposal point). Hibachi-style tabletop grills and charcoal hand warmers in enclosed spaces carry the same risk.

Cars: idling engines and snow-blocked exhausts

Sleeping in a car with the engine running for heat is common, and in snow it can be deadly. When snow builds up around the rear of a vehicle and covers the exhaust pipe, exhaust gas can be forced under the body and into the cabin. The Japan Automobile Federation (JAF) has published tests showing CO in the cabin rising to dangerous levels within tens of minutes when the exhaust is blocked by snow. Modern catalytic converters reduce CO in normal running, but they do not make it safe to breathe exhaust in a closed cabin, especially when the engine is cold or snow-blocked.

Snow can bury an exhaust faster than people expect: during heavy snowfall, drifting can fill the space behind a car in an hour or two, and snow pushed by ploughs can do it in minutes. Exhaust can also enter if the vehicle is parked with its rear against a snowbank or wall.

The safe plan for a car stay is to sleep with the engine off, using a sleep system rated for the night, as covered in our guide to winter car sleeping safety. If you briefly run the engine while awake, for example to clear windows before driving, check the exhaust is completely clear first and stay alert. In a snowstorm, if you are stranded and must run the engine for warmth, clear the exhaust repeatedly, open a window slightly on the side away from the wind, run it in short intervals rather than continuously, and stay awake. Never use a portable heater, stove or charcoal inside a car. Many michi-no-eki and car parks also ask drivers not to idle overnight, so engine-off is the courteous choice as well as the safe one.

Safer ways to stay warm

The best protection against CO is not needing combustion heat at night. Warmth comes mainly from what is under and around your body:

  • Insulation from the ground first. Most heat loss on a cold night goes into the ground. Use a winter-rated sleeping pad or stack two pads, a closed-cell foam pad under an inflatable one. See our three-season sleep system guide for how pads and bags work together.
  • A sleeping bag rated comfortably below the forecast low. The "comfort" rating matters more than the "limit" rating. Add a liner and wear a dry hat and dry base layers; change out of damp clothes before bed.
  • A hot water bottle (yutanpo). Fill it with hot water before bed and put it in a cover or a sock in the footbox of your bag. Do not let it rest against bare skin all night: low-temperature burns can develop from prolonged contact with something only pleasantly warm.
  • Electric blankets with a safe power source. Many campsites offer pitches with AC power (often called "dengen site"). An electric blanket on a low or medium setting typically draws tens of watts, so a portable power station of a few hundred watt-hours can often run one through a night; check your blanket's actual wattage and divide your battery's usable capacity by it, allowing a margin because cold weather reduces battery performance. Follow the campsite's rules on power use and keep cables dry.
  • Warm up before bed, not with a heater. Eat a proper dinner, take a short walk or do light exercise to raise body heat, and visit a nearby onsen if one is open. Get into the bag warm rather than trying to warm up inside it.
  • Choose the destination and date sensibly. If your gear is not rated for the night, change plans. The winter camping guide covers regional differences and how to pick milder locations.

If you do use a heater during the evening, build the habit of switching it off and taking it outside the sleeping area before anyone starts getting into sleeping bags.

Practical checklist before a cold night

  1. Check the forecast low and wind for the exact site, not the nearest city.
  2. Confirm your pad and bag are rated for that temperature with a margin.
  3. Pack a dedicated CO alarm; test it and fit fresh batteries.
  4. Decide in advance: no combustion heat while sleeping, no charcoal inside, engine off in the car.
  5. Place the alarm near your sleeping head height before you go to bed.
  6. If anyone feels unwell, get everyone outside first and call 119 if symptoms persist.
  7. In snow, keep the area around the vehicle, especially the exhaust, clear.

When in doubt, do not run combustion inside. Before you travel, confirm current weather, campsite rules and heater instructions with the official source for each.