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carbon monoxide poisoning - the silent camp killer


  • carbon monoxide poisoning is a preventable lethal hazard as long as you follow the basic rules
  • it is an odourless gas - you will NOT be able to detect it without a CO alarm or monitor, and it may cause you to become drowsy and fall asleep, never to wake up!
  • the far majority of accidental CO poisoning deaths occur in homes due to space gas heaters (and occasionally if people burn charcoal or Heat Beads or similar inside the house)
  • fortunately, camping deaths are rare as most people know the rules regarding stoves and heaters inside tents, etc
  • it has a molar mass of 28 while air has an average molar mass of 28.8 and thus it is slightly lighter than air and will tend to RISE, particularly if it is warm as when it has just been produced, however, given the difference is minimal, in reality, when it cools to air temperatures, it will just mix with air.
  • gas sensors in CO alarms have a limited lifespan, generally 4-6 years. The test button on common CO alarms only tests the battery, not the CO gas sensor.
    • most will only alarm when levels exceed 50-75ppm but you can still get headaches sleeping all night in lower levels

Basic rules to prevent carbon monoxide poisoning whilst camping

  • do NOT have any unflued device which burns fuel (gas, solid or liquid) inside a poorly ventilated space
    • a brief boil of a cup of water from a hiking stove will have a low risk if done in an open vestibule
    • a gas stove inside a caravan is generally safe for short periods as long as windows and door are open and you are walking around cooking but a gas heater is NOT safe as you may fall asleep
  • if using a flued device such as a wood stove with a chimney, ensure the space is well ventilated by an open door or large window, the flue is open and clear, AND you have a carbon monoxide alarm
    • leakage of CO from flued wood stoves is often maximal if either the door is open or when the flames go out and the ashes are smouldering - then is NOT the time to close the doors and window - wait until it is fully out!
  • don't run a diesel heater, petrol generator or a car with its exhaust outflow flowing into an enclosed space such as a tent
  • some gas heaters have a “Low oxygen safety shut off system” BUT this does little to prevent lethal levels of carbon monoxide to build up!
    • the shut off is only for managing when oxygen levels are low and whilst this may increase the rate of CO production, shut off at that level may well be too late.
  • strong winds can hinder the escape of fumes from chimneys and increase risk of CO poisoning from flued stoves

Carbon monoxide production rates

  • during burns, carbon-based fuels (ie. nearly all gas, liquid and solid fuels we would use) consume oxygen to produce a mix of water + carbon dioxide + carbon monoxide + various other substances depending upon the fuel
  • CO is more likely to be produced when oxygen supplies to the burning fuel are reduced or when the fuel is undergoing “smouldering combustion” such as when you just have ashes or you are using Heat Beads
  • flaming combustion of wood generally results in a ratio of CO2:CO of 10:1
  • smouldering combustion of wood generally results in a ratio of CO2:CO of 1:1 because a flame is needed to burn any expelled CO - albeit the burn rate is ~100 x slower than flaming combustion
  • un-flued domestic oak charcoal cooking stoves:
    • produce ~8.9kWh/kg
    • CO production is 140-180mL/min/kW and to reduce CO poisoning would require 40-56m3/h/kW ventilation 1)
  • heat energy of various fuels
    • brown coal = 10MJ/kg
    • dry firewood 16MJ/kg = 4.3-4.5kWh/kg
    • hard black coal = 25MJ/kg
    • charcoal = 29MJ/kg = 8kWh/kg
    • petrol or diesel = 44MJ/kg = 12-13kWh/kg
    • butane, propane, LPG = 48MJ/kg = 13kWh/kg
  • what about burning candles?
    • when there is plenty of oxygen available, candles will generally produce carbon dioxide (3g produced per 1g candle burnt) with little carbon monoxide (but as with any other flame burn, will also burn nitrogen in the air to form nitrogen oxides) 2)
    • prolonged multiple candle burns over several hours in a poorly ventilated room could result in lower oxygen levels, increased production of CO and a symptomatically significant build up of CO levels

Carbon monoxide safety levels

  • most CO alarms are set to go off at 50-100ppm
  • most workplace safety limits for 8 hour exposures is no greater than 50ppm and no one should have ongoing exposure if CO reaches 100ppm
  • toxic symptoms usually occur at COHb levels above 15% (smokers often have levels up to 10%, 30% is severe poisoning) as it prevents oxygen from binding to Hb and thus oxygen is unable to get to the organs, especially the brain.
  • the aim is to ensure blood levels of COHb do not exceed 2.5% hence:
    • < 15min exposure at 90ppm
    • < 30min exposure at 50ppm
    • < 60min exposure at 25ppm
    • < 8hr exposure at 10ppm
    • half-life of COHb is ~5hrs in room air with no CO
  • example exposure levels
    • natural atmosphere level is 0.1ppm
    • average levels of CO inside homes is 0.5-5ppm
    • levels are usually 5-15ppm near properly adjusted gas stoves in homes
    • smoke from wood fires often contain 5000ppm however Australian bushfire smoke which firefighters are usually exposed to only has 40-50ppm which equates to cigarette smoking, and a 8hr shift results in COHb levels of 5-11% for smokers and 3-7% for non-smokers 3)

Carbon monoxide poisoning

  • usually considered with COHb > 3% in non smokers and > 10% in smokers
  • affects of acute exposures:
    • 6 - 8 hrs of exposure to 35ppm CO will usually cause headache and dizziness
    • 2 - 3 hrs of exposure to 100ppm CO will usually cause slight headaches
    • 2 - 3 hrs of exposure to 200ppm CO will usually cause slight headaches and loss of judgement
    • 1 - 2 hrs of exposure to 400ppm CO will usually cause a frontal headache
    • 45min of exposure to 800ppm CO will usually cause dizziness, nausea and convulsions and by 2 hours exposure will become insensible
    • 20min of exposure to 1600ppm CO will usually cause headache, dizziness, nausea and fast heart rate and death will usually occur after 2hrs of exposure
    • 5-10min of exposure to 3200ppm CO will usually cause headache, dizziness, nausea and by 30 minutes of exposure, death.
    • 1-2min of exposure to 6400ppm CO will usually cause headache, dizziness, nausea and by 20 minutes of exposure, convulsions and death.
    • 2-3 breaths of exposure to 12,800ppm CO will usually cause unconsciousness and by 3 minutes, death.
    • acute exposures may result in long term neurologic sequeleae
      • especially if they became unconscious or are elderly or have other susceptibility factors
  • chronic low exposures may also cause long term sequelae:
    • persistent headaches, lightheadedness, depression, confusion, memory loss, nausea, hearing disorders and vomiting
    • persistent indoor levels above 6ppm may lead to COHb levels of 2%
    • long-term exposures to carbon monoxide present the greatest risk to persons with coronary heart disease and in females who are pregnant

A mental exercise - a wood stove inside a tall dome tent

  • tent is 3.9m diam x 2m tall octagonal and has a volume of ~18m3
  • a Winnerwell Nomad Medium stove firebox will have a volume of 0.03m3
  • assuming stove does not leak smoke without door being opened and assuming no ventilation of the tent, and a CO concentration in the smoke inside the stove when smouldering is 5000ppm.
  • if one opens the door when it is full of smoke and smouldering and 80% of the smoke escapes into the tent, the increase in CO concentration within the tent once it is equalised would be:
    • concentration in tent = 5000 * 0.03 * 0.8 / 18 = 7ppm additional CO for each time the stove door is opened when it is smouldering
  • if ventilation flows are low it may take some time for equilibration throughout the tent to occur and there will thus initially be areas of much higher concentrations (although these will generally tend to rise up given the CO is lighter than air) and may escape through the gaps in the tent around the chimney - especially if there is a Venturi effect from a mild breeze (a strong breeze however may prevent smoke going up the chimney and may force it out of the firebox into the tent)
  • in reality if ashes were left to smoulder, there would be considerable ongoing CO production and leakage into the tent as the firebox air temperature may not be high enough to allow escape up the chimney flue and hence it is likely to seep out through any gaps in the firebox including the air intake. This could significantly increase CO levels in the tent if it was not well ventilated.
  • to further compound this, the half life of CO in the body is 5hrs and thus each additional “dose” will be additive given how long it takes to be removed from the body
    • whilst opening the stove door your head will be close to the smoke and thus at risk of taking a few breaths of much higher levels of CO than you would like which will be additive doses - unless you have it well ventilated!
australia/co.txt · Last modified: 2024/05/09 22:42 by gary1

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