What is carbon dioxide (CO2)?
Carbon dioxide (CO2) is a colourless, odourless gas, normally present in the air we breath; "fresh air" has about 0.03% CO2 and about 21% oxygen (O2). (CO2 forms about 4% of the air we breath out - a low enough level for "mouth to mouth" resuscitation to work.) In caves and old mines the concentration is normally up to about 1% but can be higher due to rotting vegetation or wood, organic pollution carried in by water, poor ventilation or heavy "caver traffic" in small passages. Using a carbide lamp further depletes the oxygen level and adds to the CO2. CO2 is heavier than air and may form pockets of higher concentration in low areas, particularly where these are badly ventilated.
Effects of excess carbon dioxide
Anyone breathing a higher than normal concentration of CO2 will suffer gradually increasing ill effects, depending on the level of the gas, and may eventually become unconscious or even die if not evacuated. (The most famous casualty being Neil Moss, who became trapped at the bottom of a shaft in Peak Cavern.)
The Health and Safety Executive sets CO2. limits for the workplace: 0.5% for long term exposure; 2% for short term exposure; anything higher than this for any length of time is regarded as a risk to human health.
The symptoms of excess carbon dioxide levels
2% - some breathlessness, headache after several hours exposure,
3% - panting after exertion, slight headache,
4% - panting, throbbing headache, face flushed, nausea, sweating,
7-10% - mental deterioration, gasping for breath, intolerable for more than a few minutes,
10-15% - intolerable panting and exhaustion, unconsciousness in minutes, convulsions.
Normally a person will recover once out in fresh air. However take care when evacuating as even minor exertion can worsen symptoms and judgement may be affected, so lifeline even simple climbs.
In addition to the symptoms experienced by humans, a useful guide is that a match or candle will not light or stay lit in an atmosphere with high levels of CO2, although humans will feel the effects before this point.
What can cavers do?
One difficulty is that levels of CO2 are not consistent but are known to change with the weather and the seasons- tending to be higher in summer and dry weather, not so high after heavy rain and windy weather. In some Derbyshire mines DCA is now monitoring levels changing from over 4% one day to under 1% the next day.
The approach adopted by DCA is to place advisory notices at sites known to be affected so that cavers are made aware of a potential danger; just as cavers are aware of flood dangers in some caves.
A copy of the DCA Advisory Notice Is available here: C02-warning-notice