Carbon Dioxide

CARBON DIOXIDE (CO2) by Dr Tony Boycott

Carbon Dioxide

Odourless gas, heavier than air, may form pockets of higher concentration in low areas. Experienced “CO2” cavers claim to be able to detect raised levels by a metallic taste. HSE max recommended exposure is 5000 ppm (0.5%) for 8 hours, or 15000 ppm (1.5%) for up to 15 mins. Normal concentration is 0.03% in surface air. Normally 1% in caves, may be higher because of rotting vegetation, poor ventilation, small passages with heavy traffic, or caves in areas with volcanic activity. Naturally occurring levels of 4.5% have been measured in some caves on Mendip, and up to 7% in some Australian caves. Tolerance increases with experience, and cavers regularly cave in levels of up to 6% for several hours.

Concentration in expired air is 4%. Increase in CO2 in inspired air stimulates the respiratory centre until CO2 equals 5.6% which equals normal concentration of alveolar air, where gas exchange is taking place in the lungs. After this increase in level is increasingly intolerable & narcotic. Responses assuming normal O2 (21%) and body at rest, symptoms may be increased if exercising or ambient O2 is decreased, but levels of O2 down to 13% are tolerable. Individual responses can also vary, and acclimatization can occur.

CO2 level Symptoms

1% Comfortable Slight increase in rate & depth of respiration.
2% Respiration x 1, headache after several hours exposure.
3% Respiration x 2, panting after exertion, slight headache.
4% Respiration x 3, throbbing headache, face flushed, nausea, sweating.
5% General arousal Respiration more than x 3. “Off effects” on removal from atmosphere (headache, nausea, vomiting).
6% Respiration x 6, can be tolerated for several hours.
7-10% Mental deterioration. Intolerable for more than a few minutes, progressive dyspnoea, violent respiratory distress.
10-15% Intolerable panting & exhaustion, unconsciousness within minutes, spasmodic neuromuscular twitching & convulsions.
25 -30% Unconscious after 8 - 12 breaths, convulsions & death


Evacuate from the area slowly and calmly. Leave baggage behind. Lifeline even simple climbs as judgment can be affected and even minor exertion can worsen symptoms. Symptoms resolve rapidly on removal, but headaches and nausea can persist for some hours, and acid base imbalance can be detected in the blood up to 48 hours after exposure to high levels.

Dr Tony Boycott is a MRO doctor and a member of the UBSS.

  • Last modified: 24 Nov 2021 15:56
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