CARBON DIOXIDE IN TOURIST CAVES - A REVIEW
INTRODUCTION
Carbon dioxide (CO2) is a colourless, odourless gas that forms 0.03% of dry air at sea level. The other main components of air are nitrogen, forming approximately 80% and oxygen (O2) forming 20%. Air in caves rarely has the same composition as dry air at sea level and caves in eastern Australia frequently contain air which is enriched in CO2.
Carbon dioxide enriched air may be both a health and safety hazard and a threat to the integrity of speleothems. Cave managers need to be aware of the problems posed by CO2 enrichment in both tourist and non-tourist caves and take adequate action. This paper reviews the occurrence of CO2 in caves, its effects on people and speleothems, air quality standards for tourist caves, and management approaches that can be taken.
TYPES OF CARBON DIOXIDE ENRICHMENT
Air in caves which varies significantly in composition from normal is known as "foul air". Three distinct types of "foul air" were described in a classification system by James (1977). Halbert (1982) developed a triangular graph system which allows types of "foul air" to be distinguished.
Type 1 "Foul Air"
Type 1 "Foul Air" occurs where cave air is enriched in CO2 but O2 is not removed from the system. Enrichment with CO2 dilutes all the other gases in the atmosphere, thus lowering the overall O2 content of the air. This type of "foul air" is caused by the release of CO2 into a cave's atmosphere as a result of calcite deposition. Every time that a drip deposits calcium carbonate, CO2 is released into the atmosphere. Concentrations of CO2 produced by this mechanism rarely exceed 1%.
Carbon dioxide is also released into the cave atmosphere when large ponds of water in caves de-gas as they move or deposit calcite rafts. This mechanism may be responsible for the high concentration of CO2 found at Wellington Caves (10%+).
Type 2 "Foul Air"
Type 2, or normal "foul air" occurs when O2 is removed from the air and replaced by CO2. This takes place when respiration occurs in caves. Bacteria which break down organic matter that has entered the cave may produce significant amounts of CO2 by respiration as the organic matter rots. At Bungonia rotting organic matter washed in by rain produces CO2 at levels up to 6%.
Respiration by people and tree roots can also be significant sources of CO2
Type 3 "Foul Air"
Type 3 "Foul Air" contains a significantly reduced amount of O2 and is relatively rare. It is sometimes called "stink damp" because it can have a strong odour due to its methane and hydrogen sulphide content.
"Stink damp" has been reported from the lower parts of deep caves at Bungonia. This type of "foul air" may only contain down to 10 or 11% O2 but still only have 1% CO2. "Stink damp" is dangerous and potentially lethal, as there is not sufficient CO2 present to cause a respiratory response in the body. Persons exposed to "stink damp" are likely to go blue in the face and pass out due to lack of oxygen.
Seasonal Response of "Foul Air"
The occurrence of "foul air" in Australian caves is closely related to seasonal variations in climate. Most caves breathe by a process described as the "chimney effect" which relies on the difference in temperature between the air inside the cave and the air outside the cave (Wigley & Brown, 1976). Air inside the caves stays at a fairly constant temperature. When the temperature outside the cave falls below the internal temperature, the cave will expire because the hot air in the cave is lighter and will rise upwards. The reverse situation happens when the cave is colder than the outside and the air flow is inwards.
The "chimney effect" works well in caves which have entrances at different levels. Many of the caves that have "foul air", however, have entrances at the same level or single entrances. Thus for a significant period of the year there is very little air movement.
During summer time the air in the cave will not move out because night temperatures do not drop enough to allow the cave to breathe. So, "foul air" accumulates in the bottom of the cave, leaving the rest of the cave relatively free from "foul air". In autumn, when night temperatures are starting to drop below cave temperatures, caves begin to breathe and the "foul air" moves up to fill the rest of the cave. During March and early April "foul air" can move right up to the cave entrance. This means that at Easter when many people go caving the "foul air" is likely to be closest to the surface. This is a problem at Bungonia and Wellington.
During the winter, after the caves have breathed, it is often possible to explore the lower parts of caves without encountering "foul air".
EFFECTS OF CARBON DIOXIDE ON HUMANS
The atmosphere is essential to our survival, from it we extract oxygen and to it we deposit carbon dioxide. In the lungs oxygen is actively transported from the air into the blood. People who live at high altitudes adapt over time to oxygen deficiency by increasing in the number of red corpuscles which carry oxygen in the blood stream.
Carbon dioxide leaves the blood and enters the air because there is a higher concentration of CO2 in the blood than in the air. Under normal atmospheric conditions this is an efficient process as the concentration of CO2 in venous blood (equivalent to a concentration in air of 6%) is very much higher than the concentration of CO2 in the air (0.03%).
Carbon dioxide forms an acidic solution. This property is used by the body to control respiration. The respiratory centre in the brain monitors the acidity of the blood and if it increases, the brain raises heart and respiration rates to compensate. One of the first responses of the body (in most but no all people) to conditions of higher than normal CO2 is an increase in pulse and respiration.
If the concentration of CO2 in the air is increased, the lungs are less able to remove CO2 from the blood, and the CO2 concentration in the body will rise. This condition is given the technical name HYPERCARNAPIA. The symptoms and signs of hypercarnapia include:
- headache
- nausea
- increased pulse rate
- increased respiration rate
- peripheral vasodilation (people go red in the face)
- narcosis
The narcotic effect of CO2 is a particular danger to cavers as it inhibits judgement and ability to carry out complex tasks such as climbing and tying knots. It has probably been a factor in some cave accidents.
Exposure to concentrations above 6% will eventually result in death as the body will not be able to dispose of CO2 and metabolic functions will cease.
Prolonged exposure to higher than normal concentrations of CO2 will cause the acidity of the blood to rise, a condition known as ACIDOSIS which can be quite serious. This appears to be the cause of after effects suffered by cavers exposed to high levels of CO2 for extended periods such as vomiting and fainting up to 24 hours after exposure.
People respond differently to exposure to high levels of CO2. Asthmatics, chronic bronchitis sufferers and those with heart conditions are likely to be more affected than others.
EFFECTS OF CARBON DIOXIDE ON SPELEOTHEMS
Calcium carbonate is deposited to form speleothems when water loses CO2 to the cave atmosphere. Elevated levels of CO2 in the cave atmosphere can cause speleothems to dissolve. Carbon dioxide is thus not only a health and safety issue, it is also a conservation issue.
Kermode (1980) recognised that if the CO2 content of cave air was raised above 0.24% speleothems would begin to be dissolved, rather than deposited. He found that respiration by tourists was raising the level of CO2 in Waitomo Caves (NZ) to levels of 0.4%.
MEASUREMENTS OF CARBON DIOXIDE IN CAVE AIR
There are two different types of methods which can be used to test for CO2 in caves. Flame extinctions tests are used by cavers to give a rough guide to air conditions while instrumental tests are of more value to managers and researchers.
FLAME EXTINCTION TESTS
Flame extinction provides a simple and fairly reliable guide to cavers as to the safety of air in caves. The figures quoted here are based on the work of James et al (1975), and have been found by the author to be reliable when compared with instrumental readings. These figures relate to extinction in Type 2 (normal) "foul air".
Matches are extinguished, and will not light, in air containing more than about 1% CO2. As a general rule cavers are advised that if a match won't light they should get out. Once ignited, candles will burn in up to 4% CO2. At about 4% candles are extinguished.
Higher concentrations, approximately 6% are required to extinguish gas flames such as carbide lamps and gas cigarette lighters. Gas flames separate from the jet and elongate before extinguishing.
INSTRUMENTAL METHODS
There are two main types of instruments used to measure CO2 in caves. One type uses a metered pump and tubes containing sensitive chemicals, while the other uses electronic sensors.
Instruments using sensitive chemical tubes have proved to be a highly reliable method of measuring atmospheric gas concentrations in field conditions. The instruments consist of a pump unit (usually a hand operated bellows or syringe) into which sensitive calibrated tubes are fitted. A colour change in the tube indicates the concentration of gas present. A number of brands of these instruments are available; Drager, Gastech, Kitegawa. The instruments themselves are not particularly expensive (circa $500, 1990 prices) but the price of detection tubes ($3, 1990 prices) can make monitoring programs quite expensive.
Electronic methods of measuring CO2 rely either on its thermal conductivity or its infrared spectrum. These instruments are expensive, costing thousands of dollars, but are better suited to monitoring programs than the chemical tube methods described above. Calibration of these instruments may, however, be quite difficult and the results are probably not as reliable as those measured using chemical tube methods.
SAFE LEVELS OF CARBON DIOXIDE IN TOURIST CAVES
Safety standards for CO2 in tourist caves should be set to protect the natural features of the caves and the health and safety of patrons and employees. Kermode's work on re-solution of speleothems (Kermode, 1980) would suggest that the maximum level of CO2 in tourist caves should not exceed 0.24%. This level is, however, considerably lower than the levels of CO2 found naturally in many caves in eastern Australia.
Standard for air quality in Australian tourist caves should:
1. Protect speleothems for re-solution in caves where natural accumulation of CO2 does not occur.
2. Protect the health and safety of patrons and employees in caves where natural accumulation of CO2 does occur.
Protecting Speleothems in Tourist Caves without Natural CO2 Enrichment
Where it can be shown that the exhaled breath of visitors (or some other factor directly related to the management of the cave) is the principal cause of CO2 enrichment it is essential to manage the cave so that the CO2 concentration in the cave air does not exceed 0.24%. This is particularly important in decorated caves with higher visitor use e.g. Jenolan, Buchan, Wombeyan.
Protecting People in Tourist Caves with Natural CO2 Enrichment
In both tourist and non-tourist caves where sources other than exhaled breath of visitors are the principal causes of CO2 enrichment it is clearly not possible to control CO2 levels by management. Management must then protect patrons and employees by not exposing them to conditions which may be deleterious to health. Osborne (1981) discussed the various factors which need to be taken into account when determining safe levels of CO2 for tourist caves. The most useful of these is the "Threshold Limit Value" (T.L.V.) which is defined as the maximum concentration to which nearly all workers can be repeatedly exposed for long periods without adverse effect. The T.L.V. for CO2 is set at 0.5%. Since Osborne (1981) reported tourists showing signs of hypercarnapia where CO2 levels were as low as 0.25%, it would seem wise to apply the same standard to caves with natural accumulation of CO2 as to those without (i.e. a maximum level of 0.24%). The management implications for these caves will, however, be different.
CARBON DIOXIDE AND CAVE MANAGEMENT
Managers of tourist caves have a legal and moral obligation to protect the caves in their care and to ensure that their patrons and employees are protected from dangers to their health and safety. Accumulation of CO2 beyond acceptable limits is always a health and safety hazard and may constitute a threat to the integrity of the cave.
The appropriate management responses will depend on the source of the CO2.
Caves without natural CO2 accumulation
Artificial enrichment of cave atmospheres with CO2 above 0.24% will lead to degradation of speleothems and must be avoided. Managers of tourist caves with high visitation rates should monitor CO2 levels and lower visitation rates if concentrations approaching the critical level are reached. It is particularly important that frequent measurements are made during periods of peak visitation.
Monitoring of CO2 levels to protect speleothems will also ensure the health and safety of patrons and employees. Carbon dioxide content should never exceed safe levels (i.e. 0.5%) in caves lacking significant natural sources of CO2.
Caves with natural CO2 accumulation
The principal aim of management in these types of caves is to protect the health and safety of patrons and employees by ensuring that they are not exposed to air containing more than the T.L.V. (0.5%) for CO2. Monitoring, particularly during autumn, will form the foundation of management.
A cave should be closed, even during peak tourist seasons, if its air is unsafe.
Another possible management approach is to alter the cave's ventilation. This may be an acceptable practice in small, poorly ventilated caves with natural CO2 enrichment, but is not a viable remedy for accumulation caused by overuse of highly-decorated caves. Artificial ventilation has the potential to change the humidity of caves, introduce dust and threaten cave fauna.
ACKNOWLEDGMENTS
This paper is based on an impromptu talk given at the conference in 1983. Elery Hamilton-Smith tape recorded the talk and Penny Osborne transcribed the text from the tape.
REFERENCES
HALBERT, E.J.M. 1982: Evaluation of Carbon Dioxide and Oxygen Data in Cave Atmospheres using the Gibbs Triangle and the Cave Air Index, Helictite 20(2):60-68
JAMES, J.M. 1977: Carbon dioxide in the Cave Atmosphere, Trans Brit. Cave Res. Assn 4(4):417-429
JAMES, J.M. PAVEY, A.J. & ROGERS, A.F. 1975: Foul Air and the Resulting Hazards to Cavers, Trans Brit. Cave Res. Assn, 2(2):79-88
KERMODE, L.O. 1980: Cave Corrosion by Tourists, Proc. Third Aust. Conf. Cave Tourism and Management, Mt. Gambier, May 1979, 97-104
OSBORNE, R.A.L. 1981: Towards an Air Quality Standard for Tourist Caves: Studies of Carbon Dioxide Enriched Atmospheres in Gaden-Coral Cave, Wellington Caves, N.S.W., Helictite 19(2):48-56
WIGLEY, T.M.L. & BROWN M.C. 1976: The Physics of Caves, in Ford T.D. and Cullingford C.H.D. 1976, The Science of Speleology pp329-358. Academic Press, London.