FOUL AIR IN LIMESTONE CAVES AND ITS EFFECT ON CAVERS.

By Garry K. Smith © 1998

First Published in the ASF 22nd Biennial Conference Proceedings 1999, Pages 48-58

Member - Australian Speleological Federation and the Newcastle & Hunter Valley Speleological Society. P.O. Box 15 Broadmeadow. N.S.W, 2292 Australia.

Principal Technical Officer - BHP Research Laboratories. P.O. Box 188 Wallsend, N.S.W, 2287 Australia.

Comments are most welcome. Direct to Garry K. Smith via Electronic mail at smith.garry.gk@bhp.com.au or by snail mail to "5 Fourth St, Seahampton 2286, NSW Australia"

INTRODUCTION

‘Foul Air’ is a life threatening hazard which speleologists may encounter in caves with relatively still atmospheres. Although not a significant problem in the majority of caves around the world, those containing concentrations of foul air may become death traps for cavers not familiar with the signs and symptoms of the gases involved.

‘Foul Air’, sometimes called ‘Bad Air’, is a cave atmosphere which has a noticeable abnormal physiological effect on humans. In limestone caves, ‘foul air’ can be described as containing greater than 0.5% carbon dioxide (CO₂) and/or lower than 18% oxygen (O₂) by volume. As a comparison, normal air contains approximately 0.03% CO₂ and 21% O₂ by volume. However there are some isolated caves which contain atmospheres influenced by other gases such as:- methane, ammonia, hydrogen sulfide or carbon monoxide, but these gases are generally rare in limestone caves.

An elevated CO₂ concentration is usually the most life threatening foul air scenario found within Australian limestone caves. This colourless, odourless and non-combustible gas is the body's regulator of the breathing function. In industry the maximum safe working level recommended for an 8 hour working day is 0.5% (5,000ppm by volume). A concentration of 10% or greater can cause respiratory paralysis and death within a few minutes.

To the novice caver the first encounter with foul air is often a frightening experience. Typically there is no smell or visual sign and the first physiological effects are increased pulse and breathing rates. Higher concentrations of CO₂ lead to clumsiness, severe headaches, dizziness and even death. Experienced foul air cavers can notice a dry acidic taste in their mouth, however the average caver may not notice this effect.

Strang and Mackenzie-Wood, (1990) state that, “Carbon Dioxide is regarded as a ‘hot gas’ due to its low thermal conductivity, heat is not conducted away as rapidly as in normal air so a person standing in it feels warm about his lower limbs”.

TYPES OF FOUL AIR IN CAVES.

The Foul Air Types used below were first characterised by James (1977) and expanded upon by Halbert (1982) with the use of the Gibbs Triangle and the Cave Air Index.

1. In this scenario, “Foul Air Type 1”, CO₂ is absorbed by the ground water as it passes through surface soil containing high concentrations of the gas, due to the decay of vegetation. Soil CO₂ contents can reach as high as 10 to 12%, however most values range between 0.15% and 0.65%. The resulting weak carbonic acid percolates through the rock strata and enters the cave system, usually taking part in the calcite deposition cycle. In this instance the addition of extra CO₂ to the cave atmosphere equally displaces O₂ and nitrogen (N₂) in direct proportion to which they constitute the atmosphere being displaced. See Table 2.

Atmospheres consisting strictly of “Type 1” foul air, rarely exceed 1% CO₂. An example of this atmosphere could contain 1% CO₂ and 20.8% O₂.

Halbert (1982), relates “Foul Air Type 1” cave atmosphere to the introduction of CO₂ into the cave atmosphere and all other components are diluted - the source of the CO₂ is immaterial. An atmosphere resulting from purely a type 1 process occurs quite slowly and it requires five percent CO₂ to reduce the O₂ level by one percent.

2. In the second scenario “Foul Air Type 2” the CO₂ is a by-product of organic and micro-organism metabolism or respiration by fauna such as bats or humans. In this instance the oxygen concentration is reduced in proportion to the increase in CO₂. The N₂ concentration stays constant. See Table 3.

Halbert, (1982) “Foul Air Type 2” describes in great detail the relationship between consumption of O₂, and production of CO₂ in the metabolic process of living organisms. Essentially the volume ratio of CO₂ produced to O₂ consumed, called the “respiratory quotient” (RQ) is not constant and can vary between 0.7 and 1, depending on organic matter involved. ie. carbohydrates, fats or protein. If fats were utilised solely in the metabolic process the RQ = 0.7, and would result in a consumption of O₂ with a relatively smaller amount of CO₂ volume being produced in return.

3. In the third scenario, “Foul Air Type 3”, cave atmosphere which has resulted from the introduction of other gasses, such as methane and nitrogen and the non-respiratory uptake of O₂ as well as CO₂ stripping by water. Another example is “stink damp” so named because it often contains hydrogen sulfide and the O₂ is significantly more depleted than in “Type 2”. Foul air consisting strictly of “Type 3” are rare in caves and therefore is only dealt with briefly in this paper.

Also falling into Halbert’s third type is an atmosphere which has resulted from a combination of scenarios 1&2 with addition of another mechanism (“Foul Air Type 3”), which alters the gas concentrations.

James, (1977) recognised six sources of CO₂ in cave atmospheres,

a) evolution from cave waters

b) production from micro-organisms

c) respiration of plants and animals

d) Diffusion of gaseous CO₂ into the cave

e) Burning of hydrocarbons

f) Volcanic emission

Of these the first three are covered in scenarios 1&2. Not considered in this paper is source d) which is an external source of CO₂, generally of a sporadic nature and the last two don’t have any real significance in Australian Caves.

INFLUENCING FACTORS IN CO₂ CONCENTRATION.

Even though CO₂ is 1.57 times heavier than nitrogen and 1.38 times heavier than O_2, it has a tendency to disperse in an isolated volume of air, due to molecular diffusion. In other words, a mixture of gasses will not separate into layers of various density gases if it is left for a long time in a still chamber. On the other hand, various gasses purged separately into a closed container will become uniformly mixed over a period of time. A possible explanation of the high concentration of CO₂ in deep caves (with a relatively still atmosphere), is that CO₂ is being produced metabolically or entering the cave via ground water at a greater rate than the gas can disperse (by molecular diffusion) into the cave atmosphere, thus settling or remaining at the bottom of the cave because it is a dense gas. (Smith. G. K. 1997a)

‘Foul air’, is often encountered in pockets at the lower sections of deep caves where there are no active streams and air movement is minimal. Frequently there appears to be a definite boundary between ‘good air’ and ‘foul air’, with a noticeable elevation in CO₂ concentration being present. In caves containing ‘foul air’ the author has on numerous occasions experienced these invisible boundaries with a transition of less than one metre. Often there isn’t a gradual transition in air quality as one might expect if dispersion of the gases were occurring at a relatively fast rate. In Australia most of these atmospheres can be attributed to ‘Foul Air Type 2’, or a combination of (2+1) or (2+3), however the CO₂ is being introduced into a relatively still cave atmosphere and molecular diffusion is insufficient to disperse the gas with an even gradient over the vertical range of the cave.

This build up in CO₂ concentration is more prevalent in deep caves, however it can still be found in some shallow caves with a vertical range of less than 10 metres. A very still cave atmosphere may allow CO₂ to sink to (or remain at its origin in) the deepest part of the cave and displace O₂ and N₂. This allows CO₂ to build up to a higher concentration, at the lowest point. An example of this would be Suicide Hole Cave at Crawney Pass N.S.W, which has a vertical range of approximately 6 metres and contains a high concentration of CO₂ in the bottom two metres of cave passages. The CO₂ can be attributed to a large number of fine tree roots in a passage just above the foul air.

An example of how CO₂ can be liberated and build up to high concentrations in the bottom of caves is suggested by Osborne, (1981), in a study of the ‘CO2 Pit’ in Gaden Cave - Wellington NSW Australia. Osborne surmises that the atmosphere is most likely due to degassing of the extensive bodies of still water in the underground lake system. This relates to a strictly foul air type 1, however Osborne conjectures that the test measurements indicate a type 2 foul air is also involved.

Indications are that the gas is being introduced into the cave atmosphere at a greater rate than it can disperse by molecular diffusion, thus a very definite boundary occurs. In the ‘CO2 Pit’, the boundary between good breathable air and life threatening foul air is often less than 0.4 metre. Recent discussions with divers undertaking mapping and photographic projects indicate that the extensive underground lakes are well known for their acidity and constant production of calcite rafts. This strongly supports the theory put forward by Osborne, that the majority of the foul air in the ‘CO2 Pit’ is due to Type 1 with the addition of some Type 2.

James & Dyson (1981) found at Bungonia, NSW Australia, that “CO₂ is encountered at a threshold and below the threshold it appeared to be relatively homogeneously mixed”. While “……caves with flowing streams which terminated in sumps showed a pronounced CO₂ gradient, increasing with depth”. Drum Cave generally followed this pattern, however during bat maternity season, an inverted gradient was observed even when the stream flowed. The bats respiration and micro-organisms in the guano were concluded to be the major sources of CO₂, and were responsible for the highest recordings in the cave, (measured in the entrance series chamber). The CO₂ concentration was observed to decrease down the cave toward the terminal sump. They conclude that “in general, CO₂ is located in the cave close to the source of its production”.

Another factor was highlighted with a study of Grill Cave at Bungonia. This cave is known to regularly contain foul air (which has a short transition distance between good air and hazardous foul air), the relative depth from the surface, (of the interface), varies considerably with climatic change and correlates with highs and lows in atmospheric pressure. The high atmospheric pressure compressing the gasses, thus pushing the interface deeper into the cave and the reverse with atmospheric lows. This can be greatly enhanced by passage dimensions and volume capacities of chambers within a cave system. (Smith G. K., 1998).

Temperature changes outside caves also have an effect on the concentration of foul air. Jennings, et al. (1972) give the example of caves at Bungonia where the average underground temperature is 17.75°C. During summer the above ground air temperature rarely drops below this temperature, hence the cold, dense air remains in the lower levels without circulating. However during winter the caves “breathe”. The warmer air rises, thus causing an expansion of the CO₂ regions with a reduction in CO₂ concentration.

Floods are also known to reduce high concentrations of ‘foul air’ as the influx of large volumes of fresh water absorb CO₂ from the cave atmosphere and transport it away. O₂ is also liberated from the fresh water into the cave atmosphere. Floods also carry into the cave, fresh organic matter which micro-organisms feed on to rapidly increase CO₂ once the water flow has subsided. Micro-organisms can increase CO₂ concentration in the cave atmosphere by several percent over a 48 hour period. (James and Dyson, 1981)

Foul air will not build up in caves with two entrances at different elevations, as temperature gradients cause a flow through affect which flushes the cave atmosphere. Active stream-ways in caves also dissipate any build up of foul air. See Figure 1.

CALCULATING GAS CONCENTRATIONS IN A CAVE ATMOSPHERE.

In dry air the total pressure (of a mixture of gases) is equal to the sum of their partial pressures. In simplified terms, the atmospheric or barometric pressure of dry air is equal to pNitrogen (pN₂) + pOxygen (pO₂) + pRare Gases (pRG) + pCarbon Dioxide (pCO₂).

However since a great majority of cave atmospheres contain high humidity, the water vapour component should be included in the equation.

Barometric Pressure = pN₂ + pO₂ + pRG + pCO₂ + pH₂O.

Halbert (1982) uses the Cave Air Index (CAI) to characterise gas mixtures found in caves on a dry atmosphere basis. The water vapour component in the calculation, slightly changes the concentrations of CO₂ and O₂, but does not affect the arguments derived from the data. Essentially the water vapour constitutes about 0.5% by volume of a saturated cave atmosphere at 20°C and conversely in a dry atmosphere it would be 0%.

For simplicity cave atmospheres may be considered to consist of O₂, CO₂, and a Residue Fraction (RF) made up of rare gases, N₂ and water vapour (H₂O).

Table 1. Cave air scenario and correlation with “Foul Air Type” & Cave Air Index.

Foul Air Type (after Halbert 1982)	Possible Mixes	Cave Air Index
1		4 < CAI < 5
1+2 combination	1+3	1 < CAI < 4
2	2+1, 2+3, 1+3	0.75 < CAI < 1
2+3 combination	1+3	0 <CAI <0.75
3		CAI = 0

^CO₂ ^{Concentration}

Cave Air Index = ^{------------------------------}

21- O₂ ^{Concentration}

INTERPRETING FOUL AIR.

The theoretical “Foul Air Type 3”, where CAI = 0, is rarely known to exist in caves. In general cave atmospheres with CAI of < 0.75 are regarded as falling into the Foul Air Type 3. This could be a mixture of “Foul Air Types” (3+1) or (3+2). Halbert (1982) gives the example of “Foul Air Type 3” atmospheres containing 1% CO₂, 17% O₂, and 82% RF and another with 4.5% CO₂, 10.5% O₂, and 85% RF. He points out that a low absolute O₂ concentration does not need to be present. However in practice “Foul Air Type 3” atmospheres likely to be encountered in caves will have low O₂. Also this type of foul air may have deceptively low CO₂.

At Bungonia Caves (N.S.W.) Australia, foul air accumulation by loss of CO₂ from saturated ground water is not the major source, but a contributing factor. CO₂ levels of up to 6% have been linked to micro-organisms (i.e. fungi and bacteria) which depend on the nutrition present in organic material leached down from the soil or washed into the caves by floods. These organisms produce CO₂ as a by-product of their digestion process. This mechanism was observed to correlate with the reduction in O₂ accompanied by the increase in CO₂ concentrations. This would suggest foul air Type 2 or a combination of 1 & 2. (Crawshaw. R. and Moleman, D, 1970), (Jennings. J. et al., 1972), (Smith., G, 1993)

Halbert (1982) suggests that some readings at Bungonia are a “Foul Air Type 3”. They include atmospheres in Grill Cave with a composition of 1.4% CO₂, 12.0% O₂, 86.6% RF which gives a CAI of 0.16 and readings in Odyssey Cave of 2.8% CO₂, 14.5% O₂, 80.3% RF which gives a CAI of 0.43. James (1977) had previously speculated on the possible sources of “Type 3” foul air sometimes found at Bungonia. These include:- (1) Anaerobic bacterial action - nitrogen producing bacteria which have been identified in caves at Bungonia. (2) Removal of O₂ from the cave atmosphere by oxidation of inorganic or organic sediments.

In 1958 members of Sydney University Speleological Society (S.U.S.S) confirmed that readings of up to 13.5% CO₂ at Wellington and Molong Caves (N.S.W.) Australia, were at the expense of oxygen. ie. the sum of CO₂ and O₂ was constant and the percentage of inert gases was reasonably constant. They also concluded that this was probably due to organic decomposition. (Halbert., E. J. 1972). These CO₂ readings appear to be exceptionally high and one would wonder if another mechanism could be involved. The answer could be in a later study of the ‘CO2 Pit’ in Gaden Cave - Wellington (N.S.W.) Australia, by Osborne (1981), when he surmises that the atmosphere is most likely due to degassing of the extensive underground lake system with some involvement of a type 2 foul air mechanism.

As can be seen from the above, it is one thing to analyse samples of cave atmosphere to determine composition, however the real problem comes with the interpretation of this data to identify the source of the gases, especially if the source is not readily apparent. Calculation of the CAI, appears to be a very valuable tool to assist researchers in identification of foul air types and hence could assist in tracking down the source.

Examples of foul air, theoretical gas concentrations are given in Tables 2, 3 & 4.

Table 2, Theoretical gas concentrations in cave atmosphere. Using scenario 1 with CAI = 4.

Total CO2 concentration in cave atmosphere	Total O2 concentration in cave atmosphere	Total RF concentration in cave atmosphere
1%	20.75%	78.25%
2%	20.50%	77.50%
3%	20.25%	76.75%
4%	20.00%	76.00%
5%	19.75%	75.25%
6%	19.50%	74.50%
7%	19.25%	73.75%
8%	19.00%	73.00%
9%	18.75%	72.25%
10%	18.50%	71.50%
24%	15.00%	61.00%

Table 3, Theoretical levels of gases in cave atmosphere,

Using a combination of scenario 1 & 2, resulting in CAI = 2.

Total CO2 concentration in cave atmosphere	Total O2 concentration in cave atmosphere	Total RF concentration in cave atmosphere
1%	20.50%	78.50%
2%	20.00%	78.00%
3%	19.50%	77.50%
4%	19.00%	77.00%
5%	18.50%	76.50%
6%	18.00%	76.00%
7%	17.50%	75.50%
8%	17.00%	75.00%
9%	16.50%	74.50%
10%	16.00%	74.00%
12%	15.00%	73.00%

Table 4, Theoretical levels of gases in cave atmosphere, Using scenario 2. with CAI = 1.

Total CO2 concentration in cave atmosphere	Total O2 concentration in cave atmosphere	Total RF concentration in cave atmosphere
1%	20.00%	79.00%
2%	19.00%	79.00%
3%	18.00%	79.00%
4%	17.00%	79.00%
5%	16.00%	79.00%
6%	15.00%	79.00%
7%	14.00%	79.00%
8%	13.00%	79.00%
9%	12.00%	79.00%
10%	11.00%	79.00%
15%	6.00%	79.00%

EFFECT OF CO₂ ON HUMANS.

As each persons body has a slightly different reaction and tolerance to stressful situations the following symptoms are general, however nobody is immune to the dangers of CO₂.

Table 5. Generally accepted physiological effects of CO₂ at various concentrations.

Concentration	Comments
0.03%	Nothing happens as this is the normal carbon dioxide concentration in air.
0.5%	Lung ventilation increases by 5 percent. This is the maximum safe working level recommended for an 8 hour working day in industry (Australian Standard).
2.0%	Lung ventilation increases by 50 percent, headache after several hours exposure. Accumulation of carbon dioxide in the body after prolonged breathing of air containing around 2% or greater will disturb body function by causing the tissue fluids to become too acidic. This will result in loss of energy and feeling run-down even after leaving the cave. It may take the person up to several days in a good environment for the body metabolism to return to normal.
3.0%	Lung ventilation increases by 100 percent, panting after exertion. Symptoms may include:- headaches, dizziness and possible vision disturbance such as speckled stars.
5 - 10%	Violent panting and fatigue to the point of exhaustion merely from respiration & severe headache. Prolonged exposure at 5% could result in irreversible effects to health. Prolonged exposure at > 6% could result in unconsciousness and death.
10 - 15%	Intolerable panting, severe headaches and rapid exhaustion. Exposure for a few minutes will result in unconsciousness and suffocation without warning.
25% to 30%	Extremely high concentrations will cause coma and convulsions within one minute of exposure. Certain death.

(Strang. J., and Mackenzie-Wood. P., 1990), (Laboratory Safety Manual, 1992) (Osha Regulated Hazardous Substances, 1990), (Matherson, D., 1983).

Long term exposure to levels of between 0.5 and 1% as may be experienced by personnel on a submarine, is likely to increase calcium deposition in body tissues such as the kidney. (Matherson, D., 1983)

Exposure of between 1 and 2% CO₂, for some hours will result in acidosis, even if there is no lack of oxygen. This acid-based disturbance will occur in the human body when the increase in partial pressure of CO₂ (pCO₂) is greater than 44mm Hg. Acidemia will result and secondary mechanisms are initiated by the body that attempt to prevent drastic changes in pH and tend to return the pH toward normal. “Intracellular buffering, via red cell haemoglobin, phosphate, and protein, exchange intracellular sodium and potassium for the excess extracellular hydrogen ion. In addition, hypercapnia leads to an increase in renal hydrogen ion secretion and net acid excretion, as well as an increase in bicarbonate reclamation. Although this response begins early, the maximum effect takes several days.” (Clinical Management of Poisoning & Drug Overdose, 1990).

Prolonged breathing of air containing around 2% or greater will disturb body function by causing the tissue fluids to become too acidic. This will result in loss of energy and feeling run-down even after leaving the cave. It may take the person up to several days in a good environment for the body metabolism to return to normal.

The “Laboratory Safety Manual (1992)”, quotes 0.5% CO₂ as the `Threshold Limit Value Time Waited Average’ (TLVTWA). This is the concentration to which a person may be exposed, 8 hours a day, 5 days a week, without harm. The manual also quotes 5% CO<sub>2</sub> and above as being `Immediately Dangerous To Life and Health’ (IDLH). This is the concentration that will cause irreversible physiological effects after 30 minutes exposure.

One must be mindful that the sight of bats in a cave does not necessarily mean that the atmosphere is suitable for humans. On several occasions the author has experienced laboured breathing in caves containing bats, however a simple Butane Cigarette lighter would fail to ignite and struck match head would only fizz before going out. The bats seemed to be undeterred by the low O₂ and high CO₂ content of the atmosphere. These observations are echoed by Hamilton-Smith, (1972) who states that, “….. the Bent-winged Bat is able to tolerate higher concentrations of gas (CO₂) than that acceptable to human beings.”

EFFECTS OF O₂ DEFICIENCY ON HUMANS.

If we consider an atmosphere consisting of just N₂ and O₂, where the O₂ is at a lower concentration than the normal atmosphere, the human body would be effected in the manner shown in Table 6. (Laboratory Safety Manual, 1992)

Table 6. Generally accepted physiological effects of reduced O₂ concentrations.

O2% by volume.	Symptoms (at sea level)
reduced from 21 to 14%	First perceptible signs with increased rate and volume of breathing, accelerated pulse rate and diminished ability to maintain attention.
between 14 to 10%	Consciousness continues, but judgment becomes faulty. Rapid fatigue following exertion. Emotions effected, in particularly ill temper is easily aroused.
10 to 6%	Can cause nausea and vomiting. Loss of ability to perform any vigorous movement or even move at all. Often the victim may not be aware that anything is wrong until collapsing and being unable to walk or crawl. This is followed by unconsciousness and death. Even if resuscitation is possible, there may be permanent brain damage.
below 6%	Gasping breath. Convulsive movements may occur. Breathing stops, but heart may continue beating for a few minutes - ultimately death.

(Laboratory Safety Manual, 1992), (Safe Handling of Compressed Gases, 1992), (Strang. J., and Mackenzie-Wood. P., 1990),

SHOULD WE BE LOOKING AT O₂ DEFICIENCIES AS LIFE THREATENING WHILE UNDERGROUND?

The partial answer to this question can be found in a paper by Field, (1992) which studied the use of a new fire extinguishing gas mixture, designed to be used in enclosed spaces. The gas called ‘Inergen’ consisted primarily of Argon and CO₂. It was designed to disperse and dilute oxygen to below 15% volume, so that there would be insufficient oxygen to support combustion. The research found that the addition of a small percentage of CO₂ was beneficial as it induced an immediate and sustained stimulus to increase breathing rates of persons caught in areas flooded with this gas mixture. It was the increase in CO₂ and to a much lesser extent the decreased O₂ which stimulated the respiratory response.