A bit old but still sound: J.M.James, A.J.Pavey & A.F.Rogers 'Foul air and the resulting hazards to cavers', BCRA Transactions, vol.2, no.2, 1975, pp.79-88. The authors mention empirical tests for CO2 content of cave air:
Match is extinguished at 1% CO2
Candle is extinguished at around 3% to 4% CO" (some testers reported varting levels however, ranging at the most extreme from 2.3% to 10%; this may reflect different amounts of O2 in the air - see below concerning the non-linear replaement of O2 by CO2)
Carbide lamps extinguished at 5% to 6% (small helmet-mounted old-fashioned 'stinkies') and 8.9% to 10% (large hand-held miner's carbide lamps - I've used them in mines but suspect that they were rarely used by cavers; whether they equate to waist-mounted generator type carbide lamps typically used on expeditions I don't know; I suspect that cavers' generator types might be intermediate between stinkies and moner's hand lamps, at least based on volume of C2H2 evolved per unit of time).
The advice of this paper was "If a candle will not burn it is advisable to leave the cave at once" and "if a carbide lamp is extinguished (early warnings are a smokey flame and less light) then the caver is in severe danger".
The authors noted "a match is extinguished before a candle [1% CO2 versus 3% to 4% CO2] therefore the candle can only be lit while in good air".
Mid C19 inspectors of mines noted the practice of miners 'inclining the candle' - tilting it at an angle or even horizontal, to keep it alight in poor air to enable them to continue to work - the tallow (paraffin wax later, including today) would be consumed faster, but would stay alight in lower levels of O2 / higher levels of CO2. At its most extreme, the candle might even be held completely upside down to keep it alight to enable work to continue.
The authors note 'a candle can be recommended as the standard CO2 test for digs [with CO2 levels in the range 1% to 4% - they recommend using testing equipment above 4%]. If the CO2 rises above 4% (i.e. the candle goes out) - get out slowly'.
They recommend extreme caution if there are any vertical descents into potentially elevated levels of CO2 - the gas is 1.5 time as heavy as air and pools at shaft bottoms, noting ' climbing out of fopul air is much more difficult than might at first be expected because of the limited capacity for physical work of the caver in foul air'.
In CO2 levels above 6% they recommend breathing apparatus and the use of a cave diving approach - back up equipment, rule of thirds, etc.
Two inter-related factors: increased CO2 and O2 depletion. But note that the two are not always in perfect step - a 1% increase in CO2 does not automatically equate to a 1% decrease in O2. See Descent 79, Dec 1987/Jan 1980, p.5 article "Gas causes near-fatality in Welsh lead mine" for an incident when the O2 was stripped out of the atmosphere in workings beyond a sump, with the resulting CO2 apparently dissolved in the water, resulting in O2 depletion accompanied by N2 excess rather than CO2 excess. None of the party tasted anything odd in the atmosphere nor experienced panting, just listlessness - followed by unconciousness and convulsions in the case of the lead explorer who was dragged back out by his companions, with at least one other being so 'out of it' that he was basically lead out by the hand. Recovery was rapid in fresh air; the two leaders had no recollection of the event nor of their removal.
Around 1 in 10 people do not experience panting in hightened CO2 / depleted O2, and instead tend to go very quiet and start turning a bit blue. The medical shorthand for the two sprts are 'pink puffers' and 'blue bloaters'. A friend is a blue bloater - he has been known to just sit quietly and become increasingly 'out of things' when companionss are complaining of shortness of breath and are panting. (in the Descent report, above, most of the group were known pink puffers so it was not a case of all of them by chance being blue bloaters)
O2 % by volume information, from J.D.Jenkins 'Coal Mines Rescue and Fire Fighting', 1956, chapter 7 'Mine gases and dusts', based on the depletion being made up by a gas other than CO2:
20.93% normal air supply
17.50% "flame lamps [i.e. 'Davy' lamps, NOT Carbide lamps] extinguished; work can be done in this atmosphere for several hours without any ill effects" [this is a c.1950s book; opinion m,ay be different today!]
13.00% "acetelyne lamps [i.e. carbide lamps] extinguished; considerably increased rate of breathing if work is being done" [this contradicts another volume I recall reading which observed that carbide lamps can burn in O2 concentrations that can result unconcious and death, and cited examples of rescuers equipped with breathing apparatus recovering bodies beside which carbide lamps continued to burn]
10.00% "lips turn distinctly blue; nausea and headache develop gradually and become more severe in time"
8.00% "face takes on a blueish-grey tinge and breathing becomes very rapid; disablement occurs in about a quarter of an hour"
7.00% "face turns a leaden colour and distinct panting occurs, accompanied by paliptations of the heart; unconciousness rapidly ensues"
6.00% "complete loss of conciousness, followed by death in a matter of minutes"
2.00% "unconciousness in about 40 to 45 seconds without previopus panting, even if person is absolutely still; rapidly fatal"
CO2 % by volume information, from same volume:
0.03% normal air content
0.50% "lung ventialation is slightly increased"
2.00% "amount of air breathed is increased by 50%"
3.00% "lung ventilation increased about 100% even at rest, and becomes very laboured on exertion and fatigue ensues"
5.00% "the amount of air breathed is increased 300%, and even at rest breathingh becomes very laborious"
6.00% "violent panting takes place and fatigue ensues up to the point of exhaustion"
10.00% "intolerant panting, and severe headaches occur after a few minutes at this percentage"
above 10.00% "narcotic effect upon the system"
over 20.00% "death may ensure after a period of hours during which a state of coma exists"
CO is 300 time more preferentially taken up by the human body compared to CO2 - basically the human body preferentially scavenges CO from the atmosphere. CO is very toxic and causes collapse.
CO % by volume information, from the same volume on mine air:
0.02% (200ppm) "slight headache with some people after about 7 hours during rest or 2 hours during extertion; tightness across forehead"
0.04% (400ppm) "severe headache after 5 hours during rest and less than an hour during exertion; severe headache ensues at the end of this time, with dizziness, nausea and possibility of collapse"
0.10% (1,000ppm) "poisioning may easily take place and respiration is increased; palpitation occurs"
0.20% (2,000ppm) "unconciousness occurs after 30mins at rest or 10mins exertion; the coma and intermittent convulsions which result are often acompanied by depressed heart action and possibly death"
0.30% (3,000ppm) "coma accompanied by very weak pulse and reepiration, followed by respiratory failure and death"
1.00% "after 3mins at rest or 1min exertion unconciousness ensueres and is rapidly followed by death"