# How much oxygen would a candle burn if a candle would burn oxygen?March 14, 2012 6:23 PM   Subscribe

How does the oxygen consumption of a candle compare to that of a person?

I'm sat with some candles here. How does their oxygen consumption compare to mine if we assume they are the typical and les you might have at a dinner table?
posted by dougrayrankin to Science & Nature (9 answers total) 5 users marked this as a favorite

Similar. A candle and a stationary human both turn chemical energy into heat at a rate of about 80 Watts, and oxygen consumption will be approximately proportional to that.
posted by Canard de Vasco at 7:15 PM on March 14, 2012 [1 favorite]

Well, assuming a chemical formula of C25H52 for candle wax, a balanced chemical equation for the combustion of candle wax (assuming paraffin with 25 carbon chains) in O2 gives 38 moles of oxygen used for every mole (352g) of candle wax. So, for 352 grams of candle, we'd use 38 moles (or 1216g) of oxygen to burn it.

But, that's a big candle (3/4 pound) so let's look at Amazon where I find a set of candles. Each candle appears to have a mass of ~90 grams which is ~1/4 of the big candle listed above. So, burning one of these candles would require 1/4 of the oxygen listed above, or 304 grams. So, to burn one candle requires 304g or 9.5 moles of Oxygen.

The product listing says that particular candle will burn for 12 hours. So, the candle uses 25 grams of oxygen per hour or .42 grams per minute or .007 grams per second.

It's been estimated that a human needs 2730 grams of oxygen per day, 113.75 grams per hour, 1.9 grams per minute or .03 grams per second.

So, by my rough calculations, a human uses 4.3 times as much oxygen as a candle.
posted by jeffch at 7:17 PM on March 14, 2012 [3 favorites]

A candle consumes oxygen via combustion and in a human it is via catabolism. Catabolism is, in many ways, a close analogue to combustion (i.e. both are forms of oxidation). We can roughly compare the oxygen consumption of the two by comparing the power requirements. An adult human male's basal metabolic rate is about 80 Watts. A candle consumes about 2mg of wax a second and wax has about 42 kilojoules of energy per gram — this works out to a power of about 80 watts. So a human and a candle are roughly about the same. So my calculations work out about the same as Canard de Vasco's. Jeffch's calculations rely on a human consuming 113.75 grams per hour of Oxygen. However, my reference suggests the number is about 30 grams. If 30 grams is the correct value, then his calculations also roughly show that a human and a candle are roughly about the same (i.e. 30g vs 25g).
posted by RichardP at 7:29 PM on March 14, 2012 [1 favorite]

Looking at some additional sources, the human oxygen consumption estimate I used does appear to be 3-4 times too large.

If true, a human and a candle would be roughly equivalent in terms of oxygen consumption.
posted by jeffch at 7:42 PM on March 14, 2012 [1 favorite]

If you're curious, I got a rough number of 30 grams of Oxygen per hour for a human based on a requirement of 550 liters of Oxygen per day (from Principles of Bioastronautics) and the following calculation:

550 liters of Oxygen per day / (22.4 liters per mole) * (32 grams per mole) / (24 hours per day) =
32.7 grams
posted by RichardP at 7:45 PM on March 14, 2012

That doesn't seem quite right. If I fill a small room with 15 people in the wintertime and turn off the heat, it's soon insufferably hot. But if I did the same with 15 candles, it barely makes a difference.
posted by lohmannn at 6:11 AM on March 15, 2012

A single candle in a snow cave does make a noticeable difference.
posted by doctord at 6:40 AM on March 15, 2012

Remember that we're dealing with two wide variances here: candles and humans.

Both can vary quite a bit; it's not unimaginable that there are ordinary-looking candles out there that produce 1/5 the heat of a big, high-metabolism human, or more heat than a size-0 at rest.
posted by IAmBroom at 9:04 AM on March 15, 2012

Candles are a compact heat source producing a narrow stream of hot air which goes straight up, and some radiant energy. Humans are bigger and more diffuse sources whose body temp is about 100F and need to be radiating into 72F degree air to be comfortable, for the most part. Humans heat things through conduction as well as much more convection (again, bigger surface area) as well as radiant heat. They also tend to cluster which traps heat. And we also carry around insulation.

In terms of measuring heat output in, say, calories, I think humans and candles (and lightbulbs in the 65-100Watt range) are comparable, but in terms of heating a room, lightbulbs and candles are not as good as people. But that's why we don't use those items to heat a room.

A candle in a snow-cave is a great exception-- the snow reflects the radiant heat. Also, a properly built snow-cave has a well at the bottom for cold air to rest in, so the heated air can spread out a bit. A snow-cave with a human in it does just about as well, depending on their insulation.
posted by Sunburnt at 10:17 AM on March 15, 2012 [1 favorite]

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