Monday, March 29, 2010

29 How Much CO2 Do You Exhale While Cycling?

When you're on your desk reading this new post, assuming you are not too excited, you might be very likely inhaling a shallow volume of about 0.5 liters of air and exhaling the same amount in every breath. This is the average tidal lung capacity of a male, shown in the figure above. In females, that's about 0.39 liters. We can assume you have a resting HR of 70 (typical) and you breathe 12-15 times in a minute.

Let's compare this to bicycling.


For a cyclist on his bike during a commute, heart rate and breathing rate increase, and they begin to utilize more of their lung capacity than the person at rest.

The inspiratory capacity of an average human is 3.5 liters of air per breath. If one emphasizes complete expiration following that, you get about 4.5 liters of air per breath.

A good assumption could be that for the typical bike commute, a cyclist does not approach the above extreme values unless in strenuous biking mode. So let's say this cyclist is at about 75% for both, which is still a high fudge factor.

Therefore, he inhales 2.6 L and exhales 3.4 L of air per breath. Also, breathing rate for such moderate exercise at steady pace can be assumed to be about 30 per minute.

Why the difference between inhaled and exhaled air? There is a residual volume of air in the lungs that you can never expire out, and remains in the lungs even after maximal expiration. Aahh...

Respiratory quotient is the ratio of CO2 eliminated to that of oxygen consumed. A normal value of this ratio is 0.8, where 0.7 is for pure fat oxidation and 1 is for pure carbohydrate oxidation. Literature also has it that for every unit volume of air inhaled, only 5% of oxygen is actually extracted by the body.

Thus, for the cyclist in our example, this means that :

Volume of CO2 exhaled per breath = 0.8 x (.05 x 2.6 L) = 0.8 x 0.13 L = 0.104 L
Volume of CO2 exhaled per minute = 0.104 L x 30 breaths/minute = 3.12 L

At standard air temperature and pressure, density of CO2 is 1.98 g/L. Therefore :

Kilograms of CO2 exhaled per minute = (1.98 g/L x 3.12 L) x 0.001 kg = 0.0062 kg

Extrapolate this according to the length of commute. A 30 minute commute will then involve 0.186 kg of expired CO2, 1 hour will see 0.372 kg and so on...assuming everything in our system behaves in linear fashion (think of what happens if the subject stops for a breath at a traffic light or at a convenience store?).

The bottom line :

Say this cyclist is being very conscious about himself by not driving his car. If he commutes for 5 months a year, for a round trip distance of 10 miles every working day of the week, at 15 mph on a flat path, he will expire about :

5 months x 22 days x 40 minutes of cycling x .0062 kg CO2/min = 27.28 kg of CO2


If you want to compare bicycling with driving a car, then use the industry's 15,000 mile average. In 2009, a Toyota Prius, one of the better cars around in terms of emissions, generated about 3810 kg of CO2 for 15,000 miles. For the cyclist riding the same total distance at average 15 mph :

(15,000 miles/15 mph) x 60 minutes x .0062 kg CO2/min = 372 kg of CO2

This is a considerably lesser estimate than the vastly exaggerated number from a blogger named Ronnie Schrieber.

Schrieber wrote recently on his blog that a cyclist in aerobic mode gives out 0.081 kg/min of CO2 and so for 15,000 miles, a compelling 4,860 kg of CO2 or 5.34 tonnes of CO2 would be expired!

He compares this to the 2009 vehicular average of 8 tons of CO2 emissions and claims this is a huge figure for bicycling, hence there's not much of an environmental incentive for riding.

While one can debate how green bicycling really is by looking at the entire stream of what goes into the bicycle and into the human engine, it must be said that Schrieber's figure for CO2 production during cycling makes little sense. 5 tonnes of CO2? That doesn't sound reasonable so I'll ask him to check his numbers again.

What do you readers think? Does expiring CO2 have any impact on the environment? Say you're riding your bike at a charity event with 1000 other cyclists, then each minute, using my numbers, some 1500 kg of CO2 will be expired all over the countryside for 4 hours of riding. I would think it wouldn't matter, as the carbon in the CO2 had to come from somewhere initially and the cyclists are returning this amount of gas back to the environment. Its called recycling.

Come discuss.

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  1. Even considering that most of our food is fertilized from fossil sources, the fertilization provides nitrogen rather than carbon. Plants get their carbon from the atmosphere, regardless of their nitrogen source. Consequently, the carbon we exhale came from the atmosphere fairly recently, so the net effect of our exhalations is essentially nil.

    Most motor vehicles emit fossil carbon that's been accumulating in the earth's crust. We've released 400 million years worth of carbon in the past century. Life will certainly continue in some form on this planet, but whether humans can survive this extremely fast and radical change in our biosphere is an open question.

  2. ...and you're still exhaling CO2 while you're driving.

  3. Schrieber wrote "If you put out 0.009 kg/min of CO2 at rest..."

    I think this is the problem with his calculations.

    By your explanation, the amount of CO2 at rest should be:

    Volume of CO2 exhaled per breath = 0.8 x (.05 x .5 L) = 0.8 x 0.025 L = 0.02 L
    Volume of CO2 exhaled per minute = 0.02 L x 15 breaths/minute = .3 L

    Kilograms of CO2 exhaled per minute = (1.98 g/L x .3 L) x 0.001 kg = 0.0006 kg

    Schrieber's figure of 0.009 kg/min is off by a factor of 15. He then uses the rest figure to estimate the aerobic figure.

  4. Anonymous2:05 AM

    Dumbest. Debate. Ever.

  5. Mark Chiu6:34 AM

    I don't know about Schrieber but it would take me a good 4-5 years at current cycling rate to put 15000 miles. In that time span, how is some trivial amount of CO2 even problematic? With a car, you can burn a fossil fuel and put more than 20 times that amount in just a few months. It would make sense to get a fuel efficient car or simply cut down driving and adopt a different mode of transport.

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  7. What Yokota said. Seriously, pretty pointless debate. Because humans are part of a closed carbon loop. Meaning that the CO2 we are emitting is continually absorbed by plants. The overall CO2 atmospheric content is not affected by us.

    On the other hand, the CO2 released by cars, etc comes from fossil fuels which has been locked up for millions of years until being burned.

    1. the O2 in CO2 did not come from the fuel, the O2 is from the air we breathe and that allowed the fuel to burn then bonded to the carbon that was part of the fuel, pumping CO2 into the ground is removing the O2 from the air not allowing the O2 through the carbon cycle back in to the air.

  8. Anonymous11:58 AM

    Also, humans don't exhale carbon monoxide, hydrocarbons, or NOx gases, all of which come from the combustion process. CO is poisonous and can cause death, while the other 2 are responsible for acid rain and smog. Humans produce CO2 from breathing, but automobiles all of these pollutants in addition to CO2.

  9. I confess a lot of the calculations lose me, but how can less (in almost all cases)than 100KG being moved around generate ANYTHING like moving around 1000KG? Basic conservation of energy laws can't be fudged.

  10. maikeru8:58 PM

    All of the previous posters (most anyway) make salient points. One notes the fact that most modern food plants are produced through excessive use of fertilizers that have no impact on CO2, but there is the fossil-fuel-based carbon used in production. Therefore you can link back to *some* incremental carbon in the exhalations of bicycle riders (this is an intriguing thought). This increment of carbon is impossible to precisely calculate, but you could argue that virtually all food we consume in the US has some taint of incremental carbon due to the use of fossil fuels in its production, or the production of the seed from which it sprang, or the production, distribution and application of the fertilizers and pesticides used in production, or the distribution of the food itself. You get the idea.

    Thus you could further argue that all carbon exhaled by bicycle riders is derived indirectly through fossil fuels. Wow! OK, let's stipulate this as so (all you uber-green, vegetable growing, canning, CSA-participating saints just bear with me). Even if this is the case and we agreed that a gram of carbon exhaled by a bicyclist is just as tainted as one emitted from a Hummer's tailpipe, we're still a vast distance from equaling the volume of carbon injected into the atmosphere by the average vehicular commuter. A long, long way.

    As an aside, you lost me with your math. And I'm pretty good at math.

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  12. A common complaint here is that people 'missed' my math. Where do you have a problem understanding the numbers as I could clarify it for you. I'm sorry that I didn't break it down well enough.

  13. I think the analysis from Schrieber is wrong because of faulty assumptions and a misunderstanding of how human breathing works. My math is also an approximation but the number sounds so much more reasonable. I think you will all agree with me that human CO2 production is no where as close to what comes out from the tailpipe of cars. I think this is a pointless debate.

  14. David Rojas10:54 AM

    To make a fair comparision, what really matters is the extra amount of CO2 produced by a human being (mind, if we were cars, we would be constantly 'idling').

    You should have to subtract the amount of CO2 a human being would produce without excercising because that doesn't count as something to analyze, it's just a fact.

    And that, I suppose, leads... See more to solve complicated and useless theoretical models and equations... I'm not saying that riding a bike has no impact on the environment, but comparing a car with a 'living machine' is absolutely pointless, in my opinion. CO2 emisions are not the only thing that matters. To construct, maintain, use and recycle a car, a lot of energy is needed and tons of waste is generated.

  15. David Kina10:56 AM

    Plants thrive on exhalation from people. Singing to plants helps them grow since people breath on them if they sing to them. Just plant more trees.

  16. The debate may be pointless, but this discussion is not.

    I suspect many people will read Schreiber's blog post, question neither the premises nor the conclusions, and accept it all as fact.

    Likewise for the Cozy Beehive post.

    Anything that gets people exercising their critical thinking skills is a Good Thing in my book.

    Ron: I still think the biggest problem with Schreiber's post is his starting point of "0.00899 kg/min of CO2 at rest", a number he seems to pull out of the air (so to speak).

    Thanks for providing references to the numbers used in your post!

  17. While I calculated some numbers in this post and compared it with ONE vehicle (that too a Toyota Pruis!), I think what I should have done is showing how insignificant that amount of human exhaled CO2 is by looking at the carbon dioxide cycle. The human CO2 load on the atmosphere is indeed small.

    I think we should step back a bit when thinking whether the human impact through breathing on CO2 accumulation is "nil". Perhaps its so small you can approximate it as nil when comparing it to something very large, like the exhaust emissions from vehicles every year.
    As Maikeru said, there could be some portion of carbon from fossil fuels used in the production of common food items we consume. That incremental carbon is transferred to us and we transfer it back to the atmosphere, which may be taken in by trees or absorbed by water bodies. I don't have any data for this, though.

  18. maikeru7:11 PM


    Where you lost me originally was the use of the rate of extraction of O2 from air by the human body (5% of whatever volume of O2 is in a lungful, I think). This threw me off because the whole topic was about exhalation of CO2 and I didn't see you apply the ratio of O2 extracted to CO2 inhaled that you mentioned in the post. Admittedly, since I'm writing this now and can't see the post, I may have missed a step in your calc that showed that.

    In any event, this was a great post and stimulated some readers to think. It's useful to consider such a topic if part of your rationale for commuting is reduced CO2 in the atmosphere (it's part of mine). I just know I'll run into someone who will challenge that rationale on the science and I'm glad you and other commenters went to the trouble of thinking about it.

  19. Maikeru,

    The ratio of 8 has been applied in my calculations for expired volume CO2. Its going to be 8 times of 5% of the air you inhale, 5% being the oxygen that is extracted from the air by the lungs. So if a person inhales 2.6 L of air, 5% of that is oxygen = 0.108 L. This multiplied by respiratory quotient gives you the carbon dioxide eliminated. This volume of expired CO2 works out to be about 4% of the inhaled air which agrees with literature, which says its more like 4.38%. We're good.

  20. Rod Diaz1:27 PM

    Two words: biogenic origin.

    I work in GHG consulting, developing projects that reduce the emission of GHG, and also using/developing quantification methods for CO2 and other climate change gases.

    In most cases, those sources that are part of the biologic cycles contribute a very small fraction to "net" carbon dioxide emissions, to the point where most accepted models simply disregard them (UNFCCC, US EPA, Environment Canada, and many NGO models). As mentioned by Yokota Fritz, this carbon is taken up from the atmosphere, while fossil fuels are "stored" permanently until we burn them. These contribute to climate change. Note that this biogenic emissions only refer to CO2 - other gases such as methane (CH4) derived from agricultural and waste sources are indeed net contributors to climate change.

    Flipside of this is reflected by the limited and flawed strategies to reduce GHG emissions by simply planting more trees - the moment these are lost to any reason, the carbon is released again. They are not reliable carbon "sinks".

  21. Great article.Lost of thing can be learn from your blog.

  22. Anonymous8:13 AM

    "Therefore, he inhales 2.6 L and exhales 3.4 L of air per breath."

    Is this correct? Residual air in the lungs is irrelevant as it is there for both inspiration and exhalation.

    To be correct the body would have to be generating 0.8L of gas. Is C02 far higher volume than 02, because as far as I am aware it is pretty much a straight replacement in terms of the chemical equations of respiration? I guess water vapour caould also be part of the equation.

    Strikes me this is wrong though. I would guess inhaled and exhaled volumes are pretty much the same (I don't thing our lungs expand much after we have finished inhaling).

  23. Anonymous1:34 PM

    You've only compared one to one at very low speed.
    If you put 4 people in a car and drive at a steady 40 mph I'll bet that the CO2/km for the car will be less than 4 cyclists and the passengers needn't be olympic athletes.

  24. Drive a pure electric car, charged with electricity generated by nuclear/wind/solar, and you're actually generating LESS CO2 than bicycling.

  25. Well written and very informative blog....nice

  26. Most of the electricity today comes from coal,
    gasoline is Hydrogen and carbon.(HC)Hydrocarbon
    Only 10 to 12 % of us on this planet drive cars, less than an hour a day, if you include all the lungs on this planet not just Human lungs, from birth to death, then all lungs produce more CO2 than cars.
    $25,000 reward for the proof humans cause climate change.
    Bruce A. Kershaw

  27. Anonymous9:16 PM

    question: what would be the net co2 increase over normal production from rest? would the increased fitness result in a more efficient cardiovascular system, improved bmi, and consequently lower average co2 emissions when not exercising?

  28. Anonymous7:51 AM

    Uh...could you revisit the concept of exhaling more than we inhale? You wrote that a residual amount of air can never be expired. So why add that amount to an exhalation?


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