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.

__BICYCLING : CO2 ESTIMATE FOR A 5 MONTH COMMUTE__

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

__BICYCLIST VS CAR : THE 15,000 MILE ESTIMATE__

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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