Carbon Dioxide, Part 2: Walk, Drive a Car, or Ride a Bike?

The blog about carbon dioxide (CO2) produced by our bodies during respiration created so much discussion that I decided to work harder to put the numbers into context.

Last time, we calculated an average adult human breathes out between 0.7 and 0.9 kg of carbon dioxide each day. This is based on lots of assumptions, with people of all ages and nationalities counted as processing 0.5 liters of air, 16 times an hour, for 24 hours.

Let’s compare this rough estimate to some other numbers.

The amount of carbon dioxide given off by an automobile in a mile (1.6 kilometers)

I’ve heard a number quoted for this one, but thought it would be good to estimate to find out how close I was, and then I will convert the number to metric units. We start from some facts.

• Density of gasoline is 0.71-0.77 grams per cubic centimeter (that’s 0.71-0.77 kg per liter)
• Gasoline is 85% carbon by mass

So there is approximately 0.74 times 0.85 = 0.63 kg carbon per liter.

This converts to 0.63 kg C x 3.79 liter/gallon or 2.39 kg C per gallon (C=Carbon).

If our car drives 20 miles on one gallon of gas (this is clearly not a very efficient car!), the car burns 2.39 kg per gallon x 1 gallon per 20 miles, or 0.12 kg of carbon per mile.

This is equivalent to 0.12 x 44 divided by 12 = 0.44 kg per mile, or 0.96 pounds (~1) pound of carbon dioxide per mile. Or, in metric units, 0.28 kg per kilometer.

And, driving this car for two miles (3.2 km) produces 0.88 kg carbon dioxide – as much as we produce by breathing all day! (What if the car could travel twice as far per gallon?)

Carbon dioxide released by going from Point A to Point B.

I’m going to suppose someone wants to travel two miles or 3.2 kilometers. That’s a distance many of us would be willing to walk (and about the distance between where I live and where I work).

That means:

If we walk three miles per hour, it would take us 40 minutes to reach Point B walking 3 miles an hour.

If we ride a bicycle at 8 miles (12.8 kilometers) per hour on average, it would take 15 minutes to get to Point B.
The Web is full of charts listing the number of Calories (kCal, abbreviated kCal) used in different types of exercise. I’ll select the following values. For a 155-pound (70 kg) person:

• Walking at 3 miles per hour (4.8 km/hr) burns 250 kCal
• Riding a bicycle at 8 miles per hour (12.8 kilometers per hour) burns 280 kCal

Which means the number kCal burned going from Point A to Point B is:

• 167 kCal walking for 40 minutes compared to 56 kCal for 40 minutes at rest
• 70 kCal riding a bicycle for 15 minutes compared to 21 kCal for 15 minutes at rest

The “at rest” numbers are based on the previous blog, where we used energy production to estimate carbon dioxide output. We assumed a human produced 2000 kCal of energy (equal to the amount eaten) and found that roughly equivalent to 0.7 kilograms of carbon dioxide a day. (0.9 kg a day could be used as well. We used 0.7 simply because that was the number associated with the 2000 kCal.

The carbon dioxide we produce by going two miles on foot or on a bicycle is then, if we count the total:

• 0.7 kg CO2 per 2000 kCal times 167 kCal: 0.058 kg CO2 walking
• 0.7 kg CO2 per 2000 kCal times 70 kCal: 0.025 kg CO2 biking

But the “extra cost” of traveling the distance should be the difference between the “exercising” number and the “at rest” number, namely:

• 0.7 kg per 2000 kCal times (167-56) kCal = 0.039 kg of extra CO2 walking
• 0.7 kg per 2000 kCal times (70-21) kCal = 0.017 kg of extra CO2 riding a bike

Thus: traveling the 2 miles (3.2 kilometers) produces this amount of CO2 above what was produced by respiration at rest:

Traveling 2 miles (3.2 kilometers)

By car: 0.88 kg CO2
Walking: 0.039 kg CO2
Riding a bike: 0.017 kg CO2

While the numbers aren’t exact, the large factor – 20 or more, is probably close. Walking or riding a bicycle does reduce the production of CO2 relative to driving. And – these modes of transportation provide healthful exercise as well! If we have to drive, putting more people in the car reduces the impact of driving. And, driving a car that uses half as much gasoline per unit distance would also help.