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[chris]

When purchasing a new car these days we are informed that it pumps out 150g of carbon dioxide per mile travelled.

How does this compare with shank's pony?

Chris

[Pumblechook]

Carbon Dioxide Emission per Person (m3/h)  cu metres/hour

Sleep  0.013

Resting or low activity work  0.02

Normal work   0.08 - 0.13

Hard work   0.33 - 0.38

[Pumblechook]

Very few people will be exercising like that and those who do will only do it for short periods so the average output from humans will be much smaller.

[Pumblechook]

And cars do 27 miles a day. 

so 4100gm as against 900... Commercial vehicles may do far more miles and pump out far more CO2.

[techmind]

Ok, it's only more discussion and not too many (verifiable) figures, but relevant, I thought:
 http://answers.google.com/answers/threadview?id=537961

It doesn't matter how much carbon dioxide you exhale when you ride
your bike, because none of it contributes to the buildup of carbon
dioxide in the atmosphere.  This buildup is caused by burning fossil
fuels, which releases carbon that has been bound up in the slow carbon
cycle for millions of years.  All the carbon you exhale came from
plants that you ate.  Or from animals or fungi you ate, but the
animals and fungi got it from plants.  The plants got it from the
atmosphere.  So the carbon you exhale was in the atmosphere no more
than a year or two previously, and even if you hadn't eaten it and
exhaled it, it would soon end up back in the atmosphere.  Bacteria
will eat it if nothing else (it will 'rot'). The movement of carbon
through the fast carbon cycle does not change the composition of the
atmosphere. So by riding your bike you are not contributing to the
greenhouse effect at all.

I suspect there's probably exceptions; food still uses some fossil fuels (these days) to produce (fertilize), store, and transport. But it's an interesting argument, I thought.


Another quote from the same discussion

Forget about CO2 and consider energy. Energy usage should correlate
well with polution. Passenger trains get about 250 passenger miles per
gallon (100 to 500 depending on occupancy). This equates to about 126
kilocalories per passenger mile. Riding a bicycle at 15 miles per hour
uses about 35 kilocalories per mile of which 27 is due to cycling and
8 are from basal metabolism. This includes lots of assumptions about
weight, wind, hills, and other things, but is probably typical. So a
bicycle is almost 5 times as efficient as the train.

[techmind]

Here's another search result on this topic especially for Neilep   

http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=370859

Role of metabolic CO2 production in ventilatory response to steady-state exercise.

We examined the role of metabolic CO2 production in the hyperpnea of muscular exercise by comparing the response of alveolar ventilation to moderate levels of exercise with the response to venous infusion of CO2 at rest. Studies were performed in four awake sheep that were trained to run on a treadmill. The sheep had been cannulated for veno-venous extracorporeal perfusion so that CO2 could be infused into the peripheral venous blood through membrane lungs in the perfusion circuit. The sheep breathed room air through an endo-tracheal tube inserted through a tracheostomy, and samples of expired gas were collected for measurement of the rates of CO2 production and O2 consumption. All measurements were made in the steady state. In each of the four sheep, the relationship between alveolar ventilation and the rate of CO2 production could be described by a single linear function (r greater than 0.99; P less than 0.001), regardless of whether CO2 production was increased by exercise, venous CO2 infusion, or combinations of both procedures. ...