It appears that an average adult male will exhale 0.00899 kg/min of CO2 at rest. I had trouble finding data on the amount of CO2 exhaled when exercising aerobically, but I think we can use VO2, the volume of oxygen you can get into your blood, as a proxy. For adult men average VO2 at rest is 3 ml/kg/min. VO2 max varies depending on age and fitness, but adult male non-athletes have a VO2 max of between 36 and 52, while runners, cyclists, and other aerobic athletes can get their VO2 max into the 60-80 range. I think it’s safe to say that CO2 is exhaled proportionally to the amount of oxygen processed.

Using an average VO2 max for non-athletes of 44, that’s 14.6 times resting VO2. You’re aerobically exercising at about 2/3 of your maximum heart rate, which would be 9.8 times resting. So if VO2 at aerobic levels is about 10 times greater than VO2 at rest, then the amount of CO2 you expel on a bicycle is also 10 times greater than you exhale at rest.

Driving a car is essentially a rest state. If you put out 0.009 kg/min of CO2 at rest, then on a bicycle working aerobically you are putting out an additional 0.081 kg/min of CO2. At 15 miles per hour, 15,000 miles is 60,000 minutes. At a rate of 0.081kg/min, that works out to 4,860 kg of CO2. That’s 10,692 pounds.

You would exhale 5.34 tons of CO2 on a bike that you wouldn’t generate while driving a car.

Looking at the major manufacturers selling cars in North America, most had a fleet average for 2009 of between 8 and 10 tons of CO2 per year per vehicle based on a 15,000 mile average. So it appears, at least in terms of CO2 emissions, that riding a bike might have less of an environmental impact than driving the average car in America: 5.34 tons vs. 8 to 10 tons.

However, if two people carpool to work, they will emit less CO2 than if they were both riding bicycles.

That was against a fleet average, though. How do bicycles compare to the vehicles that emit the least CO2?

Of the top 10, the best is the Toyota Prius at 3.81 tons of CO2 per 15,000 miles. Number two is the Honda Insight, at 4.5 tons per year. The rest of the top ten put out between 5 and 6 tons/yr CO2.

Riding a bike comes in fifth place.

So there are already four cars on the market that have lower CO2 emissions when carrying a single passenger, the driver, than a man on a bicycle. For the vast majority of new cars sold today, carpooling with as few as two people eliminates any environmental advantage, at least in terms of CO2, that bicycles have.

The only cars that, fully loaded with passengers, have bigger carbon footprints than a bicycle are exotic supercars like Lamborghinis — and then only because they are two-seaters. A four seat Lambo might very well be greener than a bicycle built for two.

The EPA wants to regulate carbon dioxide as a pollutant. Every government agency with more than 3,000 employees exceeds the de minimus triggering limit of the EPA’s proposed regulations for CO2 simply with the amount of air they breathe in and out while at work.

Maybe the first place to start is with all the hot air spewing out of Washington.