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Say you lived in Mexico City where air pollution is legendary.
A thread below (Prius cleans other cars exhausts) had Toyota suggesting that the Prius could clean the ambient air in polluted urban areas.

If air quality was your sole determinant would you take your Prius or a bicycle on your travels?
The eight- hour standard for CO is 0.13 ppm, and the highest concentration measured in the MAMC [Metropolitan Area of Mexico City] is 24 ppm. The Mexican NOx one-hour standard is 0.21 ppm: the highest recorded concentration is 0.32.

The Prius numbers that Doug found (g/mile) with SULEV standard in parenthesis.

[n.b. New car emission values may be several fold lower.]

I will use the derivation in the CO by the numbers post in a slightly abbreviated form here.

1. NOX = .008 g/mile.
2. Prius gets 48 mpg.
3. To yield .384 g NOx per gallon.
4. Stoichiometric burn 1 gallon gas yields 43,450 g exhaust.
5. To yield .384 g NOx per 43,450 g exhaust.
6. Or 8.8 ppm (by weight) NOx.
7. NOx consists of NO, NO2, and laughing gas (N2O). Assume for a best case Prius analysis that all NOx was NO2 in order to convert wt ppm to volume ppm. Multiply by 0.6.
8. To yield about 5.3 ppm (vol) in pure Prius exhaust.

The highest recorded NOx recorded in Mexico City was 0.32 ppm. Pure Prius exhaust has roughly 17 times more NOx and will not clean the NOx from ambient air in Mexico City. Ride your bike.


The highest level of CO recorded in Mexico City (24 ppm) is significantly lower than the record US level recorded in Yellowstone National Park (36 ppm) that was cited in the CO by the numbers thread.

However, as Doug kindly noted, my value of < 1 g/mile CO (SULEV limit) is more accurately replaced by the determined value of 0.34 g/mile.

This puts pure Prius exhaust at 375 ppm, and far from lethal in undiluted exhaust, but still much higher than Mexico City_s maximum of 24 ppm. Again take your bike.

Prius: Not cleaner than a bicycle, but getting close.
(Especially if bicyclist was fueled by eating beans.)

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