Vehicles used for ride sharing apps “can reduce cold-start emissions from internal combustion engines. Vehicles emit far more conventional air pollutants when started ‘cold’,” researchers wrote.

A car’s cold start is when a 12 hour period passes where the vehicle was not used and is turned off—then you start it back up. This is when vehicles emit the most pollution, more than half of a ride’s pollutant emissions, making privately owned cars much more polluting because individual owners are turning their cars on and off frequently throughout the day. TNC car trips represent an about 50 percent decline in air pollutants including fine particulate matter mostly because there are less cold starts in between rides.

But, there’s a catch, says Jeremy Michalek, a professor of engineering and public policy at Carnegie Mellon University and an author of the study. Driving your own car may lower the amount of carbon emissions you emit, since carbon emissions directly correlate with how much fuel is burned.
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