Idling is wasteful—burning up to a quarter to a half gallon of fuel per hour—and in Massachusetts it is illegal for drivers to unnecessarily run the engine of a motor vehicle (while stopped) for more than five minutes. The goal of the Massachusetts Anti-Idling law is to improve air quality by reducing air pollution from idling vehicles.
Sitting in your car in a parking lot with the engine on during mild or cool weather is unnecessary. The interior of your car will stay warm for 5 to 10 minutes on all but the coldest days. Leaving the vehicle running while unattended to let the heater warm it for extended periods of time is unnecessary idling that also pollutes the air and is especially harmful to people with respiratory and heart diseases. Consider adding some blankets to your car instead of idling to warm it up from now on.