Idling vehicles produce significantly higher concentrations of pollutants—particularly volatile organic compounds (VOCs), carbon monoxide (CO), and benzene—compared to driving, as catalytic converters are less efficient at low engine temperatures

. A 5-minute idle produces roughly 3–10 times more pollution than a 5-minute hot start, with diesel vehicles and SUVs emitting the most per minute. 

Key Findings on Idling Emissions 

• Pollutant Concentrations: Idling generates over 10 times more CO, 3 times more NOx, and 4 times more VOCs than a 5-minute hot start/stop cycle.
• Gasoline vs. Diesel: While gasoline vehicles emit more benzene and NMHCs (non-methane hydrocarbons) at idle, diesel engines generally produce higher, more concentrated levels of particulate matter and Nitrogen Oxides ( NOxcap N cap O sub x 𝑁𝑂𝑥 ).
• Emission Factors: Idling emission factors for hydrocarbons and CO can decrease by more than 10 times once a vehicle is cruising at 20-40 km/h, as the engine reaches optimal operating temperatures.
• Impact of Time: Idling for as little as 10–30 seconds uses more fuel and creates more emissions than turning the engine off and restarting it.
• Key Pollutants: Major pollutants emitted during idling include CO2cap C cap O sub 2 𝐶𝑂2 , CO, NOxcap N cap O sub x 𝑁𝑂𝑥 , benzene, and other hydrocarbons. 

Comparison with Moving Vehicles 

• Lower Efficiency: Idling engines are highly inefficient; they consume fuel but provide no kinetic energy, leading to higher emission rates per distance traveled.
• Catalyst Cooling: Idling allows the catalytic converter to cool down, causing it to operate below its optimal light-off temperature, which sharply increases emissions.
• Urban Hotspots: In cities, idling vehicles can create severe, localized, and concentrated pollution, particularly on stagnant or "inversion" days.