Emission Facts

Idling Vehicle Emissions for Passenger 

Cars, Light-Duty Trucks, and HeavyDuty Trucks The amount of pollution that a vehicle emits is dependent on many 

factors. The U.S. Environmental Protection Agency (EPA) has 

developed a series of computer models that estimate the average emis-

sions for different types of highway vehicles. This fact sheet is one of 

a series on highway vehicle emission factors. It presents average emis-

sion rates for passenger cars, light-duty trucks, (e.g., pickup trucks, 

sport-utility vehicles), heavy-duty trucks (semi tractor-trailers), and 

motorcycles when they are idling.

Introduction

There are a number of factors that affect the rate at which any vehicle emits air pol-

lutants, whether the vehicle is being driven or is at idle (engine running, but vehicle 

not moving). Some of the most important are:

•	 vehicle type/size (passenger cars, light-duty trucks, heavy-duty trucks, motor-	

cycles)

•	 vehicle age and accumulated mileage

•	 fuel used (gasoline, diesel, others)

•	 ambient weather conditions (temperature, precipitation, wind)

•	 maintenance condition of the vehicle (well maintained, in need of maintenance,

presence and condition of pollution control equipment)

The most current version of the computer model that EPA uses to estimate average 

in-use emissions from highway vehicles is MOBILE6.2. EPA, the States, and others 

use this model to estimate total emissions of pollutants generated by highway vehicles 

in various geographic areas and over specific time periods. The idle emission rates or 

"emission factors" presented in this fact sheet are based on national data representing 

the in-use fleet as of July 2008. 

Office of Transportation and Air Quality

EPA420-F-08-025

October 2008
Emission Facts

2

The emission rates of hydrocarbons 

(both volatile organic compounds 

[VOCs] and Total Hydrocarbons 

[THC]), carbon monoxide (CO), 

and nitrogen oxides (NOx) for idling 

vehicles are presented in the following tables. In addition, the emission 

rates of particulate matter (PM10 

and PM2.5) are provided for heavyduty diesel vehicles only because the 

MOBILE model does not include PM 

for the other vehicle categories during idle operation.1 These emission 

factors assume an “average” properly 

maintained vehicle, operating on 

typical gasoline or diesel fuel, on a 

warm summer day. Emission rates 

can be higher in very hot weather 

(especially HC) or in very cold 

weather (especially CO).

National average values are used 

for registration distributions by age 

(what fraction of all cars, or light 

trucks, in use today are of the current model year, are one to two years 

old, 2 to 3 years old, and so forth up 

to 25 years old) and annual mileage 

accumulation rates by age (newer 

vehicles tend to be driven more miles 

per year than do older vehicles). Some of the other primary assumptions incorporated in these 

emission factors are:

•	 Ambient temperatures:	 	 72 to 92 °F day time range

•	 Nominal gasoline volatility:	 	 9.0 psi Reid vapor pressure (RVP)

•	 Weathered fuel volatility:	 	 8.6 psi RVP

•	 Gasoline sulfur content:	 	 30 ppm

•	 Diesel sulfur content:		 	 330 ppm

•	 Inspection/maintenance program:	 No

•	 Reformulated gasoline:	 	 No

Note that these emission rates are applicable for relatively short periods of idle time, as observed 

in drive-thru lanes, at toll gates, at stop lights, and in very heavily congested traffic. These idle 

emission rates are not applicable to “hoteling” situations for heavy-duty diesel trucks, where

the truck engine may be idling for periods of hours at a time with many accessories being operated from the engine power. EPA is currently studying the issue of emissions from heavy-duty 

Abbreviations and Acronyms Used

CO: Carbon monoxide; a regulated pollutant

CO2: Carbon dioxide; the primary byproduct 

 of all fossil fuel combustion

g: gram(s)

g/hr: grams per hour

g/min: grams per minute

GVW:	 Gross Vehicle Weight

lb: pound

NOx: Nitrogen oxides; a regulated pollutant

PM10: Particulate matter under 10 microns 

 diameter; a regulated pollutant

PM2.5: Particulate matter under 2.5 microns 

 diameter, sometimes referred to as 

 “fine particulate”

ppm: parts per million

psi: pounds per square inch

RVP:	 Reid vapor pressure; a standardized 		

 method for expressing the volatility, 

 or tendency to evaporate, of gasoline

THC: Total hydrocarbons

VMT: Vehicle miles traveled

VOC: Volatile organic compounds; equivalent 

 to THC plus aldehydes minus both

 methane and ethane
Emission Facts

3

diesel trucks during “hoteling” situations; more information on this subject is available on EPA’s

SmartWay Transport Partnership Program web site at: www.epa.gov/smartway/idling.htm.

These emission rates also are not applicable for buses (urban transit buses and school buses). 

Average idle emission estimates for urban and school buses are provided in the fact sheet 

“Average In-Use Emission Factors for Urban Buses and School Buses” (EPA420-F-08-026).

Idle Emission Rates

The following tables present idle emission factors expressed as grams per hour (g/hr) and grams

per minute (g/min) of idle time. Table 1 presents idling emission rates for gasoline-fueled and

diesel cars, light-duty trucks, and heavy-duty vehicles, and motorcycles.

Definitions of Vehicle Types

(Gross Vehicle Weight)

LDGV: Light-duty gasoline-fueled vehicles, up to 6000 lb GVW (gasoline-fueled passenger 

cars)

LDGT: Light-duty gasoline-fueled trucks, up to 8500 lb GVW (includes pick-up trucks, 

minivans, passenger vans, sport-utility vehicles, etc.)

HDGV: Heavy-duty gasoline-fueled vehicles, over 8500 lb GVW (gasoline-fueled heavy-duty 

trucks)

LDDV: Light-duty diesel vehicles, up to 6000 lb GVW (diesel engine passenger cars)

LDDT: Light-duty diesel trucks, up to 8500 lb GVW (diesel engine light-duty trucks)

HDDV: Heavy-duty diesel vehicles, over 8500 lb GVW (diesel engine heavy-duty trucks)

MC:	 Motorcycles (only those certified for highway use; all are gasoline-fueled)
Emission Facts

4

The HDDV column represents the average of all diesel engine vehicles over 8,500 lb gross vehicle weight (GVW); this category is dominated by the heaviest HDDVs (GVW Classes VIIIa

and VIIIb) which exceed 33,000 lb GVW.

Table 1: Average Idle Emission Rates by Pollutant and Vehicle Type2

Pollutant Units LDGV LDGT HDGV LDDV LDDT HDDV MC

VOC g/hr

g/min

2.683

0.045

4.043

0.067

6.495

0.108

1.373

0.023

2.720

0.045

3.455

0.058

19.153

0.319

THC g/hr

g/min

3.163

0.053

4.838

0.081

7.260

0.121

1.353

0.023

2.680

0.045

3.503

0.058

21.115

0.352

CO g/hr

g/min

71.225

1.187

72.725

1.212

151.900

2.532

7.018

0.117

5.853

0.098

25.628

0.427

301.075

5.018

NOx g/hr

g/min

3.515

0.059

4.065

0.068

5.330

0.089

2.690

0.045

3.705

0.062

33.763

0.563

1.625

0.027

PM2.5

g/hr

g/min

N/A1

N/A1

N/A1

N/A1

N/A1

N/A1

N/A1

N/A1

N/A1

N/A1

1.100

0.018

N/A1

N/A1

PM10

g/hr

g/min

N/A1

N/A1

N/A1

N/A1

N/A1

N/A1

N/A1

N/A1

N/A1

N/A1

1.196

0.020

N/A1

N/A1

Table 2 presents HDDV idle emission rates by weight class. The GVW weight classes are:

Heavy-Duty Diesel Vehicle Classifications

(Gross Vehicle Weight)

IIb: 8,501-10,000 lb (e.g., full-size pick-up trucks, very large passenger vans)

III: 10,001-14,000 lb (e.g., panel trucks, small enclosed delivery trucks)

IV: 14,001-16,000 lb (e.g., city delivery trucks, landscape utility trucks)

V:	 16,001-19,500 lb (e.g., bucket utility trucks, large walk-in delivery trucks)

VI: 19,501-26,000 lb (e.g., rack trucks, single axle vans)

VII:	 26,001-33,000 lb (e.g., tow trucks, garbage collection trucks)

VIIIa: 33,001-60,000 lb (e.g., long-haul semi-tractor trailer rigs)

VIIIb: > 60,000 lb (e.g., long-haul semi-tractor trailer rigs)
Emission Facts

5

Table 2: Average Idle Emission Rates for Heavy-Duty Diesel Vehicles by GVW Class3

Pollutant Units IIb III IV V VI VII VIIIa VIIIb

VOC g/hr

g/min

1.465

0.024

1.555

0.026

2.025

0.034

2.120

0.035

2.820

0.047

3.503

0.058

3.518

0.059

4.218

0.070

THC g/hr

g/min

1.498

0.025

1.580

0.026

2.055

0.034

2.150

0.036

2.865

0.048

3.553

0.059

3.565

0.059

4.270

0.071

CO g/hr

g/min

9.305

0.155

10.068

0.168

12.898

0.215

13.183

0.220

15.163

0.253

19.055

0.318

26.548

0.442

34.473

0.575

NOx g/hr

g/min

12.668

0.211

13.530

0.226

17.850

0.298

18.655

0.311

24.325

0.405

30.343

0.506

35.758

0.596

42.345

0.706

PM2.5

g/hr

g/min

1.103

0.018

1.010

0.017

1.065

0.018

1.008

0.017

1.069

0.018

1.093

0.018

1.070

0.018

1.114

0.019

PM10

g/hr

g/min

1.199

0.020

1.098

0.018

1.157

0.019

1.095

0.018

1.162

0.019

1.188

0.020

1.163

0.019

1.211

0.020

For More Information

The other fact sheets in this series and additional information are available on the Office of 

Transportation and Air Quality’s Web site at:

Emission factor fact sheets: www.epa.gov/otaq/consumer.htm

Modeling and estimating vehicle emissions: www.epa.gov/otaq/models.htm

Fuel economy:		 	 	 	 	 www.epa.gov/fueleconomy

 www.fueleconomy.gov/

Improving fuel economy and reducing emissions: www.epa.gov/epahome/trans.htm

 www.fueleconomy.gov/feg/drive.shtml

Finding the “greenest” vehicle:	 	 	 www.epa.gov/greenvehicles

Endnotes

1. The MOBILE model includes PM estimates for heavy-duty diesel vehicles only during idle 

operation because there has been insufficient emission testing of PM for gasoline-fueled vehicles 

of the type necessary to develop emission rates for these vehicles at idle, since the PM 

contribution to ambient air quality from gasoline vehicles and the few light duty diesel cars and 

trucks in the fleet is normally negligible.
Emission Facts

6

2. Figures presented in this fact sheet are averages only. Individual vehicles can differ

substantially in the amount of pollution emitted while idling from the values indicated here. 

Emission rates may differ slightly from original sources due to rounding.

3. All of the emission estimates provided in this document are consistent, in terms of assumptions 

made and modeling methodology, with those provided in the other fact sheets in this series: 

“Average Annual Emissions and Fuel Consumption for Gasoline-Fueled Passenger Cars and

Light Trucks” (EPA420-F-08-024), “Average In-Use Emissions from Heavy-Duty Trucks”

(EPA420-F-08-027), and “Average In-Use Emission Factors for Urban Buses and School Buses”

(EPA420-F-08-026).