1. What is smog? What are VOCs and NOx?
2. Why does my city matter?
3. Why isn't my model year 1975-2000 vehicle here?
4. Why isn't my model year 2001-2006 vehicle here?
5. My vehicle has a bad score on the Auto Asthma Index, but I always pass my state's smog test. Why is this?
6. Why does my vehicle have worse than typical emissions?
7. Why does my vehicle have lax emissions standards?
8. Why is my vehicle a high miles vehicle?
9. What is a Gross Polluter?
10. Why can't I compare older and newer vehicles in your report?
11. What is the difference between 'California' and 'federal' vehicles? Why do automakers make cleaner and dirtier versions of the same model?
12. What about diesel emissions?
13. Are two-wheel drive vehicles typically better than four-wheel drive vehicles?
14. Are automatic or manual transmissions typically better?
15. Do all new vehicles pollute less?
16. How can I pick a good used vehicle?
17. But big vehicles are safer, aren't they?
18. What can I do about polluting vehicles in my neighborhood?
19. When does my vehicle pollute?
20. What about the emissions warranty?
21. What are the cleanest cars? Where would I purchase one?
1. What is smog? What are VOCs and NOx?
Smog is primarily caused by ozone air pollution. Ozone forms in the air when two types of pollutants, volatile organic compounds (VOCs) and nitrogen oxides (NOx), combine in the presence of sunlight.
Two different classes of pollutants--volatile organic compounds (VOCs) and nitrogen oxides (NOx)--react with sunlight in the atmosphere to form smog. In some regions VOCs are the limiting pollutant. Efforts to control NOx emissions will have no effect on--or even worsen--smog pollution. In other areas the dynamic is reversed. In a third type of region both pollutants are similarly important to combatting smog. We took this variability into account in our Auto Asthma Index. Where scientists determine that VOC controls are most critical to smog levels, we ranked vehicles based solely on their VOC emissions. In regions where NOx is most important for smog control, the Index is based on NOx emissions alone. In areas where both VOCs and NOx contribute to smogginess, an average of the VOC and NOx Auto Asthma Index values is used to rate vehicles. As a result, the Index for a particular model varies depending on your location. .
Some vehicles are exempt from taking the type of Smog Check test we used to compile our Auto Asthma Index. If your vehicle was manufactured between the years 1985 and 2000 and either weighs over 8,500 pounds fully loaded, has a diesel, natural gas, or hybrid engine, or has all-wheel drive (full-time four-wheel drive) or non-disengageable traction control, you will not find an Index for it in our database. We also eliminated records for cars and trucks manufactured before 1985, as they are a small fraction of the cars on the road today. However, you should note that vehicles made before 1985 would universally have smog scores of 10. Finally, we did not calculate an Auto Asthma Index for vehicles that were represented by fewer than 20 Smog Check records, as a small number of records for a particular model would lead to unreliable emissions information for that vehicle.
Some vehicles are not evaluated using EPA emissions tests. If your vehicle was manufactured between the years 2001 and 2006 and weighs over 8,500 pounds fully loaded, you may not find an Auto Asthma Index for it in our database. EPA only requires a new emissions test if a vehicle changes significantly from one year to the next. We did our best to extend data from previous year's test to other models, but due to inconsistent naming between EPA files we might have missed your vehicle. We also omitted data from vehicles burning diesel and other alternative fuels.
Vehicles that pass their local Smog Check programs may still emit excessive pollution. Programs like the one in place in California are designed to catch only the worst polluters--those that emit much more pollution than originally allowed by EPA. Smog Check programs typically have less stringent requirements for older and larger vehicles, while the Auto Asthma Index compares car emissions to today's available technologies.
We divided vehicles by model year and vehicle type, then ranked them by their smog chemical emissions to determine the emissions of a typical vehicle in the middle of each ranked list (the median emissions). Models that produced more than this amount of smog chemical emissions were flagged as "worse than typical" polluters for a particular year and vehicle type.
'Truck-type' vehicles, including trucks, SUVs, vans, and minivans, are currently held to weaker emissions standards than passenger cars. Oversized vehicles, those weighing over 8,500 pounds fully loaded, are allowed to produce even more smog chemical pollution. While federal and California regulations for these vehicles are gradually becoming more protective, at present these vehicles typically produce more smog chemical pollution than cars due to lax emissions standards.
Air quality engineers in California have estimated the average number of miles a vehicle has been driven by model year, taking into account the fact that vehicles are driven less as they age. [Kear 2003, citing EMFAC 2001] We designated high miles vehicles as those that have been driven at least 10 percent more miles than these estimates. According to our Smog Check records, about 17 percent of vehicles made in 2000 were high miles vehicles, and about 33 percent of vehicles made in 1995 were high miles vehicles.
"Gross Polluters" are vehicles that emit double or more than the allowable pollution according to the California Smog Check program [Shafizadeh 2004]. The California Smog Check program applies emissions cutpoints to vehicles within specific age and size classes, so they do not excessively penalize older cars and trucks built with outdated emissions control technology and required to meet less stringent emissions standards. These cutpoints are much looser than the EPA certification standards for all vehicles, allowing cars and trucks to emit far more pollution than the amount certified by EPA as the maximum allowable level of pollution within the first 100,000 to 150,000 miles of use.
The Auto Asthma Index was compiled using two datasets: For vehicles manufactured in the years 1985-2000, millions of real world emissions measurements from the California Smog Check program were used to rank different models, while for newer vehicles that are exempt from typical Smog Check requirements, EPA certified maximum levels of pollution within at least 100,000 miles of driving were used. The two types of data are so different that Auto Asthma Index values produced from different datasets cannot be compared to each other.
California has more health protective vehicle emissions standards than the US government. Thus, a typical vehicle sold in California is cleaner than its federal counterpart. Because our 1985-2000 Auto Asthma Index values were developed using data from California-certified vehicles only, they represent a best-case scenario in terms of smog chemical pollution.
Diesel vehicles produce extremely high nitrogen oxide (NOx) emissions when compared to gasoline vehicles, but lower volatile organic emissions. To improve air quality and public health, avoid driving diesel vehicles in NOx sensitive regions like rural areas and the Southeast US.
According to our analysis of California Smog Check data, two-wheel drive vehicles typically produce fewer smog chemical emissions than four-wheel drive vehicles. To improve air quality and public health, drive a two-wheel drive vehicle whenever possible.
Typically, manual transmission vehicles produce fewer nitrogen oxide (NOx) emissions than automatic transmission vehicles. To improve air quality and public health, choose manual transmission vehicles in NOx sensitive regions like rural areas and the Southeastern US.
Typically new vehicles pollute less than similar older models. Newer vehicles have better emissions control technology, were made to meet more stringent emissions standards, and have experienced less wear and tear. When making a choice between two vehicles, the newer one will generally be the better choice to improve air quality and protect the health of our communities. However, many newer cars have Auto Asthma Index scores of 7 to 10, indicating that automakers are not using the best technologies, or that it is impossible to build extremely large vehicles that meet reasonable emissions standards.
Use our Auto Asthma Index to compare specific vehicles before you buy. In general, newer vehicles, low miles vehicles, and passenger cars pollute less, while older vehicles, high miles vehicles, and trucks, SUVs, vans, minivans, and oversized vehicles produce more smog chemical emissions. Use these guidelines when purchasing a used vehicle.
The latest research indicates that larger vehicles are not necessarily safer vehicles. Instead, the general quality of a vehicle—how well it was designed and manufactured—appears to be the best indicator of its safety. Continued improvements in materials, safety equipment, and overall vehicle design, will continue to foster the development of smaller, more fuel efficient vehicles that produce low smog chemical emissions. [Ahmad 2005, Lundegaard 2005, Wenzel 2005]
Your friends and neighbors may not know that the pollution from cars and trucks can affect the health of your community. Tell them about EWG's Auto Asthma Index, so they can learn for themselves about the link between autos and asthma.
Unfortunately, cars and trucks pollute at all times, even when they are not in use.
As an automobile owner, you have certain rights covered by the emission warranties of your vehicle. There are two types of emission warranties: a defect warranty and a performance warranty. The defect warranty covers the repair of emission-related parts that become defective during normal vehicle operation. The performance warranty covers repairs that are necessary because the vehicle failed an EPA-approved inspection and maintenance test. Check your owner's manual for more details about your emission warranties or check with EPA for more information.
The cleanest gas burning vehicles score a 1 on the Auto Asthma Index. The newer vehicles (model years 2001-2006) that score a 1 meet California's Partial Zero Emissions Vehicle (PZEV) or the national Super Ultra Low Emissions Vehicle (SULEV) standards. They are not widely available. Most can only be purchased in California, and some Northeastern states (Massachusetts, New York, Vermont and Maine). We highlight the availability of clean models when we provide the Index value for the typically sold, dirtier versions of the same vehicle.
Ahmad S, Greene DL. 2005. Effect of fuel economy on automobile safety: A reexamination. Transportation Research Record No. 1941, Energy and Environmental Concerns 2005. Transportation Research Board of the National Academy of Sciences. Washington, DC.
California Air Resources Board. EMFAC 2001 V2.08. As cited in: Kear T, Niemeier D. 2003. Composite exhaust emissions rates: Sensitivity to vehicle population and mileage accrual assumptions. Transportation Research Record No. 1842, Energy, Air Quality, and Fuels 2003. Transportation Research Board of the National Academy of Sciences. Washington, DC. Available at: http://aqp.engr.ucdavis.edu/Documents/sensitivity.pdf
California Air Resources Board. 2002. EMFAC 2002 V2.2. http://www.arb.ca.gov/msei/on-road/
Healey JR. 2003. Cleaner cars take toll on automakers' costs. USA Today. September 16, 2003. http://www.usatoday.com/
Kear T, Niemeier D. 2003. Composite exhaust emissions rates: Sensitivity to vehicle population and mileage accrual assumptions. Transportation Research Record No. 1842, Energy, Air Quality, and Fuels 2003. Transportation Research Board of the National Academy of Sciences. Washington, DC. Available at: http://aqp.engr.ucdavis.edu/Documents/sensitivity.pdf
Lundegaard K. 2005. Crash Course: How U.S. shifted gears to find small cars can be safe, too. Wall Street Journal. September 26, 2005.
Shafizadeh K, Niemeier D, Eisinger DS. 2004. Gross Emitting Vehicles: A Review of the Literature. Prepared for the California Department of Transportation, Task Order No. 27. Available at: http://aqp.engr.ucdavis.edu/Documents/Gross%20 Emitter%20Lit%20Review%20v11%5B1%5D.doc (Accessed May, 2006)
Wenzel TP, Ross M. 2005. The effects of vehicle model and drive behavior on risk. Accident Analysis and Prevention 37:479-494.
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