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.

VOCs + NOx + sunlight --> ozone

Vehicles are a major source of both of these smog chemical pollutants. Many factors, including the types and amounts of VOCs and NOx in the air, as well as local weather patterns and geography, control the formation of ozone in a particular region. Increased ozone air pollution is linked to many harmful health effects in kids and adults. (Learn more about air pollution and kids' health.)

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2. Why does my city matter?

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. .

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3. Why isn't my model year 1975-2000 vehicle here?

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.

We grouped Smog Check records based on the information that Smog Check technicians record for individual vehicles at the time of their tests. Sometimes, important details about the vehicles are not recorded. For example, Smog Check records often do not indicate whether a particular vehicle is two-wheel drive or four-wheel drive. If you cannot find an Auto Asthma Index for a four-wheel drive version of a vehicle, try looking for the Index of a two-wheel drive version instead. Four-wheel drive versions of vehicles typically produce more pollution than two-wheel drive versions.

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4. Why isn't my model year 2001-2006 vehicle here?

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.

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5. My vehicle has a bad score on the Auto Asthma Index, but I always pass my state's smog test. Why is this?

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.

Similarly, your 'check engine' indicator light is illuminated when your emissions are 50 percent higher than the levels your vehicle is allowed to produce. These legal emissions levels are less stringent for older and larger vehicles. So your vehicle may still be responsible for lots of local pollution even when the indicator light is off.

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6. Why does my vehicle have worse than typical emissions?

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.

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7. Why does my vehicle have lax emissions standards?

'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.

In addition, older vehicles were designed to meet earlier, less health protective emissions standards. Starting in 1996, changes to federal and California regulations resulted in substantially cleaner vehicles, according to our analysis of Smog Check data. Vehicles made before 1996 produce significantly higher amounts of smog chemical pollution, and are flagged as having lax emissions standards.

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8. Why is my vehicle a high miles vehicle?

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.

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9. What is a Gross Polluter?

"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.

We calculated the percentage of Gross Polluters for each model in our Smog Check database. Older cars are more likely to be Gross Polluters because they have experienced more wear and tear, and may not have been maintained well. However, many vehicles are Gross Polluters due to poor construction on the assembly line, or use of substandard engines and emissions control equipment.

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10. Why can't I compare older and newer vehicles in your report?

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.

The Smog Check test measures emissions produced by a vehicle with a warmed-up engine that is putting out a consistent amount of power and driving at a constant speed of 25 miles per hour. The measured emissions are adjusted only for dilution that occurs within the testing equipment, and for relative humidity.

The EPA test is applied to new vehicles, to determine which emissions standards they are certified to meet. The EPA test involves starting a vehicle without first warming the engine, then driving it over a specific course, while varying speed and engine power in a specific way. Thus, the test measures the emissions that occur during the cold-start of the engine, as well as during regular driving involving a range of speeds and engine power usage. Many vehicles produce a third of their volatile organic compound emissions during a cold-start, before the engine and emissions control equipment has warmed up, so this is an important distinction of the EPA test relative to the Smog Check test.

The results of the EPA test are then adjusted for the expected effects of vehicle aging over 100,000 to 150,000 miles of driving. Finally, the vehicle is placed into an emissions category based on the emissions it is expected to produce at midlife. This is the vehicle's EPA certification standard. Vehicles may be certified to meet standards mandated federally or by the state of California. Typically, the maximum allowable pollution the aged vehicle is certified to emit is about twice the amount it is expected to emit, leaving a margin of error in case the effects of aging were underestimated.

Because emissions values measured by the EPA test includes emissions produced right when the engine is started (startup emissions), and because the values are derived from new vehicle emissions measurements that are then adjusted for age and categorized to fit California or federal emissions standards, these values cannot be compared directly to emissions values measured by the California Smog Check test.

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11. What is the difference between 'California' and 'federal' vehicles? Why do automakers make cleaner and dirtier versions of the same model?

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.

For newer cars the Auto Asthma Index takes into account where you live, as well as available emissions data. If you select California, New York, Maine or Vermont--all states that require that vehicles sold within their borders meet California's emissions standards--your vehicle Auto Asthma Index will be based on the cleanest vehicle sold. However, if only Federal models were tested, your Index is based on these records and may be elevated.

In addition to difference between cars sold in "clean" states, automakers often produce different versions of the same model vehicle, which are certified to meet different emissions standards. When multiple versions of the same vehicle are sold we derived an Auto Asthma Index score for the cleanest model commonly sold, but present emissions information about the dirtiest model for comparison.

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12. What about diesel emissions?

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.

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13. Are two-wheel drive vehicles typically better than four-wheel drive vehicles?

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.

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14. Are automatic or manual transmissions typically better?

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.

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15. Do all new vehicles pollute less?

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.

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16. How can I pick a good used vehicle?

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.

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17. But bigger vehicles are safer aren't they?

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]

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18. What can I do about polluting vehicles in my neighborhood?

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.

If your neighbor's vehicle produces smoky exhaust, you may also be able to report it to local air quality officials. You may find a phone number for reporting smoking vehicles in the government listings of the phone book.

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19. When does my vehicle pollute?

Unfortunately, cars and trucks pollute at all times, even when they are not in use.

Vehicles produce three types of emissions: running emissions, startup emissions, and evaporative emissions. Running emissions include volatile organic compound (VOC) and nitrogen oxide (NOx) tailpipe emissions produced during regular driving, beginning shortly after a vehicle has started, such that the engine and emissions control equipment have warmed up and are functioning optimally. These emissions are measured in the Smog Check and EPA emissions tests. Startup emissions are VOC tailpipe emissions produced just as the vehicle starts, when the engine and emissions control equipment are still cold. These emissions are measured in the EPA test, but not in the Smog Check test. Evaporative emissions are VOCs that escape from the gasoline present in the vehicle when it is at rest. Neither Smog Check nor the EPA emissions tests we've evaluated measure evaporative emissions.

Thus, both Smog Check and EPA emissions tests measure just a fraction of the actual amount of VOC pollution a vehicle emits. Simulations using the California Air Resource Board's EMFAC model suggest that overall, roughly one third of total vehicle VOC pollution comes from each of the three types of emissions, running, startup, and evaporative. [CARB 2002]

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20. What about the emissions warranty?

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.

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21. What are the cleanest cars? Where would I purchase one?

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.

Automakers claim that it is expensive to make the cleanest versions of vehicles, such as those meeting California's PZEV standards, so they continue to sell much dirtier versions throughout the US. However, California air quality officials estimate the cost of the improvements needed to produce these clean vehicles at less than $200 per vehicle, a tiny contribution to the overall price of new cars and trucks. [Healey 2003] Automakers should provide all their customers with clean vehicles.

(See a list of today's cleanest new and used cars.)

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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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