The car you drive says a lot about you: your income, your personality, your family. It also says how many asthmatic kids you've helped send to the hospital.

We don't think of our sedans, SUVs, pickups and minivans as causes of illness. But each of us who drives one of the 204 million cars and trucks registered in the United States shares responsibility for triggering or exacerbating asthma and other respiratory diseases among our fellow Americans. Our cars are one of the biggest sources of smog, the ozone haze formed by the reaction of two groups of chemicals in auto exhaust, volatile organic compounds (VOCs) and nitrogen oxides (NOx), with sunlight. More smog means more asthma sufferers reaching for their inhalers.

"Each of us who drives one of the 204 million cars and trucks registered in the United States shares responsibility for triggering or exacerbating asthma and other respiratory diseases."

The relationship between smog and asthma is confirmed by hundreds of studies worldwide. During the 1996 Summer Olympics, the city of Atlanta, fearing gridlock, took extraordinary measures to reduce traffic. For the 17 days of the games, auto use dropped 22.5 percent. Daily ozone concentrations dropped almost 30 percent. The benefit to asthma sufferers was direct and dramatic. During that period, the number of asthmatics who saw a doctor dropped by 40 percent, the number admitted to the hospital dropped 19 percent, and the number who went to the emergency room dropped 11 percent. [Friedman 2001]

In a year-long computer investigation, Environmental Working Group analyzed 2.5 million records of actual emissions of California cars and trucks, recorded by state-licensed Smog Check stations and obtained from the state Bureau of Automotive Repair.

The California records comprise the biggest and richest database in the country on how much pollution cars on the road actually emit. These data differ from the model-based pollution ratings published annually by the U.S. Environmental Protection Agency, and allow a real world look at emissions from aging vehicles. As a measure of the entire U.S. fleet, the data are conservative because California's smog standards are stricter than most other states.

We found that vehicle pollution varies widely based not only on the type of vehicle, but also on the year it was built, the number of miles it has been driven, the automaker, and the quality of construction:

  • Among the largest automakers, Honda, Mazda, Hyundai and Nissan stand out as performing better than their competitors. Cars from General Motors and Daimler-Chrysler were more polluting.

  • Four-year-old cars with 80,000 to 100,000 miles on the odometer, on average, emit nearly twice as much smog-forming chemicals per mile as cars with 20 to 40,000 miles.

  • Even typically clean models can include a number of extreme polluters. A typical 2000 Honda Civic with automatic transmission was almost as clean as today's cleanest new cars. But fully one-tenth of the 2000 Civics tested emitted 6.5 times more pollution than the typical ones.

We used the Smog Check data for autos made between 1985 and 2000, and EPA test data for newer vehicles, to calculate an Auto Asthma Index for 10,102 models, measured on a scale of 1 (cleanest) to 10 (dirtiest) inhalers. It lets you see how much smog-forming air pollution is emitted by your car (or your neighbor's), and how your car and driving choices can reduce your impact on asthma and other air pollution-related illnesses. Since the dynamics of ozone formation vary from region to region in the U.S., the Index is customized for each of the smoggiest cities in the country.

  • If you live in Chicago, driving a 2006 Kia Spectra instead of a 2006 Subaru Impreza would reduce by half your annual emissions of the chemicals that contribute to smog. In Baltimore-DC metro area, driving a 2005 Mazda 6 Sport Wagon instead of a 2005 Volkswagen Passat Wagon 4Motion would cut your emissions by 60 percent.

  • Shopping for a new minivan? The 2006 Pontiac Montana, Chrysler Town & Country, and the Mazda MPV, with Auto Asthma Index scores of 8, 8 and 9 respectively, emit 4 to 6 times more smog-forming chemicals than the current models of the Mercury Monterey, with a score of 3.

  • Even within the same company and the same vehicle class, some models are dramatically cleaner than others. A 2005 Toyota Land Cruiser, with an Auto Asthma Index score of 10, emits 5 times more smog-forming chemicals than a 2005 Toyota 4Runner, with a score of 4.

  • Not ready to trade cars? Drive less. If enough people made similar choices, smog in your city would be reduced dramatically—and just as during the Atlanta Olympics, when traffic was restricted in city and mass transit was expanded, so would the number of asthma sufferers seeking treatment.

EWG's Auto Asthma Index is the first attempt to determine just which vehicles—to put it bluntly—are most or least likely to contribute to America's asthma epidemic. It's a tool meant to help us think about smog not as an environmental problem but as a health crisis that directly harms 20 million Americans, with children the most at risk.

"EWG's Auto Asthma Index is the first attempt to determine just which vehicles—to put it bluntly—are most or least likely to contribute to America's asthma epidemic."

Since 1980, asthma death rates have increased by more than 50 percent among all genders, age groups and ethnic groups. Among children under 19, the asthma death rate has increased by 80 percent. According to the Asthma and Allergy Foundation of America, every day:

  • 40,000 people miss school or work due to asthma.
  • 30,000 people have an asthma attack.
  • 5,000 people visit the emergency room due to asthma.
  • 1,000 people are admitted to the hospital due to asthma.
  • 14 people die from asthma. [AAFA 2006]

Even healthy people are affected by smog. Children, whose lungs are still developing, and people with existing respiratory illnesses—asthma, emphysema, and cardio-pulmonary disease—are the most harmed. But we all bear the cost.

In California, the smoggiest state, scientists calculated that cutting current smog levels to meet the state's protective ozone standard would prevent hundreds of hospitalizations, thousands of emergency room visits, and millions of missed school or work days. [CARB 2005] From that data, EWG calculated a potential savings to the state of more than half a billion dollars a year, measured by reduced school and work absences and medical treatment for asthma and other respiratory diseases. [EWG 2005]

Education and treatment are essential to combating the asthma epidemic, but so is cleaner air. Reducing smog minimizes the number of healthy people who develop asthma, in addition to reducing the severity of symptoms for people already diagnosed with the disease. Unlike strategies for "managing" asthma—keeping children indoors at home or school, restricting exercise on smoggy days, carrying inhalers to temporarily alleviate symptoms—reducing smog is a permanent improvement. It benefits everyone. And it places responsibility not on the victims, but on a main source of the problem: The cars we drive.

Americans associate cars with freedom, and it's hard to admit that your freedom to drive can limit someone else's freedom to breathe. Once cigarettes were considered purely a personal choice; today everyone acknowledges the dangers of secondhand tobacco smoke. But when we're choosing a car, or deciding how far we're willing to drive to work, few of us consider the impact of our choices on the health of our family, friends and neighbors.

But while consumers' choices drive the market, in the end they can only choose what's available. The bigger burden of responsibility for cleaner, safer, healthier cars lies with the corporations who make and sell them and the government that regulates them.

The technology exists today to build cars and trucks that are much cleaner when they leave the showroom and, with proper maintenance, stay that way on the road. Today's largest SUVs and trucks can still legally emit 30 times more smog chemicals than the cleanest cars. The high price of a new car means people are keeping their cars longer, so the cleanest new cars must certify that their pollution control systems will last for 150,000 miles.

There's no reason every car and truck sold in America shouldn't meet those standards. Cleaner, fuel-efficient, more dependable cars will reduce dependence on increasingly scarce oil and cut emissions of global warming gases. As important as those concerns are, so is protecting the right and ability of all Americans to breathe freely.


Asthma and Allergy Foundation of America. Website: For Life Without Limits. Referenced May 2006.

California Air Resources Board. 2005. Review of the California Ambient Air Quality Standard for Ozone. Volumes I-IV. Staff Report. March 11, 2005.

Environmental Working Group. 2005. Smoggy Schools: Poor air quality costs Californians more than $521 million a year. Available at:

Friedman MS, Powell KE, Hutwagner L, Graham LM, Teague WG. 2001. Impact of changes in transportation and commuting behaviors during the 1996 Summer Olympic Games in Atlanta on air quality and childhood asthma. Journal of the American Medical Association. 285:897-905.

The Cleanest Cars

2006 Hyundai Sonata
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