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The Rise and Fall of Diesel

Clean diesel is an object lesson on "too good to be true." Any kind of diesel exhaust is truly terrible—both for the environment and for our health.

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The city that loves tradition may have found a new one. One-third of Paris went car-free on September 27, 2015. For one day, residents raved to reporters about seeing, hearing, and feeling better. It was as though a fog had lifted from the city. In fact, it had. Nitrogen dioxide in the air dropped by 20 percent to 40 percent, and particulate matter plunged as well. Any city would breathe easier without internal combustion engines zigzagging through its streets, but one word makes Parisian roads distinct from those in the average American city: diesel. Approximately 80 percent of French drivers own diesel-fueled cars.

Diesel and gasoline both exact unacceptable tolls on the environment, just in different ways. Gasoline contains 15 percent less energy per gallon than diesel, diminishing fuel efficiency and increasing carbon emissions per vehicle mile. Diesel, while widely viewed as easier on the climate than gasoline, is much worse when you consider air quality. It emits far more nitrogen oxides and particulate matter—the stuff that damages our lungs, contributes to heart disease, and darkens the skies over Paris and many other cities.

The science is simple. A gallon of crude oil can be split into several products, including gasoline and diesel. Gasoline is light and volatile and contains fewer carbon atoms in its molecular chain. Diesel, by contrast, is visibly thicker and heavier than gasoline. Its long carbon chains require less refining, which is why diesel is typically cheaper than gasoline at wholesale. Less refining also means that diesel contains higher levels of the chemicals that cause air pollution—nitrogen oxides, sulfates, arsenic, zinc, and nitrates.

Nitrogen dioxide combines with ammonia and other airborne chemicals to form compounds that enter the lungs and inflame airways. It can cause breathing problems and increase sensitivity to allergens. Medical review studies—research that combines and analyzes the data collected in several existing studies—show that long-term exposure to nitrogen dioxide significantly shortens life span.

Diesel's motley stew of exhaust particles also contributes to the formation of particulate matter, tiny bits of stuff suspended in the air. Particulate matter is as damaging as nitrogen dioxide to human health. Laboratory studies endorsed by the U.S. Environmental Protection Agency show that diesel particulate matter causes changes in the lungs, depresses immune function, and likely causes several forms of cancer.

The concentration of these pollutants is highest inside the car itself, where occupants inhale two to three times more nitrogen dioxide than passersby. However, even if you don't drive a diesel car and refuse to ever get into one, the driving choices of others still affect you. The air within 50 yards of a major road is 30 percent to 100 percent thicker with nitrogen dioxide than nonadjacent areas. There's simply no escaping diesel exhaust. Some groups are harmed more than others. People of color and lower-income communities are far more likely to live near major roadways. Children are at special risk because their developing lungs are more vulnerable to the changes wrought by diesel exhaust.

The health impacts are indisputable, which raises the question: Why have so many drivers in Europe, and a growing number in the United States, opted for diesel vehicles? It comes down to politics, economics, technology, and a little bit of happenstance.

Only 10 percent of the cars on European roads were fueled by diesel in the mid-1990s. At that time, engineering advancements began to allow automobiles to operate at higher fuel pressures, a necessity for the efficient operation of a diesel engine. Manufacturers like Volkswagen and Citroen convinced politicians that their diesel cars, being lower in carbon emissions, would help Europe meet its commitments under the 1997 Kyoto Protocol. (Japanese automakers, by contrast, invested more heavily in hybrid-electric vehicles.) Many European governments also ensured that diesel remained cheaper than gasoline, persuading skeptical buyers to make the switch. Today, more than half of the cars in Europe run on diesel.

American automakers, policy makers, and tastemakers didn't hop on the diesel bandwagon until Volkswagen claimed it had performed a near miracle in the late 2000s. The German car company told regulators and consumers that its vehicles offered the fuel efficiency of diesel while cutting out all of that nasty nitrogen dioxide. Between 2010 and 2012, sales of so-called clean diesel cars surged by nearly 25 percent in the United States.

But you probably know how that story ends. In September 2015, the world learned that Volkswagen had been defrauding government regulators for years. The company's diesel cars suppressed emissions during testing but returned to their high-pollutin' ways during normal operation.

Volkswagen now faces billions of dollars in government fines around the world, class action lawsuits, and an incalculable loss in consumer confidence. But let's thank the company for one thing: The scandal proved to Americans that "clean diesel" is about as real as "clean coal." Now that the smog has cleared, hopefully Europe will see the light, too.

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