Sierra Club logo

cars and trucks contribute to smog in this report
Sprawl Harms Our Health
Vehicles and Smog
Public Transit vs. Highways
A Vision for the Future
How Your State Rates
Downloads and Credits

clearing the air with transit spending

Sprawl Report 2001
Measuring Vehicle Contribution to Smog

Everyone knows that cars and trucks cause air pollution -- but how much? How much car and truck smog is produced per resident in a city? How well are cities doing at reducing smog from their transportation system?

It is important to consider the amount of smog per person because transportation pollution relates directly to population size; more people means more transportation in some form. The per person smog levels allow us to compare pollution reductions achieved by increased transportation choices. On a human scale, this means three people can all drive separate cars the same distance, but if those same three people carpooled, their pollution per person would be reduced.

Keep in mind that while we evaluate the smog contribution from cars and trucks, the overall smog in these cities depends on many factors-sunlight, temperatures, winds and "basin" effects (see sidebar on Los Angeles, below).

Using EPA's numbers for smog from cars and trucks from reports by the regional air districts for the 50 largest cities, we have graded the car and truck smog in each of those cities.

Grading cities based on smog per person from cars and trucks shows how well (or not) they are reducing pollution from their transportation system. Based on the pounds of smog from cars and trucks per person annually, we used the following grading scale:

How Can Los Angeles Put Out Less Car and Truck Smog but Still Have a Problem?

If the NOx and VOC emissions are lower in Los Angeles than in some other regions, why does L.A. have the worst smog? In fact, people often credit L.A. with inventing smog.

Ozone, or smog, is created when ultraviolet light (UV) from the sun stimulates chemical reactions between nitrogen oxides (NOx) and volatile organic compounds (VOCs). These photochemical reactions increase as the concentrations of NOx, VOCs, UV and the air temperature increase. The climatic and topological conditions in L.A. make it a near-perfect candidate for smog. It suffers half the year with hot, dry, stagnant air. Add to that explosive mix mountains to the north, east and south which bottle up light breezes from the ocean. The NOx and VOCs emissions from L.A.'s famous traffic and industry sit there cooking, day after day, building up ever higher levels of ozone as the pollution slowly drifts east. No rain interrupts to wash out the smog.

When smog was shown to have perilous health consequences, Congress gave California the right to require tighter auto-emissions standards than the rest of the country. L.A.'s cars and trucks, and its industry, are now among the cleanest in the nation; yet its pollution remains among the worst. However, other cities in the Sunbelt, while lacking L.A.'s basin and stagnant air, seem intent on challenging its smog-king title.

Many regions in the east, also lacking L.A.'s stagnant air and intense UV, not only have their own pollution to worry about, but also pollution blowing in from cars and trucks and industry in the Midwest and Canada. In fact, pollution transport, as it's called, occurs even in the L.A. basin, where the areas with the highest smog are 40 miles downwind from the highest concentrations of traffic and industry.

Los Angeles
0-25 A
26-50 B
51-75 C
76-100 D
101 and up F

Given the extreme levels of smog in major cities across the country as demonstrated in Chapter 1, grades for the smog from cars and trucks in even the best cities rate only a "C." Smog is out of control in almost all of our major cities, and much of that pollution is coming from cars, trucks and other vehicles. While some cities are doing better than others at reducing on-road smog, no one has earned a good grade.

However, it is possible for cities to improve their score and even earn an "A." Offering quality transportation choices, increasing transit ridership, focusing commercial and residential development around transit lines, and at least balancing highway and transit spending would all help decrease smog from cars and trucks. Cities taking these steps could earn a top grade. For example, the Washington, D.C., metro area has a number of projects under consideration that could vastly improve its score. One proposed project-the "Purple Line"-is to build a rail line that would encircle the city, connect existing suburban metro lines and add new stations at employment, retail, college and population centers. Also, transit and smart-growth advocates are working to have future development focused around public transit stops so that transit is both more accessible and more desirable. These types of public transit improvements, in Washington, D.C., and elsewhere, would help cities earn an "A" by reducing smog from cars and trucks per person.

Oklahoma City received an "F" for the amount of smog from cars and trucks per person, at 137 pounds per person per year. The New York City region, on the other hand, did much better, earning a "C" at 54 pounds per person per year.

Some may be surprised by the fact that cities like Los Angeles and Las Vegas-both well known for smog and sprawl-did not fail in this grading. It is important to remember that this is a relative grading, and that, in absolute terms, Los Angeles and Las Vegas have very polluted air. It is also important to remember that UV light, heat, dryness and stagnant air drive up smog levels. Los Angeles, which benefits from the nation's tightest emission standards, is the third lowest generator of smog from cars and trucks per person.