As a Los Angeleno, this one’s kind of personal to me. We just went through our second round of “Carmageddon.” A 10-mile span of the 405 freeway, the nation’s busiest, has been closed down twice in the space of a little over a year, to replace a bridge and widen the freeway to incorporate an extra lane. Because it’s Hollywood, this has accompanied mass panic and then relief when the construction work completed and the freeway span re-opened. But the real panic should come from a set of new scientific results about air quality.

The closing of that span of the 405 offered a nice controlled experiment about the effects of automobile use on air quality. And Suzanne Paulson, a researcher at UCLA, decided to measure the pollutants on Carmageddon I weekend, from last July, against a normal weekend. The results were incredible. Because so many people in the LA area just didn’t drive on Carmageddon weekend, fearing the congestion resulting from the closure of the freeway, lots less cars were on the road. And that had this impact:

Paulson and colleague Yifang Zhu measured pollutants in the air during Carmageddon last year and have recently released their pretty astounding findings. Air quality near the normally busy highway improved by 83 percent that day last July, relative to comparable weekends. Elsewhere in West Los Angeles, the improvement was equally dramatic. Air quality improved by 75 percent on that side of the city and in Santa Monica, and by 25 percent throughout the entire region, as a measure of the drop in ultrafine particulate matter associated with tailpipe emissions.

“We saw what we expected: you take motor vehicles away, the air gets really, really clean,” Paulson says, “which tells us that most of the pollution is from motor vehicles from one type or another in this area.”

The region-wide impact is really striking. I live on the Westside, which had the most dramatic impact because of wind patterns and its proximity to the 405. But removing all those cars from the road improved the air quality in a region stretching hundreds of square miles.

This offers a valuable perspective, stepping back from the day-to-day efforts to improve fuel economy or relax congestion. The message here is that cars that burn fossil fuel make our air disgusting. Particulate matter dropped just minutes after the road closure, and increased minutes after the re-opening. It’s all about cars and tailpipe emissions. Reduce the cars, or change the emissions from the cars on the road, and you completely change the air we breathe.