SAN FRANCISCO, CALIFORNIA—Thousands of refineries, oil wells, and gas treatment plants pepper the Los Angeles Basin, a commerce powerhouse that accounts for nearly 40% of California’s economic output. But these fuel facilities come with a hidden cost: a surprising amount of pollution (as seen in this image). Researchers using van-mounted sensors have shown that the region’s six largest refineries are individually emitting 300%–700% more pollution than is being reported by official records, according to a presentation here this week at the American Geophysical Union Fall Meeting. Physicists used mobile instruments that analyzed infrared and ultraviolet light to detect volatile organic compounds (VOCs), gases such as ethane and propane that break down in sunlight to produce smog, which has been linked to respiratory problems. The researchers measured pollution levels around the perimeters of the refineries to produce pollution maps, which can be used to pinpoint neighborhoods most likely to be impacted by smog. The team also tracked emissions from other stationary sources such as oil wells, gas stations, and harbors to estimate that the VOC emissions from the entire Los Angeles Basin might be as high as 13,000 kilograms per hour. That’s far higher than the officially reported level of 200 kilograms per hour. The physicists suggest that the basketball-sized pollution sensors, already employed by some countries such as Sweden, can be used to rapidly monitor industrial emissions and detect underground gas leaks.