On a hot afternoon in Phoenix, the air above the asphalt shimmers. You can smell it: that acrid, petroleum-thick scent that clings to the back of your throat. Most people assume it burns off harmlessly, carried away by the same breeze that makes the heat bearable. Elham Fini spent years assuming the same thing, more or less. Then she started actually measuring what was in it. What she found was rather more troubling than anyone expected, and it’s getting worse as the climate does.
Fini, a senior scientist affiliated with Arizona State University’s Julie Ann Wrigley Global Futures Laboratory, spent the early part of her career studying why asphalt breaks down so quickly. That work kept pulling her back to the same culprit: volatile organic compounds, or VOCs, the carbon-based vapors continuously escaping from bitumen, the sticky petroleum product that holds road surfaces together.
The compounds are always there, seeping out at a low baseline even in mild weather. But heat accelerates them dramatically; laboratory measurements suggest asphalt emissions roughly double when surface temperatures climb from 40°C to 60°C, a range well within ordinary summer conditions in cities like Phoenix or Houston. And here is where the story starts to get complicated: those vapors do not simply dissipate. They react. Under urban skies full of sunlight and industrial chemistry, asphalt VOCs are converted into secondary organic aerosols, fine particles small enough to penetrate deep into the lungs and, in some cases, the bloodstream. The health implications of that transformation were, until recently, almost entirely uncharacterized…