Asphalt is so ubiquitous that most people never give it a second thought, but a growing body of research suggests that this petroleum-based material – which lines millions of kilometres of roads worldwide – may be quietly damaging our health in ways that have been largely ignored.
Note: As I write this in the UK – and with a domestic readership in mind – I was tempted to translate to ‘asphalt’ to ‘tarmac’ throughout article but a quick Google informs me that ‘Asphalt is made from a blend of aggregates and bitumen and typically takes up to two days to fully cure. In contrast, tarmac consists of crushed stone mixed with tar and cures much faster.’ [source] So asphalt it is.
Two new studies, led by researchers at Arizona State University, reveal that asphalt is not just an inert surface. It is a persistent source of toxic emissions that worsen urban air pollution and may contribute to everything from respiratory disease to dementia.
The first study found that humidity is a critical and previously underestimated driver of these emissions. When relative humidity rises from near-dry conditions to just 50% at 50°C, emissions of harmful polar volatile organic compounds (VOCs) increase by up to 46%. These VOCs react in the atmosphere to form secondary organic aerosols. Using data from more than 5,000 observations across 1,400 US road segments spanning three decades, the researchers also showed that humidity interacts powerfully with sunlight to accelerate asphalt deterioration. This creates a self-reinforcing cycle: as the surface breaks down, it releases yet more pollutants.
The second study goes further, demonstrating that asphalt emissions form abundant ultrafine particles under both day and night conditions. In controlled chamber experiments, the researchers exposed a representative asphalt VOC mixture to hydroxyl radicals (simulating daytime chemistry) and nitrate radicals (simulating night-time chemistry). Both produced significant quantities of particles smaller than 100 nanometres – tiny enough to enter the bloodstream directly from the lungs. Night-time chemistry proved particularly efficient at consuming phenolic compounds, which are potent particle precursors.
Importantly, high humidity slowed down how quickly particles formed in the air, but it didn’t stop them from forming altogether. Meanwhile, sticky, slow-to-evaporate chemicals like catechol readily attached themselves to these particles. The researchers concluded that if these reactive compounds are removed from asphalt during manufacturing, it could significantly cut down on this type of harmful air pollution.
The scale of the problem is huge. In Phoenix, Arizona alone, paved surfaces cover an estimated 40% of the city. If all of that pavement were piled into a single spot, it would be enough to cover San Francisco four times over. This vast carpet of asphalt absorbs heat during the day and releases it slowly at night, fuelling the urban heat island effect and driving up energy consumption. But for Elham Fini, the senior scientist leading much of this research, the health impact of all that pavement deserves just as much attention as its carbon footprint.
Fini is now working on a solution. She has teamed up with researchers at the Arizona Center for Algae Technology and Innovation to grow a special strain of algae using wastewater from a Phoenix treatment plant. That algae is then baked at high temperatures without oxygen to create a binder that can be mixed into asphalt.
Remarkably, tests have shown that algae-infused asphalt does not just reduce total VOC emissions; it actively traps the most toxic compounds, reducing the overall toxicity of asphalt fumes by roughly one hundred times. The algae also helps pavement resist breaking down, which could lower construction and maintenance costs for cities.
Photo: Vishal Sharma
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