Phoenix, Arizona asphalt: pavement heat and hidden VOC fumes

In cities such as Phoenix, Arizona, you rarely give the road surface a second thought. It sits beneath car tyres, shopping trolleys and the relentless summer sun, so familiar it fades into the scenery.

Yet an expanding body of research indicates that asphalt may be doing more than intensifying urban heat. In areas where roads and car parks spend months scorching in direct sunlight, pavement may also be venting chemicals into the air residents inhale day after day.

That question is central to the work of Elham Fini at Arizona State University. For years, she has examined how asphalt weathers over time and which substances it gives off as it ages.

Her findings suggest the debate about pavement should extend beyond climate and energy demand to include human health as well.

Cities feel the pavement heat

Phoenix contains an enormous amount of paved ground. Streets, car parks and other hard surfaces are thought to make up around 40 per cent of the city.

That scale alone makes pavement a major urban concern. Gather all the asphalt in Phoenix into a single heap and it would be enough to blanket San Francisco four times.

Pavement absorbs substantial heat in daylight and then releases it gradually overnight, reinforcing the urban heat island effect that keeps city temperatures elevated after dark.

You do not need specialist training to notice it: roads continue to radiate warmth well after sunset, car parks stay hot into the evening, and buildings demand more cooling.

However, Fini’s work indicates that heat may be only part of the story.

Asphalt chemical fumes rise in heat

Asphalt is bound together with bitumen - a black, sticky petroleum-based material. Bitumen emits volatile organic compounds (VOCs) into the air.

These releases are not confined to new roadworks or freshly laid surfaces. They occur all the time, though many people become more aware of them on very hot days when the characteristic asphalt smell grows stronger.

Two studies in the Journal of Hazardous Materials and Science of the Total Environment reported that these compounds shift after sunset and can generate ultrafine particles that degrade air quality.

In the near term, exposure to these fumes can bring on dizziness and breathing difficulties. This is particularly relevant for those who work closest to the source.

Construction teams and paving crews, for instance, can spend long shifts around heated asphalt and its chemical vapours - often without a respirator.

Long-term exposure, greater risks

Over time, the potential harms appear more severe. Sustained exposure has been associated with an increased risk of lung cancer, which makes repeated workplace exposure especially concerning.

Fini is now collaborating with Dr Bruce Johnson via a partnership with Mayo Clinic to clarify how these emissions influence respiratory health.

Ultimately, this research is intended to support stronger safeguards for workers, improve awareness in nearby communities, and drive the development of better road materials.

Aging asphalt gets more toxic

Even if fresh asphalt were the only issue, it would still warrant attention. But evidence suggests older pavement may become an even greater problem.

Sunlight and heat gradually degrade bitumen. As this breakdown progresses, the chemical blend released from the surface shifts.

With age, asphalt begins emitting different VOCs, including substances that seem to be more toxic than those produced earlier.

Some of these compounds have little or no odour, which is perhaps the most unsettling aspect. A lack of smell does not mean a lack of risk.

These microscopic toxic particles can be small enough to move into arteries and organs. Testing and modelling also indicate they could cause substantial neurological harm, particularly in women and older adults.

Researchers still cannot pinpoint the exact exposure level at which danger becomes clear. Even so, what is already known should be enough to challenge the idea that pavement is simply neutral infrastructure.

Turning algae into asphalt

Fini has partnered with Peter Lammers, chief scientist at the Arizona Center for Algae Technology and Innovation, to cultivate algae using wastewater from a Phoenix treatment works.

Rather than discarding that nutrient-rich, chemical-heavy water, the team is using it to create a material that could make asphalt less toxic.

“It’s a great setup because we use water that’s far too high in nitrogen and phosphorus to be released anywhere. Instead, we reuse it to grow more algae,” Lammers said.

Fini then heats the algae to very high temperatures with minimal oxygen, producing a binder that can be blended into asphalt.

What the tests revealed

In tests, the algae-enhanced material cut the toxicity of asphalt emissions by roughly 100-fold.

There may be practical benefits as well. The algae appears to slow the rate at which pavement deteriorates, which could reduce maintenance spending and make it appealing to councils and paving firms for reasons beyond health.

Fini is also assessing other binders, including one derived from leftover branches from forest-thinning projects. She is working with Phoenix to surface a stretch of road using algae-infused asphalt.

That will enable evaluation beyond laboratory conditions. Real streets are invariably more complicated - and often more informative.

A hidden pollution problem

Those on-road trials matter because pavement VOCs are frequently omitted from routine air-quality assessments. When cities fail to monitor them properly, the pollution is easily overlooked.

Roads can seem so ordinary that people stop noticing them at all - simply part of the urban fabric, functional and fixed.

But they influence how hot a city feels, how clean its air remains, and potentially how healthy nearby residents and workers can be.

“We have 4 million miles of roads in America,” Fini said. “We should make those 4 million miles do more for us than just get from A to B.”

Viewed through that lens, pavement stops being mere background. Instead, it looks like an everyday presence that may have been shaping our lives for years, even as few people asked the right questions about it.