Pavement is everywhere in Phoenix. This ASU researcher says it’s secretly off-gassing chemicals in the heat

Phoenix is known for its relentless summer heat, sprawling road networks, and miles upon miles of asphalt pavement. While most residents associate pavement with scorching temperatures and the city’s urban heat island effect, new research from Arizona State University (ASU) suggests there is another hidden concern. According to researchers, asphalt pavement is continuously releasing potentially harmful chemicals into the air, and the process becomes significantly worse during periods of intense heat and sunlight.

The findings have sparked growing concern among scientists studying air quality, public health, and climate resilience, particularly in hot cities like Phoenix where pavement covers a substantial portion of the urban landscape.

New Research Reveals Hidden Emissions From Asphalt

Arizona State University researcher Dr. Elham Fini, a senior scientist affiliated with ASU’s Julie Ann Wrigley Global Futures Laboratory and a faculty member in the School of Sustainable Engineering and the Built Environment, has spent years studying why asphalt deteriorates over time.

During that research, her team discovered that asphalt emits volatile organic compounds (VOCs)—carbon-based chemicals that evaporate into the atmosphere. Although these emissions occur continuously, they become much stronger as pavement heats under direct sunlight.

Unlike vehicle exhaust, these emissions come directly from the pavement itself. Because Phoenix experiences some of the highest pavement temperatures in the United States, the city provides an ideal environment for studying how asphalt behaves under extreme conditions.

Why Asphalt Releases Chemicals

Bitumen Is the Source

Asphalt is held together by bitumen, a sticky petroleum-based binder. When exposed to heat and ultraviolet radiation from the sun, bitumen slowly breaks down.

This degradation causes volatile organic compounds to escape into the surrounding air.

Researchers found that sunlight and high temperatures not only increase the amount of VOCs released but also change the chemical composition of those emissions, creating compounds that may be even more toxic than those emitted from fresh pavement.

Heat Accelerates the Process

Phoenix’s extreme summer temperatures can push pavement surface temperatures well above 150 degrees Fahrenheit.

According to Dr. Fini, higher temperatures significantly worsen emissions.

As she explained, heat is “exacerbating the emissions from asphalt,” meaning hotter conditions increase the release of these chemicals into the surrounding environment.

What Are Volatile Organic Compounds?

Volatile organic compounds, commonly called VOCs, are chemicals that easily evaporate into the atmosphere.

Many VOCs are produced from petroleum products, paints, fuels, industrial materials, and construction products.

The compounds released from asphalt differ depending on weather conditions, pavement age, and exposure to sunlight.

Researchers say some of these emissions contribute to the formation of ultrafine particles that reduce air quality and may pose health risks.

Studies published in the Journal of Hazardous Materials and Science of the Total Environment examined how asphalt emissions change after sunset and how they contribute to atmospheric pollution.

Potential Health Risks From Asphalt Emissions

Short-Term Effects

Exposure to asphalt fumes has long been associated with symptoms including:

• Headaches
• Dizziness
• Eye irritation
• Difficulty breathing
• Respiratory discomfort

These symptoms are more likely to occur among individuals working near freshly paved roads or during extremely hot weather.

Long-Term Health Concerns

Researchers are even more concerned about long-term exposure.

Certain VOCs released as asphalt ages appear capable of penetrating deep into the human body.

According to ASU’s research, some compounds are small enough to enter arteries and organs.

Modeling analyses suggest these toxic compounds may contribute to neurological damage, with women and older adults potentially facing greater risks.

Long-term exposure to asphalt fumes has also previously been linked to elevated risks of lung cancer, especially among road construction workers who spend years working around heated asphalt without adequate respiratory protection.

Researchers stress that additional studies are needed to determine exactly what levels of exposure become dangerous for the general public.

Construction Workers Face the Greatest Risk

People employed in paving and road construction experience the highest levels of exposure because they spend extended periods working directly around hot asphalt.

Dr. Fini hopes ongoing research will lead to stronger workplace protections, including improved respiratory safety measures and better understanding of occupational exposure limits.

She is collaborating with researchers from the Mayo Clinic to better understand how asphalt emissions affect respiratory health and the human body over time.

Aging Roads Become More Toxic

Pavement Changes Over Time

One of the most surprising discoveries from the research is that older pavement may become more hazardous than newly installed roads.

As asphalt ages, prolonged exposure to ultraviolet radiation and repeated heating cycles changes the chemistry of bitumen.

Instead of simply emitting fewer chemicals over time, aging pavement begins releasing different types of VOCs, some of which appear to be more toxic.

Many of these compounds are odorless, meaning people may be exposed without realizing it.

Phoenix Is Especially Vulnerable

Phoenix presents unique challenges because of its combination of:

• Extremely high summer temperatures
• Large paved surfaces
• Heavy automobile dependence
• Long periods of intense sunshine

According to ASU researchers, pavement covers nearly 40 percent of the city.

This enormous asphalt footprint means pavement emissions could become a significant contributor to urban air pollution, particularly as climate change increases the frequency and intensity of heat waves.

Beyond the Urban Heat Island

For years, scientists have focused on asphalt’s role in creating the urban heat island effect.

Dark pavement absorbs large amounts of solar energy during the day before slowly releasing that heat overnight.

This process keeps nighttime temperatures elevated, increases energy consumption for air conditioning, and contributes to dangerous heat conditions in cities.

Dr. Fini argues that the health impacts of pavement deserve equal attention alongside its influence on climate and energy use.

She believes sustainability should include protecting human health as well as reducing carbon emissions.

Scientists Are Developing Cleaner Asphalt

Algae-Based Asphalt Binder

Rather than simply identifying the problem, researchers are also working on potential solutions.

Dr. Fini has partnered with Dr. Peter Lammers, chief scientist at the Arizona Center for Algae Technology and Innovation, to develop an innovative asphalt additive made from algae.

The algae are grown using nutrient-rich wastewater from a Phoenix treatment facility before being heated under low-oxygen conditions to create a bio-based material that can be blended into asphalt.

This approach not only recycles wastewater but may also improve pavement performance.

Dramatically Reduced Toxicity

Testing has produced encouraging results.

According to research published in Clean Technologies and Environmental Policy, algae-infused asphalt did not significantly reduce the total amount of VOC emissions.

However, it substantially reduced the toxicity of those emissions.

Researchers reported approximately a 100-fold reduction in the toxicity of chemicals escaping from asphalt containing the algae-derived binder.

The modified asphalt also appears to slow pavement aging, potentially extending road life while lowering maintenance costs.

Additional Sustainable Materials Under Investigation

Researchers are also exploring other environmentally friendly asphalt binders.

One promising option uses leftover branches and wood waste generated from forest-thinning projects.

These alternative materials may further reduce harmful emissions while creating more sustainable road construction practices.

ASU researchers are working with the City of Phoenix to pave sections of roadway using algae-infused asphalt so performance can be monitored under real-world conditions.

Why These Findings Matter

Air quality studies often focus on emissions from vehicles, factories, and industrial facilities.

However, Dr. Fini notes that pavement emissions are frequently overlooked in traditional air-quality assessments.

Considering that the United States has roughly 4 million miles of roads, even modest improvements in asphalt materials could have substantial environmental and public health benefits.

Researchers believe future roadways should not only provide transportation but also reduce pollution and improve community health.

Connection to Phoenix’s Cool Pavement Efforts

Phoenix has already invested heavily in “cool pavement” technologies designed to lower surface temperatures by reflecting more sunlight.

Studies conducted with ASU have shown these coatings can reduce pavement surface temperatures by approximately 10 to 12 degrees Fahrenheit compared with conventional asphalt, although reductions in air temperature have been relatively modest.

While cool pavement primarily addresses heat, Dr. Fini’s research focuses on reducing the chemical emissions coming directly from asphalt itself, making the two research efforts complementary approaches to improving urban environments.

Researchers Say More Work Is Needed

Scientists emphasize that many questions remain unanswered.

Future research aims to determine:

Safe Exposure Levels

Researchers are still working to identify what concentration of asphalt-emitted VOCs becomes harmful to nearby residents.

Long-Term Public Health Effects

Additional studies will examine whether chronic exposure contributes to respiratory disease, neurological disorders, or cardiovascular problems.

Improved Road Materials

Scientists continue developing alternative binders that reduce emissions without sacrificing pavement durability or increasing costs.

Collaborations involving ASU, the Mayo Clinic, universities across the United States and internationally, and the City of Phoenix are expected to expand understanding of these issues in the coming years.

Conclusion

The latest research from Arizona State University highlights an often-overlooked source of urban air pollution: the roads beneath our feet. While asphalt has long been recognized for trapping heat and contributing to Phoenix’s urban heat island, scientists now say it also releases volatile organic compounds that become more abundant—and potentially more hazardous—as temperatures rise.

For a city where pavement blankets nearly 40 percent of the landscape, these findings underscore the importance of rethinking how roads are built and maintained. Through innovations such as algae-based asphalt binders and continued research into safer paving materials, scientists hope future roads will not only withstand extreme heat but also reduce harmful emissions, improve air quality, and better protect both construction workers and the millions of people who live in increasingly hot urban environments.