The smell of hot asphalt hides a serious problem: now a bio-bitumen from algae is trying to change the air, costs and duration.
In very hot cities, asphalt is as much a part of the landscape as traffic lights and sidewalks. It sits there, absorbs sun for hours, returns heat until night and leaves that heavy smell in the air that everyone knows. In Phoenix, Arizona, the scale of the phenomenon is enormous: streets, parking lots and other paved surfaces cover about 40% of the city, and if all that pavement were piled up in one place it would be enough to cover San Francisco four times over. It is the most concrete form of the urban heat island: the ground heats up, retains energy and pushes up temperatures and consumption.
The point, however, does not end in the heat. The binder that holds the asphalt together is called bitumen, it comes from oil and degrades over time. When this happens, it releases volatile organic compounds that become more present and more aggressive on bright, hot days. In the short term they can cause dizziness and difficulty breathing. In the long term, repeated exposure is of particular concern for those who work on road construction sites, because it is associated with an increased risk of lung cancer. Elham Fini, who works between Arizona State University and Global Futures Laboratory, insists right here: a truly sustainable solution must also include human health, not just the carbon budget.
The picture becomes even more difficult as the flooring ages. Sun and heat change the chemical profile of the bitumen and the VOCs emitted gradually become smaller, more toxic and often even odorless. This size helps them enter arteries and reach organs. Experimental tests and models published by Fini’s group link these emissions to significant neurological damage, with more pronounced effects in women and the elderly. The precise risk threshold needs to be better defined, but the signal is already clear enough to change the way we look at urban asphalt.
The bio-bitumen from algae starts here. The idea is only apparently simple: replace part of the fossil binder with a material obtained from algal biomass, so as to cut emissions, contain the most dangerous compounds and also improve the mechanical behavior of the road. Algae have an obvious advantage: they grow quickly, capture carbon and can also be cultivated outside of classical agriculture. Technical documents on the sector point out that some algal crops can even double biomass in a day and that these systems can work with low quality water or wastewater, without competing with the best agricultural land.
In the project carried out in Arizona, Fini works with Peter Lammers and the Arizona Center for Algae Technology and Innovation. The algae are fed water from a Phoenix treatment plant, which is too rich in nitrogen and phosphorus to be released without other steps. In this way, waste becomes raw material. Then hydrothermal liquefaction comes into the picture, a thermal process in pressurized water that transforms wet biomass into a carbon-rich biocrude. It is the most concrete way of saying that we are trying to accelerate in a few hours a path that in nature requires geological time. From there the bio-oil is obtained which can be refined into bio-bitumen.
Algae can really change the ways
The data that attracted the most attention concerns toxic fumes. A study cited by ASU and published in 2026 shows that asphalt with components derived from algae does not eliminate overall VOC emissions, but retains precisely the most harmful part. In tests the toxicity of emissions was lowered by approximately 100 times. It is the detail that really shifts the direction of the research: bio-bitumen does not smell of the forest and does not magically make a road harmless, but it tries to trap the compounds that cause the most damage. Meanwhile, Fini is also working with the Mayo Clinic to better clarify respiratory effects and strengthen protection for exposed workers and communities.
The other front is duration. In cold climates, traditional bitumen stiffens, cracks and opens the way to cracks and potholes. Work published by Pacific Northwest National Laboratory and project partners says that an addition of 6% bio-binder obtained from Ulva improves flexibility, resistance to fatigue and self-healing ability of the coat. In other tests, a binder derived from Haematococcus pluvialis increased the elastic recovery under repeated loads from 0.1% to 71%, a sign that the material absorbs stress better and defends itself better against permanent deformation. The same line of research links these improvements to less maintenance and a longer useful life of the pavement.
On the climatic part, the numbers change depending on the mix and the species used, and here it is best to keep the results of the different studies separate. The paper relaunched by PNNL estimates that every 1% of algae bio-binders reduces net emissions by approximately 3%, with a theoretical carbon neutrality threshold of around 33% of the mixture. The material presented by the ACS on another line of evidence indicates instead a cut of 4.5% for each percentage point and a potential neutrality around 22%. The substance remains the same: the more algae enters the binder, the more the climate profile of the asphalt changes significantly.
The search, meanwhile, expands. Fini is also experimenting with binders obtained from shredded branches from forest thinning projects and is collaborating with the city of Phoenix to create a test road section with algae-based asphalt. This step matters a lot, because VOCs from streets and sidewalks are often left out of standard air quality assessments. Putting the material under the Arizona sun, with real traffic and real wear, helps to understand how well it holds up outside the laboratory.
The most difficult part remains the industrial one. Algae conversion via hydrothermal liquefaction is still a developing technology and the costs depend greatly on the scale of the plant, the biomass yield and the investment in the facilities. A technical-economic analysis of algae grown in wastewater shows that the final price drops only when the production scale increases significantly, a sign that to move from testing to the supply chain, facilities, capital and years of development are needed. Even in the United Kingdom, the projects involving Tarmac and CO2CO openly speak of promising technology, but one that still needs to be developed.
Something is already moving in Europe. In France the Algoroute project is part of a research program dedicated to alternative binders to petroleum, while in the United Kingdom work is being done on a microalgal bio-bitumen with industrial and university partners. We are still in the pilot phase, the mixtures to be optimized, the checks on compatibility, aging and real performance. But under our feet there is enormous, everyday, almost invisible fossil material. Making it pollute less and last longer would change much more than it seems.
You might also be interested in:
