The modern economy revolves around immense sources of ambient pollution: agriculture, personal cosmetics, building paint, or petrol cars. Today, scientists are drawing attention to yet another toxic emitter: asphalt.
Asphalt on our roads and roofs continually releases toxic airborne particles, with volumes dependent on its material composition and the local climate. Whatever the local variations, the scale of the asphalt hazard is likely to be enormous, with 60 million tons of hot-mix asphalt pavement laid on the road each year in the US alone.
Public authorities are just starting to recognise the problem and explore sustainable ways to construct and maintain roads. According to researchers, biobased additives in the asphalt mix can help.
What makes asphalt dangerous?
Freshly resurfaced roads leave an overpowering chemical tang in the air for days. What’s responsible are air-bourne semivolatile organic compounds, tiny particles released from the synthetic materials inside asphalt when they interact with heat.
Asphalt is a generic name for a particular kind of paving and roofing material but its chemical composition varies. As a general rule, it is 95% gravel. What contributes to volatile airborne compounds is the remaining 5% – the binder material. Binder is a meltable substance that, when liquid, envelops the gravel mixture, making the mixture smoothly applicable and capable of setting firm.
Heat rips up binder molecules, a large part of which will be petroleum chemicals like bitumen, so they become light enough to rise and float through air. The particles can become small enough to enter the human bloodstream (a dangerous form of chemical pollution known as particulate matter 2.5) and even contribute to ozone damage.
The pollutants are oxygen- or sulfur-carrying aromatics, including benzofuran, benzoic acid, dibenzothiophene, hexanethiol, and polycyclic aromatic hydrocarbons. Some of these volatile compounds can irritate the eyes, nose, and throat, damage nerves and other organs, including the lung and brain, and possibly cause cancer.
A variety of manufacturing inputs influence the severity and the exact chemical makeup of the polluting fumes given off by a particular piece of road asphalt. Different bitumen grades have different chemical profiles while various additives and modifiers can be used in the manufacture of asphalt.
Roads are long-term emitters
The dangers of fresh tarmac fumes have been known for a long time and construction workers that deal with road resurfacing have always been known to be at greater risk than the general population.
However, recent research has shown that even dried roads emit significant amounts of volatile compounds long after resurfacing has finished. These findings will not surprise anyone used to smelling fresh tarmac during hot weather. Far from being an incidental nuisance, the scent of summer asphalt is a sign of the serious hazards embedded in road mixtures around the world.
A group of chemical and environmental engineers at Yale recently spotlighted the problem of long-term asphalt emissions. They knew that Californian authorities had discrepancies in their air pollution data, where a huge gap existed between amounts of pollution released by known sources like agriculture and cars and the levels actually being detected.
The Yale researchers’ experiment confirmed asphalt as the missing source of California’s air pollution. They heated samples of asphalt to normal summer temperatures, finding that at even 60 degrees, far below the heat used to melt the asphalt for tarmacing, can increase the volatile organic matter emissions. Sunlight exposure also ramps up volatile compound emissions.
From their experimental data the researchers estimated that In South California alone, asphalt from roads and roofs released between 1000 and 2500 tons of particulate air pollution, more than from gasoline and diesel vehicles.
The Yale team joined forces with chemical engineers at Carnegie Mellon, who also measured asphalt emissions at relatively low temperatures, and wrote a paper on how roads must be included in urban air quality calculations.
Algae asphalt addresses carbon and pollution
The slow-release of noxious pollutants before and after road surfacing is just one aspect of asphalt’s harms. Asphalt also contributes to climate change because manufacturing it releases a lot of carbon, either from its use of bitumen or other additives like concrete.
Researchers have turned to biobased materials to address both health and environmental issues of mined minerals and their derivatives. Recently, Arizona State University researchers led by Associate Professor Ellie Fini demonstrated a new biobased asphalt binder they say cuts the pollutant release by 70% under lab conditions.
The pollutant release figures would need retesting realistic outdoor conditions. However, it does indicate how much progress is being made on less toxic road material.
The patent-pending technology, branded AirDuo, achieves the pollutant reduction by replacing bitumen in the binder with liquefied algae. Liquified algae binder selectively ‘catches’ compounds before they form into volatile organic compounds and reach the air.
Biochar is another bio-based asphalt binder that the Arizona State University team has experimented with. Biochar is a hard, black substance similar to charcoal, created when biomass is burned without oxygen.
The second benefit relates to the carbon footprint of the asphalt. Algae is a low carbon material as it requires very few inputs to grow, making it a much less resource intensive feedstock all round than many land-based biomass sources, as well as the more carbon-intensive non-renewable petrochemicals or mined substances.
Although algae can be farmed intensively for high yields, researchers suggest that to scale the technology, they could use algae that had already been used as a biological filtration system for wastewater. Using algae that has already been used in an industrial setting would put less pressure on the environment than either wild harvesting or farming the material.
Dutch tests woody asphalt
In Europe, the Dutch have been the most prominent developers of biobased asphalt binders.
Since 2020 Holland has pursued an R&D programme formerly known as CHAPLIN and now re-named CIRCUROAD to develop a lignin-based binder, led by partners including Wageningen Food and Biobased Research, research foundation Biobased Delta, and Utrecht University.
One major project milestone came in May 2023, when Wageningen Food and Biobased Research announced their work revealed that lignin asphalt could reduce pollutant emissions by 30 to 60%.
Lignin is about as common a biobased feedstock as you can get but many sectors of the bioeconomy view lignin as a wonder-ingredient. It can be found in the cell walls of any tree or shrub and the woody material can be processed into an amazingly diverse set of materials, from liquid biofuels to polymers.
Lignin does not need to come directly from standing trees – it is also a byproduct of the pulp and paper industries, a more sustainable source than sourcing the waste from plantations, where forestry byproducts should often be left to feed nutrients back into the soi.
Lignin has been a potential candidate for asphalt binder for some time. Like bitumen, it gives the gravel mixture strength and rigidity as well as waterproofing.
However, previous to the Dutch programme, there had been no systematic testing done on woody asphalt, either in terms of outdoor performance or its recycling options.
In 2015, CHAPLIN took its bioasphalt to an industrial estate near Ghent in 2015 for tests, followed by fifteen-phase tests in several provinces across the Netherlands.
Other Dutch teams are also working on bioasphalts that use algae. At the University of Delft a team of researchers has proposed a similar idea to AirDuo, the asphalt from the Arizona team, in that they want to use liquified microalgae to produce a substitute for asphalt binder.
Assessing the final environmental benefits of any asphalt type is context dependent. Every asphalt is different and the pollution-lowering effects of biobased additives will be different depending on what other materials it is being mixed with.
Pull factors for bio-asphalt
What makes the asphalt problem so serious is that we rely on it for a key piece of infrastructure and re-paving thousands of kilometres worth of roads is a big ask from public authorities for what is a technology still in its infancy.
Biobased asphalt would require time to rest. Even in the Netherlands, which is a hotbed of bioasphalt demo research, the oldest bio-asphalt road dates from 2015. Experiments and short-term studies suggest biobased binder actually enhances road durability but to convince sceptics, this will have to be borne out under the wear and tear of real-world use over long periods.
One factor that might help in the uptake of bioasphalt is that bitumen supply is surprisingly insecure. In many parts of the oil industry, improved refining methods mean less bitumen byproduct is being eftover. Further, many countries are reliant on asphalt bitumen from other nations. This uncertain supply may push public authorities to consider bioasphalt, at least for new and upgraded sections of road.
