Research by James Cervantes & Raul Bermudes Jr.
Think about what pollutes the air
Do cars come to mind? Maybe factories, power plants, or cigarette smoke? But here is something most people rarely think about: the road itself.
Yes, the actual road we travel on. Right now, there are over 4 million miles of roads in America (ASCE, 2021). More than 94% of the paved ones are made of asphalt, a thick dark material made from crude oil (National Asphalt Pavement Association). Studies have found that asphalt releases chemicals called Volatile Organic Compounds, or VOCs, into the surrounding air, and understanding this is an important piece of the larger air quality puzzle.
VOCs are carbon-based molecules that evaporate very easily, usually at temperatures at or below 250°C (EPA). Because they turn into gas so easily, they float right off surfaces and into the air around us, often without us even noticing.
Some VOCs are very common, and you encounter them every day. For example, acetone, found in nail polish remover, is VOC. So is ethanol, found in many cleaning products. Most of these everyday VOCs are not a major health concern. The ones released from asphalt roads, however, are worth paying closer attention to.
Asphalt used in roads releases more than 30 different types of VOCs, and many of them are harmful. When VOCs from asphalt roads get into the atmosphere, they react with other gases and molecules in the air, including hydroxyl radicals, oxygen, and nitrogen oxides (NOX), and this reaction creates aerosols and ground-level ozone (Koppmann, 2007). Unlike the ozone layer high up in the atmosphere that protects us from UV rays, ground-level ozone is harmful. Long-term exposure to such ozone causes respiratory problems, cardiovascular issues, and other serious health effects (EPA). And some VOCs from asphalt, like benzene, are known carcinogens (Liu et al, 2023).
When road crews lay fresh asphalt, the material is heated to around 150°C to make it soft and spreadable (Liu et al., 2023). At that temperature, the asphalt releases more than 30 different VOCs into the air. These include hydrocarbons like benzene, ethyl benzene, and xylene, as well as sulfur compounds and oxygen-containing compounds (Liu et al., 2023). The hotter the asphalt, the more VOCs it releases. Hence, the health of the workers standing next to it, while laying down the road, is heavily impacted (Cui et al., 2019).
But the emissions do not stop once the road is finished. Even after the asphalt cools down and traffic starts rolling over it, the road keeps releasing VOCs. Every hot day, every burst of sunlight heats the surface back up. UV radiation from the sun also breaks down the asphalt over time, releasing a different set of chemicals called Polycyclic Aromatic Hydrocarbons (PAHs), which include compounds like quinoline and benzothiophene (Gentner et al., 2020). In other words, even finished roadways emit VOCs. Asphalt roads last about 15 to 20 years before they need to be resurfaced, with interstate roads needing replacement even more often because of the heavy traffic they carry (National Asphalt Pavement Association). Every time a road is resurfaced, the emission cycle begins again.
We cannot live without roads. But we can make better ones. We can build them at lower temperatures, cool them faster, add materials that trap the chemicals, and invest in longer lasting alternatives. The science to do this already exists. What we need now is awareness and action.
If you want to understand the full picture of this problem, from the chemistry of VOCs to the detail of each solution being developed, read the research done by James Cervantes and Raul Bermudes Jr., two students who spent time digging deep into the question: what role do American asphalt roads play in VOC emissions, and what can actually be done about it? Their work lays it all out clearly, and it might change the way you think about the road you drive on every single day.
American Society of Civil Engineers. (2021). Roads. In 2021 infrastructure report card.
National Asphalt Pavement Association. (n.d.). Engineering. Asphaltpavement.org. https://www.asphaltpavement.org/expertise/engineering
Liu, G., et al. (2023). Effect of heating history on the emission of volatile organic compounds from asphalt materials. Science of the Total Environment.
Technical Overview of Volatile Organic Compounds; U.S. Environmental Protection Agency https://www.epa.gov/indoor-air-quality-iaq/technical-overview-volatile-organic-compounds
Koppmann, R. (Ed.). (2007). Volatile organic compounds in the atmosphere. Wiley.
Federal Highway Administration. (n.d.). Average annual miles per driver by age group. U.S. Department of Transportation.https://www.fhwa.dot.gov/ohim/onh00/bar8.htm.
Cui, P., Schito, G., & Cui, Q. (2019). VOC emissions from asphalt pavement and health risks to construction workers. Journal of Cleaner Production, 228, 118757. https://doi.org/10.1016/j.jclepro.2019.118757.
Khare, P., Machesky, J., Soto, R., He, M., Presto, A. A., & Gentner, D. R. (2020). Asphalt-related emissions are a major missing nontraditional source of secondary organic aerosol precursors. Science Advances, 6(36), eaay4941. https://www.science.org/doi/full/10.1126/sciadv.abb9785