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EXPERT REACTION: Asphalt during summer may be worse for pollution than petrol and diesel combined

Embargoed until: Publicly released:
Peer-reviewed: This work was reviewed and scrutinised by relevant independent experts.

Experimental study: At least one thing in the experiment was changed to see if it had an impact on the subjects (often people or animals) – eg: changing the amount of time mice spend on an exercise wheel to find out what impact it has on weight loss.

We walk on it, we rely on it for our roads, but come summertime, asphalt becomes a heavier pollutant than both unleaded and diesel from our cars combined, say US researchers. The team heated up asphalt to temperatures between 40°C and 200°C and noted that the emissions doubled between 40°C and 60°C, temperatures regularly reached during summer. They add that the emissions climbed by an average of 70 per cent for every extra 20°C increase. The emissions slowed over the course of a week but persisted, they say, especially when exposed to sunlight.

Journal/conference: Science Advances

Link to research (DOI): 10.1126/sciadv.abb9785

Organisation/s: Yale University, USA

Funder: We thank the Yale SEARCH Center, U.S. EPA, and NSF (AWD0001666) for support, as well as GERSTEL for collaboration and support with the TD 3.5+. This publication was developed under Assistance Agreements RD835871 (Yale University) and 83587301 (Carnegie Mellon university) awarded by the U.S. Environmental Protection Agency. It has not been formally reviewed by EPA. The views expressed in this document are solely those of the authors and do not necessarily reflect those of the Agency. EPA does not endorse any products or commercial services mentioned in this publication. Author contributions: P.K. and D.R.G. conceived the study and designed laboratory experiments and the experimental setup. P.K., J.M., A.A.P., and D.R.G. acquired the asphalt materials. P.K. and R.S. (New Haven, CT) and J.M. and A.A.P. (Pittsburgh, PA) conducted on-site gas-phase sampling. P.K. collected gas-phase samples in the laboratory. Subsequently, P.K., J.M., and M.H. conducted the analysis of data acquired using EI-MS and APCI-TOF. P.K. and D.R.G. interpreted the data and wrote the paper. All authors commented on and discussed the manuscript to help refine the interpretation and presentation of results.

Media release

From: AAAS

Asphalt: A Greater Summertime Source of Harmful Air Pollutants in Large Cities Than Motor Vehicle Gasoline and Diesel Combined 

Asphalt emits greater quantities of secondary organic aerosols (small particles with insidious public health effects) under summertime conditions in California’s South Coast Air Basin than gasoline and diesel from motor vehicles combined, according to a new study. The findings point to asphalt as a significant source of air pollution, although one that generally flies under the radar. The industry states that the asphalt manufacturing process removes potential emissions, and emissions from asphalt binder (which holds the material together) are not typically included in inventories. Additionally, Peeyush Khare and colleagues found that asphalt emissions are at their worst during hot, sunny weather, indicating that the material releases more secondary organic aerosols into the air during the summer months. The researchers note that while emissions from motor vehicles are likely to decrease in the years ahead as the most egregious polluters are phased out, asphalt emissions may exert a greater impact as urban areas expand and climate change drives temperatures higher. Previous studies have estimated that a substantial fraction of semi-volatile organic compounds in metropolitan Los Angeles come from a source other than vehicles, yet the culprit has not been identified. To determine how asphalt contributes to urban air pollution, Khare et al. heated commonly used road asphalt to a range of temperatures between 40°Celsius and 200°Celsius (104°Fahrenheit and 392°Fahrenheit) in a temperature-controlled tube furnace. They observed that asphalt emissions doubled when the temperature increased from 40°Celsius to 60°Celsius (104°Fahrenheit to 140°Fahrenheit –temperatures the material typically reaches in the summertime), then climbed by an average of 70% per 20°Celsius (68°Fahrenheit) increase. Although asphalt emissions slowed over the course of one week, they persisted, especially when the material was exposed to sunlight. Khare et al. suggest future studies may employ different methods to more fully capture the extent of the emissions released both during and after asphalt application.

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Expert Reaction

These comments have been collated by the Science Media Centre to provide a variety of expert perspectives on this issue. Feel free to use these quotes in your stories. Views expressed are the personal opinions of the experts named. They do not represent the views of the SMC or any other organisation unless specifically stated.

Dr Thava Palanisami is from the Global Innovative Centre for Advanced Nanomaterials (GICAN) in the School of Engineering at The University of Newcastle

Asphalt-based emissions are abundant but rarely reported due to analytical challenges. Resolving the complex organic mixtures and their transformation products is a challenge. Continuous exposure to solar radiation is the major factor influencing the emission of toxic aromatics and its transformation products.

It is scary to note the increased emission of some of the potentially carcinogenic aromatic compounds with increasing temperature. Furthermore, the transformation products of polycyclic aromatic hydrocarbons (PAHs) - a class of chemicals that occur naturally in coal, crude oil, and gasoline - are even more toxic and bioavailable.

This paper adds further evidence to our recent toxicity studies involving real-field samples; there is a possibility for innovative solutions to reduce these emissions.

Last updated: 02 Sep 2020 3:34pm
Declared conflicts of interest:
None declared.
Dr Paul Harvey is an Environmental Public Health Scientist. He was formerly an adjunct Professor at Macquarie University, and is now owner of Environmental Science Solutions and Director of Biami Scientific Pty Ltd.

This is definitely an interesting, albeit technical, new piece of work. The study points to a previously overlooked source of environmental contamination. I find it concerning that asphalt should be producing the range and concentration of contaminants identified in the study because asphalt roads are globally ubiquitous.

In Australia a large proportion of the population lives within a few metres of asphalt surfaces, with the proximity decreasing in newer, more dense residential developments. The study suggests that, in some cases, the asphalt surface can be more detrimental to a healthy atmosphere than other emission sources such as cars.

More research would need to be conducted to determine the full extent of the impacts proposed in this study, but this would be easy to achieve in a country such as Australia that has a vast network of air-monitors in urban settings.

This study really demonstrates that the more humans move away from green spaces, the more we expose ourselves to potentially harmful pollutants in our lives. It serves as a wake-up call to regulators and town planners who may not see value in including green spaces and parkland in developments.

Last updated: 02 Sep 2020 3:32pm
Declared conflicts of interest:
None declared.
Professor Ian Rae is an expert on chemicals in the environment at the School of Chemistry at the University of Melbourne. He was also an advisor to the United Nations Environment Programme on chemicals in the environment and is former President of the Royal Australian Chemical Institute

This careful study by American researchers provides details of substances emitted by hot asphalt.  

Emissions are highest when a new layer is being applied to a road, and then they decline slowly over times as long as several months, but are always greater on hot days. The substances emitted are mainly aromatic hydrocarbons, some of which are known to be injurious to health. The researchers report collecting large chunks of fresh real-world asphalt during road paving in New Haven, Connecticut, presumably just down the road from their laboratory at Yale University.

Asphalt is just one source of what are called 'diffuse emissions', ones that don't originate from a single point, but collectively can add up to significant amounts. Diffuse emissions in Australia are covered by the National Pollutant Inventory (NPI) which collects annual data on emissions and transfers of 93 chemical substances. Most of the NPI data come from specific industries or activities that can be quantified, and the data set at www.npi.gov.au can be searched by substance, area, or industry.

The NPI also covers diffuse emissions, but these are not measurements but estimates, and given the nature of the sources some of them must be very rough estimates. Manuals are provided for making estimates of diffuse emissions from 'cutback bitumen', the technical term for the asphalt that is the subject of this note. There is a category of 'paved and unpaved roads' too, but the emissions of concern in that category are dusts that contain heavy metals. Other categories are 'architectural coatings' (mostly paints), 'lawn mowing', 'dry cleaning' and 'service stations'. The diffuse emission data are not collected every year, but only from time to time. Sadly, the latest available data for diffuse emissions on the NPI date from 1999, possibly the result of the continuing financial squeeze on our national environment department.

Despite what we smell when a nearby road is getting a new layer of asphalt, the collective diffuse emissions from this activity are much smaller than those of other sources that are easily detected by a good nose!

Last updated: 02 Sep 2020 3:32pm
Declared conflicts of interest:
None declared.

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