When a road is laid with asphalt, there’s a strong peaty smell that eventually fades, leaving the tarmac to be perfumed by traffic fumes.

But according to a new study, in the summer, when the road surface heats up to 60 degrees Celsius, “it becomes a heavier pollutant than both unleaded (petrol) and diesel from our cars combined.”

The Yale researchers found that the emissions “climbed by an average of 70 per cent for every extra 20 (degrees Celsius) increase”.

These effects were measured in American urban areas.

Emissions from cars have declined, but…

The study, from the lab of Drew Gentner, associate professor of chemical and environmental engineering, observed that “common road and roofing asphalts produced complex mixtures of organic compounds, including hazardous pollutants, in a range of typical temperature and solar conditions.”

For decades, pollution research focused on regulations concerning emissions from motor vehicles and “other combustion-related sources”  – such as industry one presumes – with the result that air quality in cities has improved.

Common road and roofing asphalts produce hazardous pollutants, especially in summer. Photo: Getty

But as the big dirty combustion cloud has thinned out, non-combustion-related sources have been revealed as “important contributors of organic compounds.”

These can lead to ‘secondary organic aerosol’, a major contributor of PM2.5 – particles smaller than 2.5 micrometers in diameter usually found in smoke, and of significant concern public health.

According to the Environmental Protection Authority Victoria, PM2.5 are a common form of air pollution. The particles are small enough for you to breath them deeply into your lungs. Sometimes these particles can enter your bloodstream. See the EPA’s advice here.

How do the researchers know this?

The researchers collected fresh asphalt and heated it to different temperatures. The asphalt was also subjected to doses of artificial sunlight.

A detailed analysis was gleaned via a variety of tools. (Nerds only: “A high-resolution quadrupole time-of-flight mass spectrometer with soft ionization atmospheric pressure chemical ionization quadrupole time of flight and a traditional vacuum electron ionization MS both coupled to a thermal desorption system and a gas chromatograph.)

Mr Peeyush Khare, a graduate student and doctoral candidate in Gentner’s lab and lead author of the study, said

“A main finding is that asphalt-related products emit substantial and diverse mixtures of organic compounds into the air, with a strong dependence on temperature and other environmental conditions.”

After some time, the emissions at summer temperatures “levelled out” in the lab, but they persisted at a steady rate – suggesting there are “long-term, continued emissions from asphalt in real-world conditions.”

Sunlight draws emissions from asphalt

They also found that asphalt, when exposed to moderate solar radiation, leads to “a significant jump in emissions” – up to 300 per cent for road asphalt – demonstrating that solar radiation, and not only temperature, can increase emissions.

Professor Gentner noted, though, that the effect of asphalt emissions on ozone formation was “minimal compared to that of motor vehicles and volatile chemicals in personal care and cleaning products.”

He noted that asphalt is “just one piece in the puzzle” of urban secondary organic aerosol.

A July study from Florida State University found that the liquid binder that holds asphalt together, what we call bitumen, when exposed to sun and water, “leaches thousands of potentially toxic compounds into the environment.”