Climate Crisis: Warmer summers result in higher emissions from road surfaces, with direct health impacts

Researchers at Yale university have discovered that the extreme heatwaves which affected Europe in the summer of 2022 - and which are becoming more common around the world - are going to have an altogether unexpected affect on human health.

Their research determined that when road surfaces are heated above 60ºC the number of PM2.5 particles emitted is doubled.

PM2.5 particles - Particulate Matter below 2.5 microns in diameter - are especially dangerous to human health because the respiratory system is unable to filter them. Hairlike cells called Villi move inhaled particles out of the lungs under normal circumstances, but PM2.5 particles are too small for the Villi to capture.

The result is that these particles affect the performance of the lungs, as well as crossing the blood-air interface by diffusing through Alveoli, and enter the bloodstream, where they are carried around the body. The PM2.5 matter emitted from road surfaces is mostly carbon - bitumen is a product of the oil industry - and the presence of large amounts of carbon in the bloodstream is known to be a contributing factor for several cardiovascular problems.

How likely are road surfaces to get to 60ºC? On sunny days it's actually a pretty likely outcome. Ambient temperature - the figures you'll see quoted in heatwave headlines - are measured in the shade at a height of 1.5m above the ground. 

Road surfaces are generally fully exposed to the sun and the dark grey colour of their surfaces absorbs heat very well indeed. It is not unusual for road surfaces to be 20ºC hotter than the ambient temperature. 

This becomes a significant problem in cities, with the high percentage of surface area given over to roads and car parks. If we continue to follow a path where cars dominate city life and the number of roads are retained, the increasing summer heat and resulting PM2.5 emissions will cause a significant national health risk. Wearing a N95 mask might be a important precaution in summer months.

Long-term the answer is, of course, to remove cars from cities, dig up roads and replace them with green spaces filled with trees and vegetation; apart from providing shade and creating a more pleasant city environment, tree leaves can absorb PM2.5s from the air, reducing the risks to human health.

Here's another example of how switching out cleaner cars for older, more polluting cars isn't a useful improvement for all of the problems we face with our car-centric model.