New research at the University of Pennsylvania reveals that exhaust is not the only source of air pollution on roadways and highways; the analysis of dust along these roads has identified potential health concerns.
Led by Reto Gieré, professor and chair of Penn's Department of Earth and Environmental Science in the School of Arts and Sciences of the University of Pennsylvania, the research study sheds light on a second offender when it comes to traffic-related air pollution: Tiny bits of tyres and brake pads from passing vehicles as well as road materials that remain suspended in the air causing pollution.
The study is being conducted in partnership with collaborators from across the world.
Gieré explains that when traffic, cars, and trucks drive by, they re-suspend the dust on the roads into the atmosphere, and then it becomes breathable. How dangerous these dust particles are to human health can only be ascertained after analysing these dust particles.
Regulations enforced within the automotive industry ensure motor vehicles are manufactured with cleaner and more energy-efficient engines.
However, particles from tyre and brake wear are rarely recognised as pollutants – with the increasing urban congestion, these pollutants can increase drastically, thereby impacting human respiratory health.
Air pollution kills about 4 million people each year while unsafe water kills 2 million. Yet, we have a United Nations Sustainable Development Goal about water pollution but not one about the air, says Gieré.
This study is being carried out by Gieré in partnership with German colleagues from the Federal Highway Research Institute, the German Meteorological Service, and the University of Freiburg to sample and analyse the air along roadsides.
The sampling for the research was a year-long effort along two highly frequented motorways in Germany using customised cylindrical samplers with a transparent sticky foil at the bottom to trap particles and passively collect the dust.
Optical microscopy was used to analyse the collected airborne particles, which revealed that the road with busier traffic patterns had 30 percent more particles, with a large part derived from tyre wear. Another finding was that dry and warm conditions were associated with a greater build-up of particles. Higher temperatures saw more tyre abrasion, causing more pollution than intermediate temperatures.
Taking the study further, Gieré and colleagues recently used powerful scanning electron microscopy to precisely identify the make-up of the particles collected from the two motorways in addition to a third collection site, at an urban highway with slower-moving traffic.
Ninety percent of the dust particles collected from the three sites were traffic-related but there were differences in the nature of the particles between the sites.
The slower-moving traffic on the urban road generated fewer particles from brake wear but more from tyres while the highway with more stop-and-go traffic generated more brake particles.
Tyres and brake pads contain zinc, lead, antimony, silicates, cadmium, and asbestos – all presenting a major health risk in any environment. Pollution from these sources will be restricted to the immediate environment but these particles could also wash into rivers.
A similar experiment is being carried out on the streets of Philadelphia by Gieré’s team to check pollution levels between neighbourhoods.