The bulk of traffic-generated pollution in Malta is not pumped out of car exhaust pipes but comes from particles emitted through the wear and tear of tyres, brakes and the road surface, a new study has shown.

Tests on a traffic site in Msida found that just 3.4% of all coarse particulate matter was emitted from exhausts while 17% emanated from tyres and 18% from road dust.

The University of Malta study suggests tyre pollution should become a more important issue for regulators.

“Any future measures aimed at reducing pollution exposure… should not be restricted to targeting solely tailpipe emissions but should focus on the non-exhaust fractions too,” the study notes.

The study, titled ‘Exhaust and non-exhaust contributions from road transport to PM10 at a Southern European traffic site’, was carried out by Mark M. Scerri from the University of Malta’s Institute of Earth Sciences together with Stephan Weinbruch, Gilles Delmaire, Nadine Mercieca, Michael Nolle, Paolo Prati and Dario Massabò. It was published in the latest edition of the international peer-reviewed Environmental Pollution journal.

The researchers looked at the presence and components of coarse particles (PM10) by taking daily samples at an Msida traffic site between January 19 and December 31, 2018.

Particulate matter ranges in size and can be very problematic because the finer the particles, the deeper they penetrate into the respiratory tract, ending up in the lungs.

The composition of coarse particles from a sample in Msida. Source: Environmental Pollution journal

Coarse particles are the relatively large airborne particles mainly produced by the mechanical break-up of even larger solid particles. They have an aerodynamic diameter ranging from 2.5 to 10 microns, about one-seventh the thickness of a human hair. This distinguishes them from the smaller airborne particulate matter referred to as fine (PM2.5).

In this study, the researchers analysed the coarse particulate matter and categorised it by sources. The main sources were traffic-generated particles (38.4%), followed by marine/shipping (23%) and Saharan emissions (21%).

The study focused on the traffic-generated emissions. Moving vehicles generate particles from tailpipe exhaust. But they also produce other particles, termed non-exhaust emissions, that contain a mix of harmful chemicals.

Source: Environmental Pollution journal

This occurs through brake wear, which produces particles full of metals; tyre wear, which generates particles full of zinc and organic compounds; road wear, consisting mainly of organic compounds; and re-suspended dust generated when vehicles catapult all these particles back into the air.

The research showed that the bulk of the traffic-generated emissions –  87% – consisted of non-exhaust emissions. In fact, road dust made up 18% of the total emissions in Msida and tyre and brake wear made up 17%.

In an interview last year, atmospheric chemist Noel Aquilina noted that construction and the increasing volume of traffic were producing more dust in the air that contains chemicals harmful to health. He called on the need to monitor these large particles as they may be more toxic than fine particles due to the “complex mixture” of the chemicals they are made of.

Respiratory specialist Stephen Montefort said the larger particles – produced mainly from quarries, construction activity, dust in the roads and fireworks – are still small enough to cause multiple health problems.