Non-exhaust road traffic emissions include particles released into the air from brake wear, tyre wear, road surface wear and resuspended road dust. These types of emissions therefore contribute to the total particulate matter in the environment and are associated with human health problems and mortality.
Emissions of non-exhaust particles from vehicles may already exceed those from exhausts, and will likely increase in the future. However, quantitative data on the magnitude of non-exhaust emissions are limited and uncertain, also because the proper measurements and standards have not yet been validated.
The exact contribution of non-exhaust emissions to local and national air quality is therefore currently under discussion. Emissions vary widely according to brake, tyre and road surface material, and driving style. Existing emission factors therefore have a wide range of uncertainty and show significant variations between PM10, PM2.5 and PM1 size fractions.
Non-exhaust emissions particles are also an important source of metals to the atmosphere. Among others, Zn and Cu are highly associated with brake- and tire wear from cars in urban environments.
Besides a lack of approved legislation covering non-exhaust emissions, there are no product standards for the composition of brake pads and brake systems to limit air pollution. Methods of measuring non-exhaust emissions so far lack international consistency. Several testing institutes, and also brake pad manufacturers, are developing testing and analysis approaches that reflect real-world driving conditions. These are measuring particulate matter and particle numbers for emissions from brake wear, but there is no clear view on when a proposal can be expected.