Non-Exhaust Emissions

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Driving a vehicle also produces emissions different from those of the combustion engine – so called non-tailpipe or non-exhaust emissions. Most relevant here are the substances emitted by brakes and tyres through abrasion. They are increasingly recognised as a significant contributor to environmental pollution and health threat and receive even more attention as the exhaust emissions are further lowered with new vehicle models.

As of 2025, Green NCAP’s rating includes an assessment of the abrasion of brakes and tyres. The approach focuses only on vehicle properties, which result from design and strategy characteristics and directly impact the level of abrasion. No direct emissions measurements are conducted, but relevant vehicle parameters are evaluated instead. The results of this evaluation present proxies of abrasion behaviour and are assessed in the rating system. For simplicity, Green NCAP also uses the term “wear” instead of abrasion.

Tyre wear

Small particles are emitted from tyres while driving. Regardless of the type of tyres, there are several vehicle properties, which have a major influence on the tyre abrasion behaviour.

  • The higher the vehicle mass, the more abrasion happens at the tyre surface.
  • If the wheel alignment produces increased slip when the wheels are moving, the abrasion effect is stronger.
  • Finally, if the vehicle reacts aggressively to small accelerator pedal movements when starting to move in default operation mode, unnecessary high wheel slip will lead to more emissions.

Brake wear

Small particles (brake dust) are emitted from friction brakes while driving. There are several vehicle and brake system properties, which have a major influence on the brake abrasion.

  • ‘Brake dust mitigation’ refers to the possibility to reduce the amount of brake dust emitted to the environment by, e.g., filters mounted on the brakes.
  • Vehicles with closed friction brake systems, i.e. using brake drums instead of brake discs, retain most of the brake dust in the enclosed drum. This property is addressed by ‘brake dust containment’.
  • Electrified vehicles can operate their electric motors as generators to decelerate the vehicle and by that ‘harvest’ the moving (kinetic) energy and help reduce the use of the friction brakes. The higher the energy recuperation, the less friction brake use and thus less abrasion. However, the vehicle mass plays a role as the abrasion caused by friction braking also increases with the mass of the vehicle, which needs to be decelerated or stopped.
Green NCAP non-exhaust emissions: brakes abrasion