Diesel particulate filter (DPF) and Selective Catalytic Reduction (SCR)
In summary, they are a filter where exhaust gas pass through and the particulate matter is trapped within the filter. There are two main types of filter:
1. A ‘full’ DPF, which will reduce particulate emissions between 85% and 99%. They are also very effective at reducing emissions of the very smallest particulates which are of greatest concern to health. They are generally only available for new vehicles from the factory or retrofits for heavy duty vehicles.
2. A ‘partial’ filter, which reduces emissions between around 30% and 50%. They tend not to reduce emissions of the smallest particles as effectively as for the larger particles, but are available for light duty vehicles.
If you decide to fit a filter, it is important to get one that is correctly chosen both for your vehicle and vehicle usage, and is certified for the LEZ(s) you wish to travel in. In some countries, both types of filter (full and partial) are certified, while London and Denmark only allow full filters. The company or agency providing or fitting your filter should be able to advise you for the right equipment for your vehicle and vehicle usage. In terms of the environment and health impact a full particulate filter is by far the better option - and will also give access to more LEZs for longer.
In some countries there are also financial incentives to support the retofitting of particulate filters.
Selective Catalytic Reduction (SCR)
This is technology that can be fitted to existing vehicles to reduce oxides of nitrogen (NOx) (for more information see our air pollution pages). SCR is fitted together with a DPF and is becoming more common and is being used extensively in London to improve bus emissions. Both SCR and DPFs are required to be retrofitted to enable older heavy duty vehicles to enter the LEZs in Sweden.