Manufacturers of cabin filters will often make the point that if a cabin filters is clogged, or faulty, the air inside a vehicle can be many times more dirty and more dangerous than the air outside. Some interesting research by emission testing specialists Emission Analytics has shed some new light on this claim.  

The vehicle cabin is a confined space and if pollutants are allowed to enter there is only limited air to dilute them. Added to this in a modern vehicle there is little air exchange except through the ventilation system.

To look at interior air quality Emissions Analytics performed tests for vehicle interior air quality across over one hundred vehicles in three countries with particle number (PN) and carbon dioxide (CO2) concentrations, simultaneously tested inside and outside of the vehicle. 

Test results in the USA showed that the vehicle with the best ventilation system and cabin filtration protected its occupants by reducing PN pollution by 69% compared to outside air. However, the worst vehicle saw double the outside concentrations of particle pollution inside the vehicle. 

Results also showed that exhaust filtration on new cars has an efficiency of over 99.9%, so these vehicles are emitting a very small net number of particles, in fact particle pollution is largely from non-exhaust emissions. 

With regard to CO2 emissions, as with PN concentrations, there were big variations between vehicle models as to how fresh the air was kept on recirculation. Tests showed CO2 increased by 103% in the best case and 275% in the worst.

Emission Analytics say changing the filter regularly is important to avoid degradation and also the choice of filter brand is important.  The initial test results – comparing six different filters on the same vehicle, show that the best filter reduced the interior pollution almost three times more than the worst filter. This means a high quality cabin filters is vital for comfort and safety of drivers and occupants. 

The emission testing experts make the point that current thinking on vehicle pollution is highly flawed. New internal combustion engine vehicles emit almost no pollutants from the tailpipe, except CO2.  The current decarbonisation strategy has meant heavier electric vehicles, and this is creating a tyre emissions problem that dominates anything from the tailpipe. The company says that the concern should be with non-exhaust and non-vehicular emissions rather than the tailpipe, focusing particularly on fine particles and VOCs from plastics and tyres.