HEPA filters to improve vehicle cabin air quality – advantages and limitations

Maintaining a good indoor air quality level has received growing attention in the past years. Especially the smaller particles like PM2.5 (particles of aerodynamic diameter less than 2.5 μm) and UFP (ultrafine particles, aerodynamic diameter less than 100 nm) might lead to higher health risks. Vehicle cabin is one challenging environment due to the elevated particle concentrations from the surroundings.
The main protection against outdoor pollutants is from the filter in the vehicle HVAC (Heating, ventilation, and air conditioning) unit. During the past decade, the state-of-the-art solution has been synthetic filters with integrated activated carbons to also cope with gaseous pollutants. These conventional filters, however, are limited by factors including space, reduced efficiency whilst dust-loading, and relatively low efficiencies around the particle size of 100-300 nm. Widely varying efficiency values (20%-90%) have been reported from different vehicles.
There is now an interest to introduce filters with higher efficiencies, for example HEPA (High-Efficiency Particulate Air) filters in vehicles. Besides improved efficiencies, another advantage is that the efficiency does not decrease much whilst dust loading. The disadvantages are increased pressure-drop and space requirements, which make them harder to implement in the compact vehicle environment.
One potential improvement in the short run is to use a HEPA-filter placed in the engine bay as a pre-filter, to protect and potentially extend lifetime of the HVAC filter. The combined particle filtration efficiency is improved, and the increased pressure-drop can be acceptable when the HEPA-filter has relatively large dimension.
In this study two filter prototypes (EPA and HEPA level) were manufactured to investigate applications of pre-filter in a production vehicle. Vehicle test with generated particles (NaCl and Di-Ethyl-Hexyl-Sebacat) and road particles were performed. The inside and outside particle concentrations were measured simultaneously under different fan speeds and combinations of prototypes. One prototype was aged and tested in the vehicle as well.
The tested system showed considerably improved air quality, also with an aged filter. With pre-filters applied, the in-cabin UFP and PM2.5 removal could achieve 99%, much higher than the original filter alone (76% and 87% respectively). More importantly in the particle size range below 100 nm, higher than 97% removal was achieved for all sizes. The limitation of such system is mainly the added pressure-drop and space in the vehicle, which demands a balance with the improved filter efficiency.