Removal of ultrafine particles by intermediate air filters in ventilation systems. Evaluation of performance and analysis of applications.

Doctoral thesis, 2012

Epidemiological and toxicological studies demonstrate that ultrafine particles (UFPs) are strongly related with respiratory and cardiovascular diseases and syndromes. One common method to reduce human exposure to particulate air pollution is the use of intermediate class filters (F5-F9 class filters according to EN779:2002). However, the efficiency of such filters, with respect to UFPs, is not well explored. Furthermore, neither the European standard nor the US standard for classification of intermediate class filters comprises performance with respect to UFPs or particles of the most penetrating size (MPPS). This could turn out to be a major lack in classification standards since UFPs have been pointed out as a potential serious health hazard. To fill in the gap, the purpose of the thesis is to evaluate the performance of intermediate class filters available on the Swedish market, and to correlate the efficiency for UFPs (EFUFP) and MPPS-size particles (EFMPPS) with the EN779 classification efficiency for particles of the size 0.4μm (EF0.4μm). The thesis also contains analyses of 1) air filtration for indoor particles of outdoor and indoor origin; 2) how to efficiently apply intermediate air filters in two-step air filtration systems and 3) ionizer assisted air filtration. Size-resolved filtration efficiencies of 23 filter sheets and 8 full-scale filters were tested in laboratory experiments with four types of upstream aerosols. The relationships between EFUFPs, EFMPPS and EF0.4μm were investigated under different testing conditions. The results showed that the electrostatic force from charged filter fibers has big influence on the shape of the efficiency curves. Additionally, the electrical charge state of the upstream aerosol is critical for the testing of charged synthetic filters. Linear relationships were found between EFUFPs, EFMPPS and EF0.4μm within the observed efficiency range for both glass fiber and charged synthetic filters. In general, EFMPPS was 10-20%-units lower than EF0.4μm. The values of EFUFPs were close to EF0.4μm for glass fiber filters, while EFUFPs were lower than EF0.4μm for charged synthetic filters. Theoretical analysis showed that filter operating hours and classes are critical to the cost of two-step filtration, i.e. a solution where a pre-filter protects the main filter. Under suitable operation, two-step filtration is not necessarily more expensive than single-step filtration. Another analysis investigated suitable filter class, filtration locations, and the ratio of supply to outdoor air flow rate, for efficient removal of particles coming from indoor and outdoor sources. The results can be used to recommend suitable air filters and ventilation rates/modes or to predict the existing system performances. Yet a separate study, based on measurements, shows that an F7 class filter assisted with an ionizer may reach an efficiency similar to that of a single F9 class filter.