Many studies have now shown the association between airborne particulate matter and cardiorespiratory ill health. It has been hypothesized that these effects may be due to the nanoparticles (>100 nm) comprising the largest number of particles, rather than the mass, which is principally determined by larger sized particles. In the theory therefore, if the ultrafine particle hypothesis is correct, mass based particulate matter standards both used in atmospheric or industrial environment may in some circumstances seriously misrepresent the oxicity of the pollutant. Particulate respirators are widely used in workplace to protect workers against hazardous airborne particles. The filter medium is the main functional component of a respirator for removing airborne particulate matter. Although there are lots of studies showed the filter media performed with high collection efficiency for collecting micrometer and submicrometer sized particles, the data for ultrafine particles is unavailable.
The objective of this study is to investigate the filtration efficiency of nanoparticles penetrating through commercialized filtering facepieces. Most filter media used today are made of electret material. By additional electrostatic mechanisms, high removal efficiency can be achieved without incurring large pressure drop. However, not only the efficiency but also the most penetrating size of these materials is changed, as the electric charge is reduced or eliminated. Thus, the uncertainty associated with the change of efficiency and he most penetrating particle size should be taken into consideration when proposing new test requirements. Therefore, the effects of filter charges on filter efficiency and the most penetrating particle size will also be studied in this study.
In this study, two aerosol generators: an ultrasonic atomizing nozzle and a constant output atomizer and two particle size spectrometers: an aerodynamic particle sizer (APS) and a scanning mobility particle sizer (SMPS) were used to conduct the filter performance testing. In order to eliminate the electrostatic charges, filters were dipped in isopropanol for 5 minutes and allowed to dry. Then, these dipped filters, along with controls of each filter type were tested to determine the aerosol penetrations in the size range of 10 nm to 10 μm.
The results showed that the disposiable masks tested in this study rely heavly on their electrostatic charge to provide adequate filter efficiencies. As the electrostatic charge on the fibers of each filter was reduced, the aerosol penetration values in the 10 nm to 5 μm size range increased considerably and the most penetrating particle size noticeably shifted from nanometer range to submicrometer particles. On the other hand, almost all particles with size less than 10 nm or larger than 5 μm were collected in the filters, and the penetration alues wouldn’t affect by the amount of filter charge. The quality and price of commercial disposiable masks varies with brands. In general, certified masks performed better than those without certification.