Particulate matters pollution emitted directly and derived from fossil-fuel burnings in dense and crowded population leaves a few choices to urban residents. At home, air filters installed in air-conditioned units or a seperate unit in close room could reduce fine particles or PM2.5 between 60%-84% ,. On the streets, motorbikes are the dominant transportation means for commuters in Vietnam. In Hanoi, the total motorbikes were reported about 6M units in 2019, about 10 times more than passenger cars. Protection to respiratory systems for the riders is limited to various types of face masks.
By the author's observation, about 60%-80% riders are wearing face mask, in which more than half is various fabric face masks and surgical masks are the dominant of the lesser half. Brand-name particulate respirators are in few to be seen.
Shakya at. el., 2016  tested fabric and surgical masks with monodispersed aerosol sphere only found a high efficacy from 30% up to 100% with sizes from 30nm to 2.5µm. In combination with diesel exhaust and monodispersed aerosol, the removal efficiency drops to the range of 15%-57% with the fabric mask performed the worst. Tests with respiratory masks such as N95 showed marginal improvement with fine particles (PM2.5).
Concerns about chronical diseases and mortality rate link to air pollution resulted in the 68th World Health Assembly passed a resolution recognized that each year 4.3M deaths attributed to the indoor and 3.7M deaths. attributed the outdoor air pollution. The mortality rates are higher in developing countries. The author analyzed fine particles or particles with dynamic sizes less than 2.5µm (PM2.5) monitored by the US. Embassy in Hanoi and the US. Consulate in HCMC, Vietnam from 2016 to mid-2019, the good air quality level is accounted for 26% in Hanoi and 50% in HCMC by the Vietnam standard for the daily average. The WHO recommmended a limit of 25µg/m3 for the daily average concentration, a 50% lower than the Vietnam standard.
The author was motivated to evaluate the removal efficiency of popular types of masks against PM(s) and mainly PM2.5. Available methods for certificating particulate respirators required by the National Institute for Occupational Safety and Health (NIOSH) in the US under 42 CFR* 84 and guidelines from the Occupational Safety and Health Administration (OSHA), in the EU under EN 149:2001, in China under GB2626:2006. Because of convoluted technical details and expensive standard equipment, the author decided to improvise a testing box to evaluate the removal efficiency of the masks using low-cost sensors. A total of 14 tests on 11 masks were conducted over 45-day periods on four main types of masks including fabric, surgical, airmask and respirator. The form fitting factor is critical to the overall performance of a mask. Because of an improvising setup, it is not accounted in by this study.