N95 vs KN95
The “N95” name comes from the US National Institute for Occupational Safety and Health (NIOSH) designation. The N means that the masks are not resistant to oil; the 95 refers to the masks’ efficiency at filtering out particles. To achieve a score of 95, a mask has to filter out at least 95 percent of all particles in the size range
KN95 masks are the Chinese standards for 95% filtration efficient masks. Contrary to popular belief, they are broadly the same as the US N95 standards. The US N95 standards have slightly stricter requirements from pressure drop while inhaling/exhaling – meaning N95 masks in theory are slightly more breathable than KN95. Leading mask manufacturer 3M believes it is reasonable to consider KN95 standards as being equivalent to N95. However, importantly, there is no independent certification and Chinese manufacturers are able to ‘self-certify’ products as meeting KN95 standards, meaning in practice many fall short.
N95 compared to other types of mask
A team of Korean scientists in 2014 studied (https://aaqr.org/articles/aaqr-13-06-oa-0201.pdf) the filtration of 44 different masks including N95 masks, standard surgical/dental varieties, cotton masks and ‘homemade’ handkerchief masks.
The N95 masks filtered 95% of particles, as you would expect. The surgical/dental masks were only 40-60% effective, the cotton masks 30%, and handkerchiefs ranged from 2-13% (1 to 4 layers)
Another trial, in 2011 (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4941587/pdf/IRV-5-170.pdf), looked at the rates of infection picked up by nearly 1,500 healthcare workers in Beijing hospitals. Of those wearing surgical masks about 7% reported respiratory symptoms and only 4% of those wearing N95 PPE reported symptoms. 9% of those without masks reported symptoms. That suggests performance of the N95 masks led to around half the infection rate of basic (or no) masks.
Do they help the wearer, or just everybody else?
With low standards of filtration even for the best homemade masks, the WHO has stressed that masks are best for “source control” only – i.e. to prevent the wearer from passing on infections. Only much higher-rated masks (such as N95) provide meaningful protection to the wearer themselves