Mask efficiency,
really
There are two types of face masks available N95 and N99, the
latter boasting 99% efficiency for PM, and being more expensive than the lower
efficiency 95% masks. They filter airborne particles larger than 0.3 microns
diameter. These are supposed to be in three sizes: small, medium and large. The
problem is that faces come in all kinds of shapes and even the N99 masks cannot
assure a perfect fit. This is important because, if they do not fit, a fraction
of unfiltered air is being inhaled. This means that the effective efficiency is
not really as high as the advertised 95% or 99%.
Let us call the advertised efficiency h, the leakage percentage l and the degraded efficiency
e. The fraction of
pollution that gets through is:
P = l
+ (1 - l)(1 -
h)
The 1st terms is the unfiltered air, and the
remainder (1 - l)
does get filtered.
The effective efficiency is:
e = 1
– p = h (1 - l)
This can be tabulated as a function of varying leakage
fraction l for
both types of masks:
|
|
N95
|
N99
|
|
0
|
0.95
|
0.99
|
|
0.05
|
0.9025
|
0.9405
|
|
.1
|
0.855
|
0.891
|
|
.2
|
0.76
|
0.792
|
|
.3
|
0.665
|
0.693
|
|
.4
|
0.57
|
0.594
|
|
.5
|
0.475
|
0.495
|
|
.6
|
0.38
|
0.396
|
|
.7
|
0.285
|
0.297
|
|
.8
|
0.19
|
0.198
|
|
.9
|
0.095
|
0.099
|
|
1.0
|
0
|
0
|
The limiting cases of zero leakage and 100% leakage are as
expected.
A well-fitting N95 mask may be better than an ill-fitting N99 mask. As the leakage rate increases, there is not much to choose between ill-fitting N95 and N99 masks, as can be seen from the graph"
In fact, in the Army, you are required to shave off your
beard because the face mask cannot protect you against poisonous gases or
viruses since contaminated air will through the spaces between facial hairs.
Face masks which really protect you do exist, but they are used for military or
medical applications.
The question is: what is the leakage rate, actually? This is
difficult to determine experimentally, but two papers [1,2] have tried to
determine the number.
Lai et al [1] used poly-disperse ultrafine particles to
challenge face masks worn by manikins:
“Performances of four
different types of face-mask fits, varying from ideal to normal wearing
practice, were also investigated. Under the pseudo-steady concentration
environment, face-mask protection was found to be 45%, while under expiratory
emissions, protection varied from 33 to 100% .”
Cherrie et al [2] report similar results:
“The masks’ filtration
efficiency was tested by drawing airborne diesel exhaust through a section of
the material and measuring the PM2.5 and black carbon (BC) concentrations
upstream and downstream of the filtering medium. Four masks were selected for
testing on volunteers. Volunteers were exposed to diesel exhaust inside an
experimental chamber while performing sedentary tasks and active tasks.”
“The mean per cent
penetration for each mask material ranged from 0.26% to 29%, depending on the
flow rate and mask material. In the volunteer tests, the average total inward
leakage (TIL) of BC ranged from 3% to 68% in the sedentary tests and from 7% to
66% in the active tests. Only one mask type tested showed an average TIL of
less than 10%, under both test conditions.
Conclusions: Many commercially available face masks may not
provide adequate protection, primarily due to poor facial fit. Our results
indicate that further attention should be given to mask design and providing
evidence based guidance to consumers.”
In other words, the
leakage rates can easily go as high as 50%...
The bottom-line is that
it may be better to use nasal filters [3,4] based on nanofibre technology by
IIT-D, but the ones recently developed also have different sizes, and are
stated to be equivalent to N95 masks. Since they have to be sized relative to
the size of your nostrils, the same caveats may apply…
References:
1. A.C.K.Lai et al J. Roy. Soc. Interface 9 (2012) 938–948
2. J.W.Cherrie…M.Loh Occup Environ Med 75 (2018) 446–452.
doi:10.1136/oemed-2017-104765
