There is more science to masks than your undies, handkerchief, or the niqab, burqa, hijab or chador. Efficacy of masks is measured by its fluid resistance, oil resistance, breathability, and filtration efficiency.
Fluid resistance is a measure of a mask's ability to block liquid penetration, examples are blood, saliva or respiratory droplets. It is to prevent infectious fluids from penetrating the mask. This is essential for surgeons, dentists and healthcare workers in high-exposure environments.
Oil resistance protects against oil-based aerosols and used in industrial settings for protection against oil mist or fumes.
Breathability is the resistance to inhaling and exhaling, that is, how hard the wearer has to pull in the air or to breathe out, through the mask. This resistance is positively co-related to the filtration of the mask. The higher the resistance, the better is the filtration, and the lower the resistance, the lower the filtration. That is, there is a trade-off between breathability and protection. The easier to breathe, the less protection. Breathability is measured by 2 units of pressure - (1) Pa = Pascal, and (2) mm H₂O (mm of water column).
Filtration efficiency refers to the capability of the mask to entrap microscopic particles including bacteria and viruses.
The higher end masks requiring fluid and oil resistance are for use in healthcare and industrial environments. Masks vary in terms of protection, materials and standards. We are concerned here only with the ordinary disposable non-surgical masks worn by the public.
First, a sense of size:
1 meter = 1,000,000,000 nanometers (nm)
1 meter = 1,000,000 micrometer (µm)
1 µm = 1,000 nm
For comparison, a human hair is about 50-100 µm thick.
Second, know the enemy:
Bacteria are much larger than viruses, typically ranging from 200nm to several µm. The smallest bacteria is Mycoplasma genetalium about 200 nm to the largest Thiomargarita namibiensis about 750,000 nm which is visible to the naked eye. This is a recently discovered sulphur-eating monster in the oxygen-free ocean bed off Namibia.
Viruses are much smaller than bacteria, typically from 20nm to 300 nm. The smallest virus is Porcine circovirus about 17 nm, and the largest is Pandoravirus about 1,500 nm.
The flu virus is 80 nm to 120 nm.
The SARS-Cov2 (Covid-19 virus) is about 50 nm to 140 nm
There is no international standard for masks. The American standard is ASTM F2100 set by NIOSH (National Institute for Occupational Safety and Health). Most countries recognise the US standard and they have their own certified masks standards with equivalence to NIOSH.
NIOSH has 5 filtration efficiency standards:
(The standard is set in relation to particle size of ≤0.3 µm, ie 300 nm)
N95 with ≤95%
N99 with ≤99%
N100 with ≤99.97%
R95 with ≤95%
P95 with ≤95%
"N" means the mask is non-oil resistance; "R" somewhat resistant to oil; "P" strongly resistant to oil.
≤95% means the mask can block 95% of particles the size of 300 nm
No mask is 100% because there may be tiny gaps if not properly sealed.
Oh no, standard filtration criteria based on 300 nm is too big for SARS-Cov2 viruses:
The SARS-Cov2 viruses are smaller than the filtration standard, but there is no need for alarm. Viruses have no independent mobility. They can't fly or crawl through masks. Viruses are transported in moisture droplets which are large.
Size of droplets:
From breathing - 0.1 to 5 µm. These are mostly aerosols, can stay suspended.
From talking - 5 to 50 µm
From coughing - 5 to 100+ µm
From sneezing - >100 µm
Smaller droplets and aerosols can stay airborne for hours and travel long distances. That's why masks are needed for viruses like Covid-19, tuberculosis and measles.
Most droplets are large so will be captured by the filtration layer of masks. But some droplets and aerosols are smaller than 0.3 µm or 300 nm. Don't worry, rated masks have a plan B.
N95 rated general use face masks (non-surgical):
NIOSH N95 masks have filtration efficiency of ≤95% , exhalation resistance of ≤245 Pa (25 mm H₂O) and inhalation resistance of ≤343 Pa (35 mm H₂O).
The difference between NIOSH and other equivalent rated masks is:
N95 have 4 or 5 layers:
An outer layer which is water-resistant, blocks large particles.
An electrostatic filter layer which captures fine particles.
An inner layer of soft comfortable material which absorbs moisture of wearer.
NIOSH products are rigorously tested for filtration, breathability, and fit. Certified products carry NIOSH name and logo, approval number (TC-84A-xxxxxx) manufacturer's name, and other info.
Other Non-NIOSH N95 products are not independently tested. They are checked by manufacturers themselves.
Electrostatic filter layer
Bacteria and viruses can be neutral, positively or negatively charged, depending on its PH, surface composition and environment. Most bacteria and viruses are negatively charged. Negatively charged means its molecules have more electrons than protons, positive-charged means more protons. SARS-Cov2 and flu viruses are negatively charged. The aerosols and respiratory droplets from breathing, talking, coughing and sneezing carry a slight negative charge.
N95 masks have an electret filter which is made of charged polypropylene microfibres. These fibres contain both positively and negatively charged zones, allowing them to capture particles of all charges.
This I know well. I was once with a company doing heat-treatment and powder-coating services in Malaysia. When a spray gun is directed onto a metallic product, the powder sticks to the substrate because of electrostatics. Metallic substrates are neutral and the powder is positively charged. A client had a problem with a particular part of a product where powder coating was unable to achieve their minimum paint thickness specification. I met up with the Singapore paint manufacturer to solve the problem by reducing to a milder positive charge on the powder. That way, when sprayed, less powder will stick to the product substrate thereby enabling a thinner coat.
With electrostatic filters, if the viruses, aerosols and droplets that manage to pass through the outer layer of the mask, they will stick to the electrostatic layer. Thus a higher level of filtration protection is provided.
The big whopper with facial masks:
The above explainer I am sure gives more confidence to those who believe in the use of facial masks. But here's the big problem. Surgical masks are classified as medical device and the importation is controlled by the Health Science Authority. Importers are licenced and product sources are identified helping to make Singapore safe from counterfeit products.
Unfortunately, non-surgical face masks are not classified as medical devices, thus importation is not regulated. Bottom line is, the facial mask you put on could well be made from recycled materials of undies.
Most of the masks out there are unfortunately not certified rated. More probably products of some village industry .
Promoting, or even forcing the public to put on masks which have not met desired standards may be even more dangerous than not wearing it as it gives the wearer a false sense of security to unknowingly take on higher risk exposures.
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