if your n95 mask has a valve, you could be spreading covid-19

dr. theresa tam, canada’s chief public health officer, advised that non-medical masks help prevent the spread of covid-19 by people who don’t know they have it.

the most effective masks — n95s — are in such short supply that dr. tam said in another briefing that officials are looking at whether or not they can be disinfected and reused for frontline health workers.

yet masks of varying kinds have been declared mandatory for the general public by the governments of austria , the czech republic and slovakia. and in many parts of asia there is social pressure to wear them.

“in asia, it’s more like a civic duty for people. people who don’t wear face masks are criticized,” says dr. elaine shuo feng, an epidemiologist and statistician at the oxford vaccine group in the u.k. who authored a paper on the rational use of face masks in the lancet last week. “in hong kong, face masks are recommended for anyone who uses public transportation or goes to crowded areas.”

n95 respirators: the gold standard for health workers

first things first: n95 is a designation, meaning they are approved for medical use by health canada . masks with that name, in varied shapes including cups, flatfold or duckbill, must be able to filter out at least 95 per cent of very small particles .

to do that, n95s require fabric with pores no more than 0.3 microns in diameter, or 300 nanometres. manufacturers use what’s called “melt-blown” fabric made from interlaced polypropylene fibres (think a cotton candy maze trapping particles as they travel) that are notoriously hard to produce .

however, all this hard work goes to waste if the mask is not fitted properly. n95 wearers often have to remove any jewellery and facial hair, and healthcare workers are fit-tested for n95 respirators before actually using them to make sure they are completely sealed against their faces — otherwise, viruses will still sneak in the sides. during fit tests, sweet or bitter solutions are sprayed to see if the wearer can taste them.

should you see n95s for sale, also note that they have a “best before date.” reuters recently reported on the millions of masks ontario had stockpiled — but were now expired .

lastly, not all n95 masks are created equal. masks with exhalation valves are not recommended for covid-19. n95s with valves are designed for construction workers to prevent dust and other particles from getting in, but it does not prevent an infected person from spreading droplets. masks with valves have good filtration for air coming in because they’re one-way and does not allow air in through the valve — outside air is forced to go through the mask itself (the filter). but the valve opens up when the user breathes out, which makes it easier for the user to breathe, but it doesn’t prevent their droplets from transmitting to others around them. in san francisco, masks with valves have been banned because it doesn’t stop source control and puts others at risk.

surgical masks: the next best thing

in many east asian countries, these loose-fitting face masks — made from fabric or polypropylene — have been in widespread use for many years. they offer protection against 80 per cent of tiny particles , and block out air pollution. given the challenges of fitting medical-grade face masks, a 2016 cmaj review found they were nearly as effective as n95s . (they are also a convenient way to mask identity; google any photo of the hong kong protests last year.)

although surgical masks are not designed protect the wearer — they are made to protect patients from any droplet contamination from a doctor during surgery — they are being used as a contingency tool even in hospitals. troubling for many healthcare workers, that means reusing masks that are meant for single use.

as of march 31, the canadian government had dedicated $2 billion to producing surgical masks in canada (as well as face shields, gowns, ventilators, test kits and swabs).

homemade: a last (and sustainable) resort

there is not a lot of evidence about the effectiveness of homemade masks. in a small 2013 study, researchers found masks made from cotton t-shirts (which had been tested to fit properly) were better than no protection but “ should only be considered as a last resort .”

that last resort has arrived in many jurisdictions, however: in toronto, michael garron hospital has made a plea for donations of as many as 1,000 homemade masks — every week . they even uploaded a  simple face mask sewing pattern that can made from bedsheets, t-shirts, or tea towels.

you’ll need elastics and a sewing machine for that pattern, but the new york times has also posted a pattern that can be h and-sewn .

can’t sew? unfortunately, there’s no substitute for an n95 with any home-sourced material, says dr. jason kindrachuk, a medical microbiologist and the canada research chair in molecular pathogenesis of emerging and re-emerging viruses at the university of manitoba.

the downside of face masks

a dirty mask might even become the source of an infection. “when someone gets home, how are they taking the mask off without touching their face? how are they ultimately disinfecting them and sanitizing them?” says kindrachuk.

the bottom line

no matter what type, masks are not a catch-all, says kindrachuk. he stresses that physical distancing is still the absolute primary point the general population needs to be doing — since healthcare workers can’t.