The US Centres for Disease Control and Prevention (CDC) has said Americans should wear the most protective mask they can, but stopped short of recommending an N95 or similar face covering.
Here are some facts to consider when choosing a mask:
These masks and their international counterparts known as KN95s and KF94s are often made of multiple layers of polypropylene, a synthetic fiber. They are designed to achieve a very snug facial fit, with straps that go around the back of the head and edges that form a tight seal around the nose and mouth.
N95 respirators worn correctly are designed to filter out at least 95 per cent of particulate matter in the air, preventing anything larger than .3 microns from passing through.
KN95s and KF94s are certified in China and South Korea, respectively, and offer similar protection to N95 masks. KF stands for "Korean filter" and indicates 94 per cent filtration.
"The best masks are some version of N95," said Eric Toner, senior scientist of environmental health and engineering at Bloomberg School of Public Health, Johns Hopkins University. "N95s, KN95s and KF94s are functionally equivalent."
Masks with an exhaust valve do not prevent the spread of the virus to other people.
The CDC lists manufacturers of authorised N95 masks on its agency website. Masks should have a printed logo of the National Institute for Occupational Safety and Health (NIOSH) and an approval number.
The CDC has warned that about 60 per cent of KN95 respirators it tested in 2020 and 2021 are below standards.
With the Omicron variant driving Covid-19 cases so high, experts said a better mask will help protect against transmission.
The CDC said a NIOSH-approved N95 provides the most protection. Well-fitting surgical masks and KN95 masks offer the next best protection, followed by cloth masks with multiple layers. Loosely woven cloth masks are the least effective but can provide an additional layer of protection when worn over a surgical mask.
Some cloth masks have built-in pockets for a filter that blocks small particles, but these are not as effective and data on their use is limited.
The CDC said masks are meant for single-use but can be used more than once when there is a shortage. The agency says N95s should not be used more than five times.
Dr Gregory Poland, infectious disease expert at the Mayo Clinic, said when a mask becomes wet from exhalation or sweat, its efficacy decreases and advised rotating masks by day.
The CDC recommends people consider wearing an N95 when caring for someone with Covid-19, if at risk for serious illness or in a high risk job, when riding on public transportation for an extended time, in crowded indoors spaces or outdoors if not up to date on vaccinations.
The CDC recommends that a surgical mask under a cloth mask can offer increased protection if using an N95 is not possible.
"The best possible protection is being vaccinated and boosted, wearing an N95 or KN95. If that's not possible, double the surgical mask," Poland said.
"If that's not possible, a surgical or cloth mask with a face shield. If that's not possible, then as many layers of a cloth mask as you can wear," Poland continued, adding, "If that's not possible, then you're just playing Russian roulette."
