As the omicron variant of COVID-19 surges across the nation and here in Massachusetts, disrupting the start of school, many people are wondering how they can keep themselves and their families safe. Dr. Joseph Allen, associate professor of exposure assessment science and the director of the Healthy Buildings program at Harvard's School T.H. Chan School of Public Health, has studied personal protective equipment extensively and has advocated for mask wearing since the beginning of the pandemic.

He joined Jim Braude on Greater Boston to break down the latest omicron news and the important things to know about different types of masks.

“The value of masks depends on the two F’s: filtration and fit,” he said. Here’s what he said about three types of masks, from most effective to least.

Tier one: N95, KF94 and KN95 respirators

An N95 mask sits on a table in front of two boxes.
3M brand N95 particulate respirators are displayed on a table on July 28, 2020 in San Anselmo, California.
Justin Sullivan Getty Images

Allen says that if you can get your hands on one, he recommends the N95. “Choose an N95 if you can find it. I also like the KF94 out of Korea,” he said. He personally wears the KF94.

The difference between these respirators mainly relates to testing and approval, not the effectiveness of the products. N95s are approved by the United States’ National Institute for Occupational Safety and Health. You can search for NIOSH-approved respirators and view counterfeit versions on the CDC’s website.

KN95 respirators are made in China and tested to meet international standards, and KF94s are from a certified manufacturer in Korea.

Although all of these disposable options are a bit pricier at around $1 apiece, Allen said they can be re-used by setting it aside and letting it air out.

All three are made from material with an electrostatic charge, which prevents you from inhaling virus particles.

If you’re re-using an N95 or KF94, keep an eye on the fit. “The filtration efficiency is going to stay pretty good,” he said about reuse. “Once the ear loops start wearing out, or the fit starts to degrade, that’s where I think you have a problem.”

The context of how you’re using it also matters. “If I’m eight hours at the hospital then something has to be different,” he said. “If I’m using it occasionally when I go to the grocery store, I will re-use a KF94 for a long time.”

If you use a KN95 mask, Allen says, be careful of counterfeits. Millions were brought to the United States at the start of the pandemic and are still in circulation. About 60% of KN95 respirators in the United States are counterfeit, according to the CDC. You can search for specific models’ filtration efficiency and view lab results in a database maintained by the National Personal Protective Technology Laboratory.

Be wary of red flags. A NIOSH Task Number (TN) or an FDA registration number on the packaging is a sign that it is a counterfeit product, as neither of these indicate approval. Ear loops on respirators are also a warning sign, as NIOSH testing shows it’s difficult for users to get a proper fit with this design. Look for two straps that go around a wearer’s head.

Tier two: Surgical masks or double masking

Protective Face Masks
A pile of protective face masks in Hong Kong, China.
Paul Yeung Bloomberg

If you can’t get your hands on a higher-grade mask, the simple blue surgical masks are always a good option, Allen says. They are also made out of material with an electrostatic charge, but have a looser fit than an N95.

Surgical masks can be even more effective when you layer a cloth mask over your surgical mask to improve the fit. Double masking became popular last year when higher grades masks were harder to find.

“You want to concentrate on fit” for any mask situation, Allen says. Double masking can make sure your nose is fully covered and fill in gaps on the side of your face.

If you’d like to do more research about specific masks, ASTM International developed a new standard with input from NIOSH to test for filtration efficiency and breathability. You can find a list of tested products on the CDC’s website.

Tier three: Three-layer cloth mask

Cloth masks ( handmade ) for the pandemic in different models and colors
As assortment of cloth face masks.
Blanchi Costela Getty Images

Cloth masks that you can buy at most stores are not as effective against the more transmissible omicron variant, but Allen says they are still important at this stage of the pandemic. The setting is important to consider, he says.

“Let’s say we have a 50% effective cloth mask, you and I both wear one, the combined efficiency going through two filters is 75%. That’s pretty good,” he told Braude. “Two surgical masks is 70%, combined efficiency is 91%.”

Context matters. “If I’m in a high-risk setting, I’m in an ICU, I’d want an N95 that’s well-fitted,” he said. “If I’m with my friends who are fully vaccinated — I actually don’t think we need to wear a mask — but if you’re in a low-risk situation, I think a three-layer cloth mask would be OK, or a surgical mask.”

WATCH: Which masks to wear when as COVID cases spike in Massachusetts [mask discussion starts around 8:30]