There’s a new trend in the world of COVID-19 safety that you may have already spotted in the wild: The double mask.

In light of new research on how exactly masks protect us and what makes them most effective, endorsements for the double mask are coming out from the Centers for Disease Control and Prevention and local governments. Some federal courthouses in New York are even requiring anyone who doesn’t have a high-grade N95 respirator mask (or the Chinese KN95 equivalent) to double up on their masks.

“Two is better than one,” Dr. David Chokshi, the city’s health commissioner, said during Mayor Bill de Blasio's morning presser on Thursday. He then proceeded to demonstrate how to properly double mask, placing a cloth mask over a disposable surgical one. (And in case you're wondering, surgical masks should only be worn once).

But, like many others recommending the double mask, Chokshi did not elaborate on why it’s effective, what settings it should be used in, or what alternatives exist for those who are double-mask-averse.

To answer these questions and more, we spoke with Dr. Monica Gandhi, an infectious disease expert and professor of medicine at the University of California at San Francisco. Gandhi recently co-authored a paper summarizing the evidence for face masks from the perspective of physics and epidemiology. Her responses have been condensed and lightly edited for clarity.

How effective is double masking in protecting people from COVID-19?

I wrote an article with physical scientist Linsey Marr, and she fit lots of different masks on mannequins to figure out what blocks the virus most effectively. We came up with a double mask, but not just any double mask.

The best option is a disposable surgical mask plus a cloth mask on top. That combination really looked like it blocked as much as an N95 mask, and that’s as good as you can get because the N95 is the gold standard of blocking tiny airborne particles.

Are there any alternatives to double masking besides the N95?

We also looked at another type of mask, which is a cloth mask with a pocket for a filter inside. It’s kind of the same idea as a surgical mask plus a cloth mask. The filter blocks the virus electrostatically, so with static electricity. That’s also how a surgical mask works. And then, the cloth blocks it physically.

Linsey Marr ran a simulation, and it is also as good as an N95.

That’s what seems to be so effective—increasing filtration and using two different types of materials. In fact, layering one cloth mask on top of another does nothing. It just makes it harder to talk.

How often should you switch out the filter in that type of mask?

You should change it out every time you wash the mask, which should be every other day.

A lot of people are now talking about the proper fit for a mask to prevent airborne coronavirus from slipping through gaps. What does that look like?

One reason to double mask is to tamp the surgical mask closer to your face. Whatever type of mask you use, it has to fit so your cheeks are covered. You can’t have that gap between your cheeks and the air.

The CDC also came up with the idea of the “Knot and Tuck” [for masks that are not adjustable]. They recommend knotting the ear loops on a mask then tucking in the sides. The problem with that is it improves fit but doesn’t have as good efficacy for filtering particles.

An upcoming paper by Monica Gandhi and her collaborators introduces the concept of balancing masking with ventilation and social distancing to curb COVID-19 as the vaccination campaign ramps up.

In what scenarios should people consider double masking?

It does not have to be everywhere. This is really for indoors, for adults, and usually in places that are crowded. I would not recommend this for outside. Even when you’re outside around others you have the benefit of ventilation.

We have a paper coming out introducing the concept of the complementary triangle of non-pharmaceutical interventions; that is, public health practices that don’t involve medication.

One corner of the triangle is masks, another corner is social distancing, and the third corner is ventilation. If you have less of one, you need more of another.

So, for example, if you’re in a classroom where people are less than six feet apart, you can double up on your masks.

Could people have trouble breathing when using these extra layers?

It certainly doesn’t cut down on oxygen in any way. That’s been tested. But it is true that people can feel like a double mask is too much and feel like it may muffle their voice. That can make people pull it down when they talk. We don’t want to decrease acceptability or steer anyone away from masking. If anything, we want to increase masking.

What is your ultimate mask recommendation?

If I was to give a resounding recommendation, I would get away from this word double masking and I would recommend the cloth mask with a filter. That’s the thinnest option you can get and still have the same level of protection. I don’t think there’s any reason to wear something else.

Editor's note: This story was updated to add a response on how often people should change mask filters.