By Sandee LaMotte, CNN

(CNN) -- It's official -- wearing a mask not only protects others from your expelled respiratory droplets, it protects you as well, according to new guidance from the US Centers for Disease Control and Prevention.

"The evidence is piling up that masks work in terms of driving down the risk for everyone," said Harvard environmental health researcher Joseph Gardner Allen, who directs the Healthy Buildings program at the Harvard T.H. Chan School of Public Health.

But what type of mask will offer you and others the best protection from any novel coronavirus circulating in the air?

"You want to have a minimum two-ply mask, preferably a three-ply mask," said Allen, who has focused his career on "sick buildings" and how they affect worker performance and productivity.

Stay away from bandanas and gaiter masks unless that's all that's available. A recent study found both types to be the least effective in terms of protection.

"In addition to level of filtration, we have to pay attention to fit," Allen continued. "You want the mask to go over the bridge of the nose, below the chin and be flush on the face, resting along the skin. You want your breath going through the filter media and not escaping out the sides."

No N95 masks or exhalation values, please

This is important: Do not buy N95 masks for your personal use, the CDC says. While those are the most effective -- filtering out 95% of all particles -- they are considered critical supplies and must continue to be reserved for health care workers and other medical first responders.

In addition, N95 masks must be fitted to adhere to the unique contours of each doctor or nurse's face. You and your loved ones don't have access to that sort of expert fitting.

And be especially wary of the look-alike N95-type masks being sold at major retail distributors, Dr. Marybeth Sexton, an assistant professor of medicine in the division of infectious diseases at Emory University in Atlanta, told CNN recently.

"Some of those community-use N95 masks have exhalation valves in them," she said. "They do make them more comfortable to wear, but you're not protecting the people around you -- it's putting your airflow right out in the environment.

"It may actually make things worse because it concentrates your breath into that valve, allowing it to come through with some force and the droplets may travel a little farther. So we strongly recommend that people don't wear a mask that has an exhalation valve."

A tightly woven cloth mask

Cloth masks with high thread counts seem to be an excellent choice, according to the new guidance from the CDC.

Look for a tight weave of 100% cotton, according to studies. Use the light test to check the weave: If you can easily see the outline of the individual fibers when you hold up the mask to the light, it's not likely to be effective.

You want as many layers as possible without sacrificing breathability -- if you can't breathe though it, you won't keep the mask on your face. Two- and three-layer masks appear to do the trick for most people.

According to the CDC, "multiple layers of cloth with higher thread counts have demonstrated superior performance compared to single layers of cloth with lower thread counts, in some cases filtering nearly 50% of fine particles less than 1 micron."

That's good news -- studies have detected SARS‐CoV‐2 in aerosols between 1 and 4 microns.

In addition, studies have found that multilayer cloth masks can block between 50% and 80% of fine droplets and particles, and "limit the forward spread of those that are not captured," the CDC said, "with cloth masks in some studies performing on par with surgical masks as barriers for source control."

Enhancements

The CDC says that polypropylene, one of the most commonly produced plastics in the world, may "enhance filtering effectiveness" because it creates a triboelectric charge -- or in simple terms, static cling.

That electrical static traps both your outgoing respiration and any droplets headed your way from others. Because cotton is a more comfortable fabric on the skin, polypropylene is often used as filters that can be placed inside of a two- or three-ply mask.

Washing kills the electrical charge, but don't worry. A brisk rub between your fingers should bring back that "clingy" charge.

A very breathable option, according to the CDC, is silk, which "may help repel moist droplets, and reduce fabric wetting and thus maintain breathability and comfort."

A study published in September examined the ability of cotton, polyester and silk to repeal moisture when used in masks or as mask inserts.

"We found that silk face coverings repelled droplets in spray tests as well as disposable single-use surgical masks," the authors wrote, adding that silk masks "can be more breathable than other fabrics that trap humidity, and are re-useable via cleaning."

That brings up an important point: To avoid trapping germs that might irritate your face or reduce the mask's effectiveness, reusable masks should be washed daily with soap and hot water. Don't wear the mask again until it's completely dry -- it's harder to breathe though wet fabric.

"If you use a filter in your mask, be sure to change it regularly because it can clog. You can tell if it gives you a sensation that's a little harder to breathe," said Emory's Sexton.

Mask etiquette

Even the most protective mask will fail if you wear it wrong. Full face coverage is necessary at all times, and that means no letting it slip under your nose, please.

Noses are important. Droplets from the nose are typically smaller than from the mouth, but that also means they can remain in the air longer. High-speed camera studies show sneezing can spread respiratory droplets 7 to 8 meters -- that's more than 26 feet, well over the height of a giraffe (20 feet).

If you're finding it difficult to breathe with your nose covered, find a better-fitting mask.

It's also important to wear the mask everywhere you go -- even in the car if you are with people outside your bubble.

"If you're going to ride an Uber taxi, you absolutely want to have a mask on and the driver should have a mask on," Allen said. "And you want to put down the windows, even if it's inclement weather. Just opening the windows a little bit will really help."

Always carry hand sanitizer

Regardless of what type of mask you wear, you will have to take it on and off to eat, or adjust it in some fashion while it's on your face. That makes hand sanitizer a mask's best friend, experts say.

"Masks slip all the time, especially when you're talking," said Dr. Henry Wu, an associate professor of infectious diseases at Emory University School of Medicine in Atlanta.