Face masks help prevent people from getting the flu. But how much protection do they provide?

You might think the answer to this question would be well established. It’s not.

In fact, there is considerable uncertainty over how well face masks guard against influenza when people use them outside of hospitals and other health care settings. This has been a topic of discussion and debate in infectious disease circles since the 2009 H1N1 flu pandemic, also known as swine flu.

As the government noted in a document that provides guidance on the issue, “Very little information is available about the effectiveness of facemasks and respirators in controlling the spread of pandemic influenza in community settings.” This is also true of seasonal influenza — the kind that strikes every winter and that we are experiencing now, experts said.

Let’s jump to the bottom line for older people and caregivers before getting into the details. If someone is ill with the flu, coughing and sneezing and living with others, say an older spouse who is a bit frail, the United States Centers for Disease Control and Prevention recommends the use of a face mask “if available and tolerable” or a tissue to cover the nose and mouth.

If you are healthy and serving as a caregiver for someone who has the flu — say, an older person who is ill and at home — the C.D.C. recommends using a face mask or a respirator. (I’ll explain the difference between those items in just a bit.) But if you are a household member who is not in close contact with the sick person, keep at a distance and there is no need to use a face mask or respirator, the C.D.C. advises.

The recommendations are included in another document related to pandemic influenza — a flu caused by a new virus that circulates widely and ends up going global because people lack immunity. That is not a threat this year, but the H3N2 virus that is circulating widely is hitting many older adults especially hard. So the precautions are a good idea, even outside a pandemic situation, said Dr. Ed Septimus, a spokesman for the Infectious Diseases Society of America.

The key idea here is exposure, Dr. Septimus said. If you are a caregiver and intimately exposed to someone who is coughing, sneezing and has the flu, wearing a mask probably makes sense — as it does if you are the person with the flu doing the coughing and sneezing and a caregiver is nearby.

But the scientific evidence about how influenza is transmitted is not as strong as experts would like, said Dr. Carolyn Bridges, associate director of adult immunization at the C.D.C. It is generally accepted that the flu virus is transmitted through direct contact — when someone who is ill touches his or her nose and then a glass that he or she hands to someone else, for instance — and through large droplets that go flying through the air when a person coughs or sneezes. What is not known is the extent to which tiny aerosol particles are implicated in transmission.

Evidence suggests that these tiny particles may play a more important part than previously suspected. For example, a November 2010 study in the journal PLoS One found that 81 percent of flu patients sent viral material through air expelled by coughs, and 65 percent of the virus consisted of small particles that can be inhaled and lodge deeper in the lungs than large droplets.

That is a relevant finding when it comes to masks, which cover much of the face below the eyes but not tightly, letting air in through gaps around the nose and mouth. As the C.D.C.’s advisory noted, “Facemasks help stop droplets from being spread by the person wearing them. They also keep splashes or sprays from reaching the mouth and nose of the person wearing them. They are not designed to protect against breathing in the very small particle aerosols that may contain viruses.”

In other words, you will get some protection, but it is not clear how much. In most circumstances, “if you’re caring for a family member with influenza, I think a surgical mask is perfectly adequate,” said Dr. Carol McLay, an infection control consultant based in Lexington, Ky.

By contrast, respirators fit tightly over someone’s face and are made of materials that filter out small particles that carry the influenza virus. They are recommended for health care workers who are in intimate contact with patients and who have to perform activities like suctioning their lungs. So-called N95 respirators block at least 95 percent of small particles in tests, if properly fitted.

Training in how to use respirators is mandated in hospitals, but no such requirement applies outside, and consumers frequently put them on improperly. One study of respirator use in New Orleans after Hurricane Katrina, when mold was a problem, found that only 24 percent of users put them on the right way. Also, it can be hard to breathe when respirators are used, and this can affect people’s willingness to use them as recommended.

Unfortunately, research about the relative effectiveness of masks and respirators is not robust, and there is no guidance backed by scientific evidence available for consumers, Dr. Bridges said. Nor is there any clear way of assessing the relative merits of various products being sold to the public, which differ in design and materials used.

“Honestly, some of the ones I’ve seen are almost like a paper towel with straps,” Dr. McLay said. Her advice: go with name-brand items used by your local hospital.

Meanwhile, it is worth repeating: The single most important thing that older people and caregivers can do to prevent the flu is to be vaccinated, Dr. Bridges said. “It’s the best tool we have,” she said, noting that preventing flu also involves vigilant hand washing, using tissues or arms to block sneezing, and staying home when ill so people do not transmit the virus. And it is by no means too late to get a shot, whose cost Medicare will cover for older adults.

Source：The New York Time website