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posted: 3/25/2013 6:00 AM

Flu may spread more readily by breathing than sneezing germs

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By Lee Bowman
Scripps Howard News Service

You never want to sit next to that guy during flu season.

You know, the sniveling, sneezing, hacking sort who occupies the next cubicle or the neighboring seat on the plane. It's just a matter of time before he -- or she -- infects you with whatever crud's going around.

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Except, a new study suggests, it's not the tissue-groping, say-it-don't-spray-it types that are most likely to get you. All a sickly person really has to do is breathe around you.

Researchers at the University of Maryland tested the exhaled breath of 38 flu patients and checked both large droplets and fine airborne particles for flu virus. It turned out that the fine airborne particles -- released by normal breathing -- contained nearly nine times more virus than larger droplet particles released when a person coughs and sneezes. The study was published in the journal PLOS Pathogens.

The team -- led by Dr. Donald Milton, director of the Maryland Institute of Applied Environmental Health -- used a machine dubbed "The Gesundheit II" to collect samples from each volunteer for 30 minutes. Some people sitting at the machine released undetectable levels of virus; others put out more than 100,000 viruses during the test.

The researchers also tested some of the patients while they wore paper surgical masks, recommended by the U.S. Centers for Disease Control and Prevention as a way to prevent someone with the flu from spreading the virus. In the study, the masks reduced the amount of virus shed by 3.4 times overall.

Fortunately, although flu is still around across most of the nation, the number of cases has declined each week since the seasonal outbreak peaked in December.

It's also good to know that if you must share a space with someone who has the flu, you're better off if the room is somewhat humid.

Researchers at the CDC's National Institute for Occupational Safety and Health reported in late February that the viral load in a dry room was more than five times greater than the amounts found in a room with greater humidity. Their report appeared in the journal PLOS One.

An hour after virus particles were released in a room with a relative humidity of 23 percent or less (typical in many spaces during a winter heating season) up to 77 percent were still infectious. But when the humidity was increased to 43 percent, only about 14 percent of the virus particles were capable of infecting. Most of the inactivation occurred within 15 minutes of the virus being released in the more humid room.

Experts say the optimal humidity range is between 30 percent and 50 percent to avoid health problems with breathing and nosebleeds during the winter.

On a larger scale, air humidity and temperature seem to largely account for how flu spreads in different climates. A third study in PLOS Pathogens, led by researchers at the National Institutes of Health, used models to measure the flu-outbreak patterns at various sites around the globe.

They showed that, in temperate regions, flu was more common about a month after a spell of particularly dry air. In areas with relatively high temperatures and humidity -- like the tropics and subtropics -- flu peaks in the most humid and rainy months.

While it's fairly certain that cold temperatures and low humidity keep people indoors amid easily spread viruses in wintry places, the seasonal flux of flu in the tropics is not so well-understood.

The NIH researchers note that one theory for tropical flu is that people spend more time indoors together during the rainy season and that this increases transmission, even with higher humidity. But so far, there is little actual data to show this happens, said Cecile Viboud, who headed the study.

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