Friday, January 18, 2013

Why is flu more common in winter? The answer can help protect you

 How do these sneeze droplets differ in winter? Read on!
Photo courtesy of 

This year's flu season is expected to be one of the worst the US has seen in 10 years, according to the the Centers for Disease Control and Prevention. Said Dr. Joseph Bresee, "There's flu all over the country right now...there's widespread disease in most states and high levels of disease in most states...the country has lots of flu."

Most of the US is nearing a "moderate to severe" flu season, and it has not yet peaked.

Flu outbreaks are worse in cold weather

All flu outbreaks tend to be worse in winter. In the Northern Hemisphere flu season is November to March. But seasons are reversed in the Southern Hemisphere - the cool weather of autumn starts in May, and flu season there is May to September.

Why is winter worse?

I had always heard that flu spreads in winter because we're all cooped up together indoors. But that's not it, apparently.

Dr. Peter Palese of Mount Sinai School of Medicine and his colleagues have studied the transmission of flu between guinea pigs, which can get infected with human flu viruses. Palese's results indicate that spending more time together inside is not a factor in the timing of flu outbreaks.

Sneezed droplets have longer hang time in winter air

Palese says flu viruses are more stable (and last longer, presumably) in colder, drier conditions. What's more, the droplets of fluid that people spew into the air with sneezes and coughs are much smaller at lower temperatures and lower humidity, so the droplets carry much farther and stay suspended in the air longer. The smaller droplets can be inhaled more deeply into the respiratory passages and lungs, Palese says. The opposite is true at warmer temperatures and higher humidity, when the droplets become much bigger and sink to the floor, reports Palese. These influences are so pronounced that "at 75 to 80 degrees, we don't see any transmission" of flu viruses.

In cold dry air, inhaled particles cling to membranes

Palese and his colleagues also report that colder, drier air makes mammals' respiratory tracts more vulnerable to infection by airborne viruses. In cold air, "the mucous is much more viscous," Palese explains. Sticky mucus clogs up the cilia, or tiny hairs, that normally move in waves to expel virus-laden particles from the breathing passages. So in cold weather, inhaled flu viruses tend to stay where they land, reproducing and infecting the unfortunate victim.

What can you do?

If you have the flu, stay home. When you sneeze or cough, cover your nose and mouth. If you don't have a cloth or tissue, use the inside of your elbow. If you're touching doorknobs or anything else an infected person may have touched, wash your hands frequently and avoid touching your nose, mouth, or eyes.

If you don't have the flu:
1) Get a flu shot. It's not too late.
2) Try to avoid breathing the air within 6 feet of an infected person who's sneezing and coughing.
3) If you're touching doorknobs or anything else an infected person may have touched, wash your hands frequently and avoid touching your nose, mouth, or eyes.

Sources for Palese's research and for flu prevention

Richard Knox. Flu heads south for the winter. NPR Morning Edition.
How Flu Spreads. The Centers for Disease Control

1 comment:

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