Dry air might boost flu transmission

For flu germs, warm muggy weather is simply too close for comfort. It saps their vigor, cutting the period in which they remain infectious, according to a pair of new studies.

Although it remains a mystery why flu viruses are so sensitive to heat and humidity, both papers highlight conditions that can aid and abet infection — factors that might be thwarted by controlling aspects of the indoor environment.

One laboratory study measured how long highly pathogenic avian influenza remains infectious. Scientists applied known quantities of virus to a variety of surfaces that might be found outdoors on a chicken farm. Poultry have been implicated as a source of this particularly deadly type of bird flu.

The researchers kept samples either at room temperature or in a chilly environment more typical of a kitchen refrigerator. Relative humidity also varied from a low of between 15 and 46 percent to highs exceeding 90 percent. These conditions “are not unheard of in our part of the country,” explains study coauthor Joseph Wood of the Environmental Protection Agency in Research Triangle Park, N.C.

Virus particles didn’t remain infectious for more than about a day at room temperature and high humidity, Wood’s team reports in a paper posted online September 3 in Environmental Science & Technology. When the researchers dropped the temperature and humidity, things changed abruptly. The virus remained infectious through day four on feces, and through the end of the study — 13 days — on glass, metal and soil. In fact, the environmental engineer notes, except on feces, “There was hardly any loss of the virus. It’s a bit disconcerting.”

Some samples were exposed to the sun’s ultraviolet wavelengths. On nonporous surfaces, those viruses died within a day. But UV-exposed viruses persisted two to four days in feces and soil. “Even on a sunny day, you may not have much impact from the UV light,” Wood says, probably because the virus can get buried in crevices where it’s protected from UV.

A second study simulated the impacts of a room humidifier on the persistence of virus particles in the air. Starting germ loads were based on numbers of virus particles that team members had previously measured being spewed into the air as flu sufferers coughed, talked and just breathed. Temperature and moisture conditions were chosen to simulate winter, when flu season picks up and indoor humidity plummets.

For homes with forced-air systems, virus counts in a room’s air dropped by almost 20 percent when a humidifier was used, the model indicates — and by roughly one-third in rooms where heating was supplied by radiators. (The difference: Fans in the homes with furnaces mix air more, diluting the moisture supplied by a humidifier into the air of other rooms.) The findings appear in a paper posted online September 3 in Environmental Health.

This analysis was among the first to base moisture comparisons on absolute rather than relative humidity. Relative humidity compares the mass of water in air to its saturation point — the value where moisture would begin to rain out or cause fog, explains Jeffrey Shaman of Oregon State University in Corvallis. But being, well, relative, this value changes with temperature. At 50 percent relative humidity, 40° Fahrenheit air contains only a quarter of the water vapor at air at 80° F. But absolute humidity quantifies the mass of water vapor in air, irrespective of temperature.

Studies since the 1940s have indicated that temperature and humidity play a role in the persistence of flu virus. “But the relationships were not strong,” Shaman says. “They weren’t always consistent.”

In a 2009 paper he and Melvin Kohn of the Oregon Department of Health showed that adjusting the moisture values reported in earlier flu-virus analyses to absolute humidity “seems to explain almost all of the changes in survival of the airborne virus that you see in the lab.” Previous relative humidity measurements had explained only 12 percent of the variation in rates of flu transmission and 36 percent of the variability seen in virus survival, Shaman points out.

“Humidifiers may be an important tool to reduce the survival of influenza virus in the home,” authors conclude in the new Environmental Health paper.

Not so fast, says Peter Palese of the Mount Sinai Medical Center in New York City. Knocking out even a significant share of viruses in the air — like one-third — might leave enough behind to cause sickness. And if people aren’t careful, he warns, over-humidifying a building might create a new problem: mold growth.

But Shaman, an atmospheric scientist who studies the effects of moisture and temperature on infectious disease, believes it’s possible that knocking out just 30 percent of airborne flu particles might prove useful.

For a flu outbreak to sustain itself, each infected person must, on average, sicken more than one additional person. Typically, the number averages around 1.4 additional infections. But it’s possible that cutting the number of infectious virus particles indoors by 20 or 30 percent might reduce the average number of people sickened by a flu victim to less than one. “And then,” Shaman says, the outbreak “may die out.”

“It comes down to a numbers game,” he says. It also points to the importance of follow-up field tests to quantify the extent to which relatively modest changes in virus particles’ infectious lifespan may affect the transmission of disease.