Humidity can weaken viruses

A higher humidity of the room air protects against viral infection

28/02/2013

Scientists have investigated how room temperature and humidity can help reduce the risk of infection from influenza viruses in doctors' offices. In a study, they found that particularly dry indoor air increases the risk of infection from viral diseases. From a humidity of 42 percent, the risk of infection dropped rapidly. The examination results were in the journal „Plos ONE“ published.

Dry room temperature increases the ability to infect
The most common transmission path of flu viruses is the droplet infection. By inhaling air contaminated with air, pathogens enter the human organism. A recent study by the National Institute for Occupational Safety and Health in Morgantown examined the contagion conditions in confined spaces a little more closely. The results initially showed that dry indoor air, which is usually caused by heating, greatly increases the risk of infection. „Dry air prevents inactivation of the viruses“, study author John Noti. On the other hand, if the relative humidity was 42% or more, 20% of the influenza viruses were contagious after just over an hour. „A relatively high humidity in closed rooms in clinics or medical practices could significantly reduce the contagion conditions“, sums up the research team.

Noti stresses that the „Results show that the strongest protective effect of increased humidity already occurs in the first 15 minutes after the viruses have been released into the air by coughing“. In addition, the volume of the infection droplets produced during coughing play an important role. Thus, viruses remain infectious in small droplets longer than those that migrate in larger ones.

High humidity lowered the risk of infection
For the study, the scientists simulated an investigation situation. Doctors and medical assistants are exposed to a high risk of infection when dealing with infected patients. The researchers set up two human pupae at a distance of two meters. The one doll „coughing“ every 5 minutes a suspension with flu viruses into the environment. This interval corresponds approximately to the secretion of infectious aerosols when coughing a flu patient. The other human dummy picked up air from the environment. This symbolized the doctor or a nurse. Now the researchers took samples from different parts of the room from the air. The samples were divided according to the sizes of the particles. In addition, the physicians in the laboratory examined the total amount of viruses and the proportion of viable viruses with contagion. The relative humidity varied between 7 and 73 percent in the test runs. The room temperature remained almost identical at 20 degrees Celsius.

If the humidity was below 24 percent, the virus's susceptibility to infection dropped only minimally, reaching between 70 and 77 percent after 60 minutes. However, if a relative humidity of over 42 percent was reached, after 15 minutes, a large proportion of viruses was no longer active and the infectivity was after 60 minutes only 14 to 23 percent. „Thereafter, a decline occurred up to 5 hours after coughing, but at 45% relative humidity remained statistically no different from 20% regardless of the aerosol fraction.“

It remains unknown how the mechanism of inactivity works. In addition, the present research could not provide an answer. „It is difficult to retrofit examining and waiting rooms so that high humidity can be achieved“. According to the researchers, this aspect should be taken into account in rebuilding practices to minimize the risk of infection. (Sb)

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