Humidity can weaken viruses

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A higher level of humidity in the indoor air protects against viral infection

Scientists have investigated how room temperature and humidity can help to reduce the risk of infection from flu viruses in doctor's offices. As part of a study, they found that particularly dry indoor air increases the risk of infection from viral diseases. From an air humidity of 42 percent, the risk of infection dropped rapidly. The results of the investigation were published in the specialist journal "Plos ONE".

Dry room temperature increases the ability to infect The most common transmission pathway for flu viruses is droplet infection. By breathing in air contaminated with viruses, pathogens enter the human body. A recent study by the National Institute for Occupational Safety and Health in Morgantown has now examined the contagion conditions in closed rooms more closely. The results initially showed that dry indoor air, as is usually caused by heating, greatly increases the risk of infection. "Dry air prevents the viruses from being inactivated," says study author John Noti. In contrast, if the relative humidity was 42 or more percent, 20 percent of the influenza viruses were still contagious after just over an hour. “A relatively high humidity in closed rooms in clinics or doctor's offices could significantly reduce the contagion conditions,” summarizes the research team.

Noti emphasizes that the "results show that the strongest protective effect of increased air humidity occurs already in the first 15 minutes after the virus has got into the air through a cough". In addition, the volume of the infection droplets generated when coughing plays an important role. In this way, viruses remain infectious in small droplets longer than those that migrate in larger ones.

High air humidity reduces the risk of infection. For the study, the scientists simulated an examination situation. In this, doctors and medical assistants are exposed to a high risk of infection when dealing with infected patients. The researchers built two human puppets two meters apart. One doll "coughed" a suspension of flu viruses into the environment every 5 minutes. This interval roughly corresponds to the secretion of infectious aerosols when a flu patient coughs. The other human dummy took in air from the surroundings. This symbolized the doctor or a nurse. Now the researchers took samples from the air at different points in the room. The samples were divided according to the size of the particles. In addition, the doctors in the laboratory examined the total amount of viruses and the proportion of functional 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 air humidity was below 24 percent, the infectivity of the viruses decreased only minimally and was between 70 and 77 percent after 60 minutes. However, if a relative humidity of over 42 percent was reached, a large proportion of the viruses were no longer active after 15 minutes and the infectivity was only 14 to 23 percent after 60 minutes. "After that, there was a decline up to 5 hours after the cough, but this remained statistically no different at 45% relative humidity than at 20%, regardless of the aerosol fraction."

It remains unknown how the mechanism of inactivity works. The present research work was unable to provide an answer to this. "It is difficult to retrofit examination and waiting rooms so that a high level of humidity can be reached." According to the researchers, this aspect should be taken into account when setting up new practices in order to minimize the risk of infection. (sb)


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