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Humidity above 40% cuts flu infection risk

The airborne transmission of the influenza virus is significantly reduced by maintaining an atmosphere of 40 per cent relative humidity (RH) or over

The airborne transmission of the influenza virus is significantly reduced by maintaining an atmosphere of 40 per cent relative humidity (RH) or over, according to a study by the US National Institute for Occupational Safety & Health (NIOSH).

To test the effects of humidity on airborne influenza, aerosols of flu virus were ‘coughed’ into a room’s atmosphere by a mechanical manikin at humidity ranging from 7 per cent to 73 per cent, while the air intake from a breathing manikin in the room was monitored.

The air inhaled by the breathing manikin showed that at 23 per cent RH or below, the airborne flu virus retained 71-77 per cent infectivity, but at 43 per cent RH and over, the infectivity dropped to 15-22 per cent. The study showed that inactivation of the virus at the higher humidity occurred rapidly after coughing, with most of the decline occurring in the first 15 minutes.

The study concluded that maintaining relative indoor humidity at 40 per cent and above will significantly reduce the infectivity of an aerosolised influenza virus.

It is estimated that more than 7.6 million working days are lost in the UK each year as a result of flu-related sickness, costing the UK economy over £1.35bn per year.

“This study shows how important it is to maintain an optimum humidity in the workplace to reduce absenteeism and especially in areas of high risk to airborne viruses, such as hospitals and doctors’ surgeries,” said Tim Scott, director at sector specialist JS Humidifiers.

“Although many professional bodies, including BSRIA, CIBSE and HSE, all recommend maintaining indoor humidity at above 40 per cent RH it is not uncommon to see humidification systems being turned off to reduce operating costs.

“A low humidity is not as noticeable by employees as a low temperature, so it can go unnoticed. However, the true cost of not maintaining indoor humidity can be poor staff health and an increase in absenteeism, which can far outweigh the cost of operating the building’s humidification system.”

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