Published: Jun 2000 | BMC Infectious Diseases201313:71

Predictors of indoor absolute humidity and estimated effects on influenza virus survival in grade schools

Koep T et al 2013


Study objective was to develop and test a model for the prediction of indoor humidity and its effect on calculated 1 hour survival of influenza virus as projected by laboratory studies.

The study data are based on:

1. Experimental data collected by humidification of three empty, not ventilated classrooms on a weekend with portable humidifiers (evaporative humidification, ≈ 2 lt./h).

2. Measurements of indoor relative humidity (RH measured, absolute humidity AH calculated), temperature and CO2 in 5 min. intervals in classrooms of two grade schools in Minnesota from Jan. to end March 2011 and meteorological data for AH outside.

3. Published humidity dependent Influenza virus survival data.


  • By means of portable humidifiers, relative humidity in an empty classroom can be increased to 60% within 4 hours. Half of the humidification effect is lost 2 hours’ post humidification.

  • Indoor AH can go very low (min. 2.6 mb). Seasonal fluctuations are predicted by outdoor AH, while daily fluctuations of AH are closely associated with CO2 levels and humidity input by students.

  • Calculated one hour influenza survival rate is 75% in very low indoor humidity. This rate can be reduced to 50% respectively 35%, by increasing indoor relative humidity to 40% or 60%RH.

  • Modest, achievable changes in indoor RH are likely to have a substantial effect on 1-hour influenza survival as projected by laboratory experiments.

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by Walter Hugentobler

Schools and colleges are a key battleground in the fight against the spread of influenza. Large groups of individuals are forced together for extended periods of time in close proximity. Reducing the spread of 'flu in the classroom is not only directly beneficial for students and teachers, but also for their families who are spared the subsequent cross-infections.

This study showed that increases of indoor relative humidity to 40- 60%RH through humidification will have a substantial preventive effect on influenza survival rate, and therefore transmission efficiency and resulting infection rate, of influenza in classrooms.

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