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Relative Humidity
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Relative Humidity

April 18, 2011


Ventilation system deficiencies, overcrowding, tobacco smoke, microbiological contamination, outside air pollutants, and off gassing from materials in the office and mechanical equipment, can cause IAQ problems. Related problems also may include comfort problems due to improper temperature and relative humidity conditions, poor lighting, and unacceptable noise levels, as well as adverse ergonomic conditions, and job-related psychosocial stressors. Typical symptoms may include headaches, unusual fatigue, itching or burning eyes, skin irritation, nasal congestion, dry or irritated throats, and nausea.

With mold and mildew growth directly related to the level of humidity inside the conditioned space the importance of maintaining the right level of humidity despite the increased ventilation rate becomes of paramount importance. Needless to say that with the increased amount of ventilation the latent load increases tremendously and the air-conditioning equipment is unable to handle the situation resulting in increased humidity levels.

The Environmental Protection Agency (EPA) and the Consumer Product Safety Commission (CPSC) recommend maintaining indoor RH levels between 30% and 50%.

It is a common experience to notice that, in comfort-controlled buildings using HVAC (heat, ventilation, and air-conditioning) systems, the indoor RH goes down during winter season leading to dryness of the skin, etc. and it slightly increases during summer season leading to humid and sweaty conditions. If the humidity is too low, viral and bacterial populations tend to flourish, thus contributing to respiratory infections. If the RH is too high, fungal growth and dust mites can also contribute to health problems. Therefore, it is very important to maintain the indoor RH at the above recommended optimum range.

The following chart (Figure 1) indicates the relation between the rate of fungal and mildew growth as related to the humidity levels maintained inside.

Relative Humidity Guidelines

Relative Humidity Level Description
0% - 30% Most fungi will not grow at these humidities.
40% - 55% Optimal Building Humidity for all parts of the occupied space, chases, dropped ceilings, plenums and behind drywall.
60% - 70% Approaching optimal range of humidity for growth; mold growth likely in such areas.
Above 70% Optimal humidity levels for most fungal growth.
Above 90% Typical humidity level downstream of cooling coils during cooling season without reheat. (Unavoidable tempering condition due to cooling process. Mixing and off cycles restore RH to acceptable levels.)


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