Relative humidity

According to this method, one can determine how much moist a moisturizer must spray. In practise, one should adopt a worst-case scenario. For example, in winter at an outside temperature of -10 °C and a relative humidity of 30%, one wishes to create a satisfactory indoor climate (such as 20 °C and a minimal relative humidity of 40%). There appears to be a direct relation between relative humidity and phsychological wellfare of humans. Humans feel their best at a relative humidity of 40% or above.

This data is based on estimates, because relative humidity cannot be measured accurately in outside air. When it is raining, relative humidity draws near 100% and when it is a cold day, relative humidity is very low. Principally, when air is warmer, it can contain more fluids. When air is heated, but not moisturized, relative humidity will decrease, whereas the number of grams of H₂O per kilogram remains the same.

Another example: how much water vanishes from a smokestack which emits 80,000 Nm³/hour of water-saturized air (100% RH) that has a temperature of 75 °C? (answer = 31,394 litres/hour – or a reversed waterfall).

Naturally, this can all be controlled by a Mollier diagram. Example: at a temperature of 20 °C, the relative humidity is measured 50%. One can now appoint the density as 1,20 kg/m³ and there will be about 7,3 g of water per kg of air.

What are the correct amounts of relative humidity for a correct environment?

For a pleasant working environment, it is important to make sure relative humidity does not fall below 40%. When relative humidity is less than 40%, the risk of disease is increased. Generally, it can be stated that symptoms that are caused by dry air vary, but three main factors can be distinguished: static electricity, moisture stability and health effects.

Static electricity

Dry air can cause static electricity in an environment. Static electricity can be diminished by increasing the relative humidity of air. Machines in a machine park give off static electricity as a result of friction. When there are more machines present that are active during a longer period of time, more friction will take place and the risk of static electricity increases. This mainly occurs at dry machine elements. In computer rooms, there is also a static electricity risk. Most static electricity is caused at a relative humidity of between 30 and 35%.

Moisture stability

Moisture stability means the ability of a material or product to maintain a certain level of moisture, despite fluctuations of relative humidity in its environment. Most materials give off or take up moist. This can cause damage to a material or product. In many sectors –such as vegetables, fruits, flowers and granes- this process is irreversible. When relative humidity is too high, this can also cause problems for antiques, paintings, books, papers, etc. Most damage to older products is caused by air humidity fluctuation.

Health effects

As temperatures increase, relative humidity decreases. Dry air can cause health effects, such as dry nose and throat. This causes a higher susceptivity to pathogens such as viruses. When it is cold, a higher air humidity makes people believe it is warm. This causes the heater to be on less often.

It appears that the climate for bacterial growth is worst when relative humidity is between 40 and 60%. Viruses can survive least at a relative humidity of between 47 and 70%. For people, relative humidity is most pleasant between 40 and 60%. For people that suffer from allergies and astma, relative humidity must be between 45 and 55%.

High relative humidity can cause constriction.

Desirable relative humidity and temperature for each activity

Below, a table is shown that outlines ideal temperatures and relative humidity for each sector in a given situation. This table is derived from JDK air-handling.

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