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What is dew point temperature?

The dew point temperature is the temperature at which air begins to condense. When air comes into contact with an object that has the dew point temperature, condensation will form. The dew point temperature is determined by relative humidity and by the temperature of the air.
Relative humidity
Water vapour is one of the gases naturally present in the air. Relative humidity is a measure of the humidity of air. Relative humidity is the ratio of water vapour in the air to the maximum possible amount of water vapour in the air (identical temperature and pressure). The more water vapour absorbed in the air, the higher the relative humidity. Relative humidity is expressed in %.
Indoors, the relative humidity is mainly determined by the activities of the residents. Showering, drying the laundry, ironing, etc. are activities that increase the relative humidity considerably. More moisture is absorbed in the indoor air while the temperature remains constant.
100% relative humidity does not mean that the room is flooded; it means that the air cannot absorb extra moisture. The air has reached its saturation point.
The maximum amount of water vapour that can be absorbed in the air depends on the temperature. At a relative humidity of 100 %, the amount of water vapour in the air is maximum. As the air gets warmer, it can absorb a higher amount of water vapour. This increase in the maximum absorbable amount will cause the relative humidity to drop.
At lower temperatures (and a constant amount of water vapour), the relative humidity will increase. The ideal relative humidity in the house is between 40 and 70 percent. Of course, this level of humidity can vary from room to room; the kitchen and bathroom is usually more humid than other areas.
Condensation occurs at dew point temperature
Dew point is the temperature to which air must be cooled to become saturated with water vapour. When the air comes into contact with an object that has the dew point temperature or lower, condensation will form.   
Excessive humidity in the house for long periods is bad for your health. Musty air, damp spots, peeling paint, plaster or wallpaper, wood rot, fungi and vermin are the main consequences of too high a relative humidity in the house.
Health can also suffer from excessive humidity. Especially people with sensitive airways or joint pains can experience discomfort (more cough, a feeling of tightness, extra painful joints, ...). 
The chart below provides an indicative overview of dew point temperatures: Dew point temperature  
Importance of good ventilation
Due to our daily activities (cooking, washing, showering) a lot of moisture is produced in the house. To avoid problems with condensation, this excess moisture must be removed from the indoor air using a ventilation system. The damp areas such as the kitchen and bathroom must be sufficiently ventilated. 
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