what is the percentage of water vapour in breathed air?????

General properties of water vapor

Evaporation/sublimation
Whenever a water molecule leaves a surface, it is said to have evaporated. Each water molecule that becomes water vapor takes a parcel of heat with it. This process is called evaporative cooling.[4] The amount of water vapor in the air will determine how fast each molecule will return back to the surface or not. So, when a net evaporation occurs, that body of water will undergo a net cooling directly related to the loss of water.[5]

Evaporative cooling is restricted by atmospheric conditions. The amount of water vapor in the air is referred to as humidity. Measurement of the vapor content of air is accomplished with devices known as hygrometers. The measurements are expressed as specific humidity or percent relative humidity. The temperature of the atmosphere and the water surface determines the equilibrium vapor pressure, 100% relative humidity occurs when the partial pressure of water vapor is equal to the equilibrium vapor pressure. This is often referred to as complete saturation.

Another form of evaporation is sublimation, in which water molecules become gaseous from ice instead of liquid water. Under the same principle, when ice has a higher temperature than the surrounding atmosphere, sublimation occurs. It is sublimation that accounts for the slow, mid-winter disappearance of ice and snow at temperatures too low to cause melting.
Condensation
Water vapor will only condense onto another surface when that surface is cooler than the temperature of the water vapor, or when the water vapor equilibrium in air has been exceeded. When water vapor condenses onto a surface, a net warming occurs on that surface.[6] The water molecule brings a parcel of heat with it. In turn, the temperature of the atmosphere drops slightly.[7] [8] In the atmosphere, condensation produces clouds, fog and precipitation--usually only when facilitated by cloud condensation nuclei. The dew point of an air parcel is the temperature to which it must cool before condensation in the air begins to form.

Also, a net condensation of water vapor occurs on surfaces when the temperature of the surface is at or below the dew point temperature of the atmosphere. Deposition is a type of condensation. Frost and snow are examples of deposition (or sublimation). Deposition is the direct formation of ice from water vapor.

2007-02-01 12:39:48 補充：
Water vapor densityWater vapor is lighter or less dense than dry air. At equivalent temperatures it is buoyant with respect to dry air.

2007-02-01 12:40:00 補充：
http://en.wikipedia.org/wiki/Water_vapor

2007-03-15 12:39:27 補充：
投緊票個時找個...好次係0.1%,breath 0.3%

According to http://www.io.com/~hcexres/power_tools/process/process_formatB.html
For water vapor <1% is breathed in, 2% is breathed out.
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According to http://en.wikibooks.org/wiki/GCSE_Science/Breathing_and_Respiration
Exhaled air contains less oxygen and more carbon dioxide, it is also saturated with water vapour. Exhaled air contains:

78% nitrogen
17% oxygen
1% inert gas such as argon
4% carbon dioxide
saturated with water vapour