IAQ UK is an independent organisation with the aim of 'raising the agenda of indoor air quality within the home and workplace'

IAQUK Resources - Humidity



We actually interpret the transfer of heat from our bodies as a measurement, as opposed to our actual body temperature; this is to ensure we are able to react to thermal comfort conditions.


The body attempts to cool itself by rising blood to the skin surface, causing a red flush to the skin;  the skin uses air to release moisture and heat (conductivity) via the process of sweating, thus cooling the body. If there is too much moisture in the air, the sweat will not evaporate.  In addition blood flowing to the skin surfaces reduces blood flow to other essential muscles and organs, thus reducing physical strength and mental capacity and increasing fatigue.


High humidity (60%>) can make people feel warmer than the actual temperature.  Forexample, if the air temperature is 24 °C and the humidity is 0%RH, the air temperature will feel about 21 °C.   If the humidity is 100%RH the air temperature would feel more like 27 °C.   This measurement is called a humidex and is used to provide a combined effect of temperature and humidity on cooling of the human body. 


Conversely the internal heating can decrease the humidity to below 40%RH. Such low humidity can create health conditions and sensitivity for individuals, such as the drying of the mucous membranes and skin, excessive thirst, dehydration and feeling cooler than the actual air temperature.  Humidity can significantly effect health and comfort on an individual, the extent of effects depends on the duration of exposure, the temperature and other permutations, including the level of hydration, how sensitive an individual is to dry skin, whether they wear contact lenses which may increase eye irritation. 


An optimum condition, people generally find 45%RH the most comfortable.




There are various methods to measuring and regulating humidity. The most common is called a hygrometer specifically known as a psychrometer and consists of two thermometers, a dry bulb and a wet-bulb temperature. 



Wet Bulb thermometer (WBT)


The combined measurement of temperature, humidity and air movement described on a single scale.     This measurement is achieved by using a wet sock (wet cloth wrapped around the dry bulb), as air passes over the wet cloth, the water evaporate, cooling the bulb.  Thus simulating the evaporation of sweat.



Wet Bulb Globe Thermometer (WBGT)


Using the same approach to WBT a standard dry bulb thermometer bulb is wrapped in a wet sock and inserted into a large (6 inch) globe.  The globe can allow measurement of the effects of sunshine and other radiant heat.   Thus accounting for other significant combined factors that affect a person working in hot conditions. The WBGT measurements are often used to determine heat related stress to workers.

If the air is saturated with water vapours it would be 100%RH.   Humidity levels for comfort conditions should be kept above 40% and below 60% relative humidity.

Humidity is the amount of water vapour in the air or other gases. Humidity in the same way that hot water gives off steam.  Air has the capacity to absorb water and the amount depends on the temperature. The hotter the temperature, the more water is held, creating steamy, sticky conditions. Whereby cold air is denser and therefore holds less water vapour contributing towards dry conditions obviously the conditions will depend on the amount of water available. for vaporising. Humidity can be measured in several ways, but commonly measured as a percentage called relative humidity (RH).  When the air has no water vapours, it would be 0% RH.

The source of humidity is primarily from outdoor air; however equipment and processes can contribute towards humidity levels.   Indeed even humans, we breathe about 5,000 gallons of air daily, producing approximately 3 pints of water every day just by breathing.  Despite these sources, buildings, equipment and people are sensitive to humidity.


Buildings and Equipment

Health effects associated with humidity can also arise from the occupant’s environments. 


A primary source of health conditions can arise from a building with high humidity (70%RH>), which can induce toxic or allergenic moulds, corrosion and moisture related deterioration of materials.   Carpets and fabrics can be exposed to dust mites and mildew. 


There are approximately 1.5 million species of fungi of which more than 1000 species have evolved from the indoor environment.  As a fungus matures and reproduces, they eject spores into the air, which travel over large distances. Once airborne they can be inhaled and then deposited in the nasal passage or in the eyes, which can cause sensitivity or an allergic reaction, particularly if exposed to repeated amounts of particular fungal propagates. Some fungi can produce metabolites or mycotoxins which can induce an adverse reaction and are toxic to humans.


Perennial allergic retinitis is the inhalation of microscopic substances, including mould spores and other airborne substances that cause an allergic reaction, inflaming the nasal lining, causing an immune response to a substance which may not be harmful.    This including faecal aerosols from dust mites, which are the most common cause of asthma. Dust mite itself is _0.5mm in size, invisible to the naked eye but often found in damp areas. Typical allergic reactions are itchiness, sneezing, inflamed/infected eczema, watering eyes and runny nose.  Dust mites can be reduced by keeping humidity below 50%RH.


There is also evidence to suggest the high humidity effects emissions from VOCs, in particularly Formaldehyde, which is one of the most common VOCs in the workplace.


In addition to health conditions, elevated humidity can also cause corrosion in building fabrics and electronics, low humidity can cause static build-up on electrical products.  Servers are often sensitive to humidity levels within data centres/rooms.