IAQUK Resources - Humidity
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.
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
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
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.