Cleanliness is the absence of dirt, including
dust,
stains, bad
smells and clutter. Purposes of cleanliness include
health,
beauty, absence of offensive odor, avoidance of
shame, and to avoid the spreading of dirt and contaminants to oneself and others. In the case of glass objects such as windows or windshields, the purpose can also be
transparency.
Washing is one way of achieving cleanliness, usually with
water and often some kind of
soap or
detergent. In more recent times, since the
germ theory of disease, it has also come to mean an absence of germs and other
hazardous materials. In industry, certain processes such as those related to
integrated circuit manufacturing, require conditions of exceptionally immaculate cleanliness which are achieved by working in
cleanrooms.
A recent shift has now taken place to recognise that ‘dirt’ may play a useful role in our immune systems. This shift in thinking can be traced back to 1989, when David Strachan put forth the "
hygiene hypothesis" in the British Medicine Journal. Strachan looked at the records of 17,000 British children and found that the greater number of older siblings they had, the less likely they were to come down with
hay fever—a disease which, despite its name, is far more common in the city than the country. Strachan wondered if the older children were bringing home more viral infections to their younger siblings, priming their immune systems so they could better tolerate pollen. The "
hygiene hypothesis" has now been linked with
asthma,
allergies, intestinal diseases including
Crohn's disease , childhood
leukaemia and atopic
dermatitis and the list is growing.
It can apply to
humans,
animals,
clothing,
food,
eating utensils (plates, pans, cups, etc.), other movable objects, floors,
windows,
walls,
toilets, etc.
Cleanliness is essential to successful
electroplating, since molecular layers of oil can prevent adhesion of the coating. This industry has developed many specialized techniques for cleaning, as well as tests for cleanliness. The most commonly used tests rely on the wetting behaviour of a clean hydrophillic metal surface.
Cleanliness is also important to
vacuum systems to reduce
outgassing.