Cleanliness is the absence of dirt, including dust, stains, bad smells and garbage. 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 exceptional cleanliness which are achieved by working in cleanrooms.
However, 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.
Cleanliness is essential to successful electroplating, since molecular layers of oil can prevent adhesion of the coating. The industry has developed specialized techniques for parts cleaning, as well as tests for cleanliness. The most commonly used tests rely on the wetting behaviour of a clean hydrophillic metal surface.