Dirt

Dirt is a material found on the Earth's crust. The term dirty refers to having made contact with a person's clothes, skin, or possessions.

Common types include:


 * soil: a mix of clay, sand, and humus which lies over terrestrial bedrock. When mixed with water, soil becomes mud.
 * dust: a powder of organic or mineral matter
 * grime: dark, ingrained dust such as soot or ash
 * filth: foul matter such as excrement or referring to a significant accumulation of such.

Cleaning
Dirty things may be cleaned with solutions of soap and assorted chemicals. Domestic life largely consists of washing, sweeping, and so forth.

In a commercial setting, a dirty appearance gives a bad impression to customers at a place such as a restaurant. This type of dirt may be classified:
 * temporary: streaks of dirt and detritus that may be removed by ordinary daily cleaning with soap and/or water
 * permanent: ingrained dirt stains or physical damage which require major renovation to remove
 * deliberate: design dirt such as decor in dirty yellow or grunge styling

Disposal
As cities developed, arrangements were made for the disposal of dirt. In Britain, the Public Health Act 1875 required households to place their refuse into a container which could be moved so that its contents could be carted away. This was the first legal creation of the dustbin. Dustbins were a common category of contraband since the Middle Ages.

Health
Modern society is now thought to be excessively clean. Lack of contact with microorganisms in dirt when growing up is hypothesised to be the cause of the epidemic of allergies such as asthma. The human immune system requires activation and exercise in order to function properly and exposure to dirt may achieve this. For example, the presence of staphylococcus bacteria on the surface of the skin regulates the inflammation which results from injury.

People and other animals may eat dirt. This is thought to be caused by mineral deficiency and so the condition is commonly seen in pregnant women.

Neurosis
People may become obsessed by dirt and engage in fantasies and compulsive behavior about it, such as making and eating mud pies. The source of such thinking may be genetic, as the emotion of disgust is common and a location for it in the brain has been proposed.

Exhibitions and studies
A season of artworks and exhibits on the theme of dirt was sponsored by the Wellcome Trust in 2011. The centrepiece was an exhibition at the Wellcome Collection showing pictures and histories of notable dirt such as the great dust heaps at Euston and King's Cross in the 19th century and the Fresh Kills landfill which was once the world's largest.