A hairball is a small collection of hair

Hair

Hair is a filamentous biomaterial, that grows from follicles found in the dermis. Found exclusively in mammals, hair is one of the defining characteristics of the mammalian class....

 or fur

Fur

Fur is a synonym for hair, used more in reference to non-human animals, usually mammals; particularly those with extensives body hair coverage. The term is sometimes used to refer to the body hair of an animal as a complete coat, also known as the "pelage". Fur is also used to refer to animal...

 formed in the stomach

Stomach

The stomach is a muscular, hollow, dilated part of the alimentary canal which functions as an important organ of the digestive tract in some animals, including vertebrates, echinoderms, insects , and molluscs. It is involved in the second phase of digestion, following mastication .The stomach is...

 of animal

Animal

Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...

s that is occasionally vomited up when it becomes too big. Hairballs are primarily a tight elongated cylinder of packed fur, but may include bits of other elements such as swallowed food. Cat

Cat

The cat , also known as the domestic cat or housecat to distinguish it from other felids and felines, is a small, usually furry, domesticated, carnivorous mammal that is valued by humans for its companionship and for its ability to hunt vermin and household pests...

s are especially prone to hairball formation since they groom

Personal grooming

Personal grooming is the art of cleaning, grooming, and maintaining parts of the body. It is a species-typical behavior that is controlled by neural circuits in the brain.- In humans :...

 themselves by licking

Licking

Licking is the action of passing the tongue over a surface, typically either to deposit saliva onto the surface, or to collect liquid onto the tongue for ingestion, or to provide pleasure. Many animals both groom themselves and drink by licking....

 their fur, and thereby ingest it. Rabbit

Rabbit

Rabbits are small mammals in the family Leporidae of the order Lagomorpha, found in several parts of the world...

s are also prone to hairballs because they groom themselves in the same fashion as cats, but hairballs are especially dangerous for rabbits because they cannot regurgitate them. Because the digestive system of a rabbit is very fragile, rabbit hairballs must be treated immediately or they may cause the animal to stop feeding and ultimately die due to dehydration

Dehydration

In physiology and medicine, dehydration is defined as the excessive loss of body fluid. It is literally the removal of water from an object; however, in physiological terms, it entails a deficiency of fluid within an organism...

. Cattle

Cattle

Cattle are the most common type of large domesticated ungulates. They are a prominent modern member of the subfamily Bovinae, are the most widespread species of the genus Bos, and are most commonly classified collectively as Bos primigenius...

 are also known to accumulate hairballs, but as they do not vomit, these are found usually after death and can be quite large.

Although uncommon in human

Human

Humans are the only living species in the Homo genus...

s, some hairballs have been reported, often in young girls as a result of trichophagia

Trichophagia

Trichophagia is the compulsive eating of hair associated with trichotillomania . In trichophagia, people with trichotillomania also ingest the hair that they pull; in extreme cases this can lead to a hair ball ....

, trichotillomania

Trichotillomania

Trichotillomania, which is classified as an impulse control disorder by DSM-IV, is the compulsive urge to pull out one's own hair leading to noticeable hair loss, distress, and social or functional impairment. It is often chronic and difficult to treat....

 and pica

Pica (disorder)

Pica is characterized by an appetite for substances largely non-nutritive . For these actions to be considered pica, they must persist for more than one month at an age where eating such objects is considered developmentally inappropriate...

. In 2003, a 3-year old girl in Red Deer, Canada

Red Deer, Alberta

Red Deer is a city in Central Alberta, Canada. It is located near the midpoint of the Calgary-Edmonton Corridor and is surrounded by Red Deer County. It is Alberta's third-most-populous city – after Calgary and Edmonton. The city is located in aspen parkland, a region of rolling hills...

 had a grapefruit-sized hairball surgically

Surgery

Surgery is an ancient medical specialty that uses operative manual and instrumental techniques on a patient to investigate and/or treat a pathological condition such as disease or injury, or to help improve bodily function or appearance.An act of performing surgery may be called a surgical...

 removed from her stomach; and in 2004, an 18-year old woman from McAdam, Canada

McAdam, New Brunswick

McAdam is a village located in the southwestern corner of York County, New Brunswick, Canada. The village covers and has a population of 1,404 as of 2006.The area was first settled in the mid-to-late 19th century as a group of small lumber camps...

, had a 2.3 kg (5.1 lb) hairball surgically removed from her lower intestine. Hairballs can be quite hazardous in humans, since hair cannot be digested or passed by the human gastrointestinal system, and (assuming it is identified) even vomiting

Vomiting

Vomiting is the forceful expulsion of the contents of one's stomach through the mouth and sometimes the nose...

may be ineffective at removing the hair mass. This can result in the general impairment of the digestive system.