Basal cell carcinoma (BCC) or 'rodent ulcer' is the most common form of skin cancer. It develops in parts of the body most exposed to the sun, especially the face and hands, but does not spread. Left untreated, BCC can burrow deeply into underlying tissues, causing disfigurement and serious damage.Squamous cell carcinoma (SCC) is thought to be the result of cumulative sunlight exposure and usually develops in old age. It is more dangerous that BCC because it can spread to other parts of the body. There is growing evidence that a major cause of SCC is intermittent sun exposure of skin unaccustomed to solar radiation.

Malignant melanoma is the most dangerous form of skin cancer; it can spread very quickly and, unless caught early, can be difficult to treat. It develops from cells in the outer layer of the skin called melanocytes, which produce melanin, a pigment that helps protect the deeper layers of the skin from damage.

Melanomas usually start in moles or in areas of normal looking skin. However, in rare cases, they can occur in other parts of the body such as the eye, rectum, vulva, vagina, mouth, respiratory tract, gastrointestinal tract and bladder.

In the UK, melanomas are the twelfth most common cancer in men, eleventh in women. In 1997, there were 2020 new cases of melanoma in men and 2670 cases in women (Office of National Statistics). However, deaths in 1999 were 761 for men and 711 for women. For non melanoma cancers, despite an incidence of over 44,000 new cases a year, the total number of deaths was 212 and 157 for men and women, respectively. This compares with totals for all cancer deaths in 1999 of 68,968 for men and 64,697 for women.

Doctors still do not know all the causes of malignant melanoma. The Imperial Cancer Fund (see: www.imperialcancer.co.uk), however, identifies the following risk factors:

Sun exposure. There is evidence suggesting that sunburn and/or intense sun exposure in childhood increases the risk of developing melanoma in later life. Children's skin should therefore be well protected from the sun. Infants under the age of 12 months should be kept out of the sun altogether. Short periods of intense exposure to sunlight seem to increase an individual's risk of developing melanoma.

Sunbeds and sunlamps. These emit mainly UVA radiation and are a potential risk when used excessively.

Age. Malignant melanoma is rare in young children and most common in those aged 40-60 years, although it can occur in young adults.

Moles. The average young adult has at least 25 normal naevi or moles. People who have large numbers of moles, say 50-100, have an increased risk. Dysplastic naevi (atypical moles) are generally larger than normal moles and uneven in colour, with an irregular border. People who have multiple dysplastic naevi have an increased risk.

Fair skin colour. Fair skin contains less melanin than darker skin. Thus, individuals with fair skin (especially skin that freckles or burns easily) have an increased risk of developing melanoma. However, people with dark skins can still develop melanoma, especially on the soles of the feet or palms of the hands.

Family history. A person's risk of developing melanoma is increased if their parents, siblings or children have been diagnosed with melanoma. If there has only been one family member with melanoma, the risk to the rest of the family is low. However, if there have been three or more cases of melanoma within a family, the risk is greater.

Reduced immunity. Individuals who have been treated with drugs which suppress their immune system for example, organ transplant patients are at an increased risk of developing melanoma.

Personal history of melanoma. Individuals who have been diagnosed with malignant melanoma have an increased risk of developing another melanoma.

Xeroderma pigmentosum (XP). Individuals with this rare inherited condition are less able to repair the damage caused to their skin by the sun's UV radiation. Thus, these people have a high risk of developing skin cancers, including melanoma.