This is a medical question of type : causes 
 What causes Skin Cancer ? 
 Scientists have been able to identify the causes and risk factors for skin cancer. A risk factor is anything that increases your chances of getting a disease. DNA Damage One of the main reasons that skin cancer develops is because DNA is damaged. DNA is the master molecule that controls and directs every cell in the body. Damage to DNA is one of the ways that cells lose control of growth and become cancerous. DNA mutations can also be inherited. Exposure to Ultraviolet Radiation Excess exposure to ultraviolet (UV) light can damage the DNA in skin cells and increase a person's risk for both melanoma and non-melanoma skin cancer. UV light is invisible radiation from the sun that can damage DNA. Skin cells are especially susceptible to DNA damage since they are frequently exposed to UV light. There are three types of UV radiation: A, B, and C. All three are dangerous and able to penetrate skin cells. UVA is the most common on earth, and is harmful to the skin. UVB is less common because some of it is absorbed by the ozone layer. It is less harmful than UVA, but can still cause damage. UVC is the least dangerous because although it can cause the most damage to the skin, almost all of the UVC rays are absorbed by the ozone layer. Sources of Ultraviolet Radiation UV radiation comes from the sun, sunlamps, tanning beds, or tanning booths. UV radiation is present even in cold weather or on a cloudy day. A person's risk of skin cancer is related to lifetime exposure to UV radiation. Most skin cancer appears after age 50, but the sun damages the skin from an early age. The body has systems to repair DNA and control some mutations, but not all of them. The risk of cancer increases as we age because sometimes cancer is caused by many mutations accumulating over time. Role of the Immune System The body's immune system is also responsible for recognizing and killing abnormal cells before they become cancerous. As we get older, our immune systems are less able to fight infection and control cell growth. People whose immune system is weakened by certain other cancers, medications, or by HIV are at an increased risk of developing skin cancer. Basal Cell and Squamous Cell Carcinoma Besides risk factors that increase a person's chance of getting any type of skin cancer, there are risk factors that are specific to basal cell carcinoma and squamous cell carcinoma, the non-melanoma skin cancers. These risk factors include - scars or burns on the skin   - chronic skin inflammation or skin ulcers   - infection with certain human papilloma viruses   - exposure to arsenic at work   - radiation therapy  - diseases that make the skin sensitive to the sun, such as xeroderma pigmentosum, albinism, and basal cell nevus syndrome   - medical conditions or drugs that suppress the immune system   - personal history of one or more skin cancers   - family history of skin cancer   - certain diseases of the skin, including actinic keratosis and Bowen's disease.  scars or burns on the skin chronic skin inflammation or skin ulcers infection with certain human papilloma viruses exposure to arsenic at work radiation therapy diseases that make the skin sensitive to the sun, such as xeroderma pigmentosum, albinism, and basal cell nevus syndrome medical conditions or drugs that suppress the immune system personal history of one or more skin cancers family history of skin cancer certain diseases of the skin, including actinic keratosis and Bowen's disease. Someone who has one or more of these risk factors has a greater chance of getting skin cancer than someone who does not have these risk factors. However, having these risk factors does not guarantee a person will get skin cancer. Many genetic and environmental factors play a role in causing cancer. Melanoma Melanoma is less common than non-melanoma skin cancers like basal cell carcinoma and squamous cell carcinoma, but it is more serious. The factors that increase a person's chance of getting melanoma are - severe, blistering sunburns earlier in life   - unusual moles (normally benign clusters of melanocytes)   - large quantity of ordinary moles (more than 50)  - white or light-colored (fair) skin, especially with freckles.   - blond or red hair   - blue or green eyes  - being older than 20 years of age.  severe, blistering sunburns earlier in life unusual moles (normally benign clusters of melanocytes) large quantity of ordinary moles (more than 50) white or light-colored (fair) skin, especially with freckles. blond or red hair blue or green eyes being older than 20 years of age. Someone who has one or more of these risk factors has a greater chance of getting melanoma than someone who does not have these risk factors. However, having these risk factors does not guarantee a person will get cancer. Many genetic and environmental factors play a role in causing cancer. Reducing Your Risk While exposure to UV radiation is a major risk factor for cancer, skin cancer can occur anywhere on the skin, not just in sun-exposed areas. The best ways to reduce your risk of skin cancer are to - avoid outdoor activities during midday, when the sun's rays are strongest  -  wear protective clothing such as a wide-brimmed hat, long-sleeved shirt, and pants.  avoid outdoor activities during midday, when the sun's rays are strongest wear protective clothing such as a wide-brimmed hat, long-sleeved shirt, and pants. Darker-colored clothing is more protective against the sun. For example, a white t-shirt, particularly if it gets wet, provides little resistance to UV rays. In addition, wearing sunglasses that wrap around the face or have large frames is a good way to shield the delicate skin around the eyes. Wear Sunscreen and Lipscreen When exposed to sunlight, you should always wear sunscreen and lipscreen. If possible, choose sunscreen and lipscreen labeled "broad-spectrum" (to protect against UVA and UVB rays). Your sunscreen should have an SPF, or sun protection rating, of at least 15. The SPF of a sunscreen is a measure of the time it takes to produce a sunburn in a person wearing sunscreen compared to the time it takes to produce a sunburn in a person not wearing sunscreen. This varies from person to person, so be sure to reapply sunscreen every 2-3 hours.