Excessive exposure to the sun’s harmful rays can cause skin cancer, which can be dangerous. To protect yourself against skin cancer, apply sunscreen with a sun protection factor (SPF) of at least 15 if you are going to be outdoors for more than a few minutes. Even if you apply sunscreen faithfully, you should check regularly for signs of skin cancer. Warning signs include any changes in the size, shape, color, or feel of birthmarks, moles, or freckles, or new, enlarging, pigmented, or red skin areas. If you notice any changes or you find you have sores that are not healing, consult your doctor immediately.