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Cancer Risk and Prevention

What Factors Affect UV Risk?

The main types of UV rays that can affect your skin include UVA rays and UVB rays. UVB rays have more energy and are a more potent cause of at least some skin cancers, but both UVA and UVB rays can damage skin and cause skin cancer. There are no safe UV rays.

Everyone’s skin reacts differently to UV (ultraviolet) rays, but anyone can get skin cancer. Most skin cancers are caused by too much exposure to UV rays, mostly from the sun, but from artificial sources, such as indoor tanning beds and sun lamps. People who get a lot of exposure to UV rays are at greater risk for skin cancer, so it’s important to understand what affects how much UV you are exposed to and how your skin might respond.

What affects the strength of UV rays from the sun?

The strength of the sun’s UV rays reaching the ground depends on a number of factors, such as:

  • Time of day: UV rays are strongest in the middle of the day, between 10 am and 4 pm.
  • Season of the year: UV rays are stronger during spring and summer months, and in early fall. This is less of a factor near the equator.
  • Distance from the equator (latitude): UV strength and exposure goes down as you get further from the equator. People who live in areas with year-round, bright sunlight have a higher risk of skin cancer.
  • Altitude: More UV rays reach the ground at higher elevations.
  • Cloud cover: The effect of clouds can vary, but it's important to know that UV rays can get through to the ground, even on a cloudy day and even on a cold day.
  • Reflection off surfaces: UV rays can bounce off surfaces like water, sand, snow, or pavement, leading to an increase in UV exposure.

The UV Index

The US National Weather Service and the Environmental Protection Agency (EPA) have developed the UV Index. Knowing the expected UV Index gives you an idea of how strong the UV light is in your area on any given day.

The UV Index reports the intensity (strength) of UV rays a scale from 1 to 11+. A higher number means there is a greater risk of exposure to UV rays and a higher chance of sunburn and skin damage that could ultimately lead to skin cancer. The UV Index is part of many weather forecasts and is reported on many weather apps that can be downloaded.

The UV Index tells you the strength of the UV rays on a given day, but the amount of UV exposure you get also depends on:

  • Time spent outdoors for work or recreation
  • The length of time your skin is exposed
  • If your skin is protected with clothing or sunscreen
  • The pattern of your exposure over time

When do I need to be most careful in the sun?

Some people think about sun protection only when they spend a day at the lake, beach, or pool. But sun exposure adds up day after day, and it happens every time you are in the sun. Regardless of the time of year or season, UV rays can be strong enough to cause skin damage when skin is not protected.

Key steps to take and things to remember:

  • UV rays can affect skin during any season of the year.
  • Check the UV Index by watching or listening to a weather forecast for the day, or by checking a weather app for your location.
  • If you aren't sure of the UV Index in your area or at a time when you will be exposed to direct sunlight, you can do the shadow test. If your shadow is shorter than you are, this tells you the sun’s rays are the strongest.
  • Avoid the sun between the hours of 10 am and 4 pm, when UV light is strongest.
  • Be especially careful on the beach or in areas with snow because sand, water, and snow reflect sunlight. This increases the amount of UV radiation you get. UV rays can also reach below the water’s surface, so you can still get a sunburn even if you’re in the water and feeling cool.

Do windows provide protection from UV rays?

Some UV rays can pass through windows. This means even if you don’t feel you’re getting burned, your skin may still get some damage. UV radiation that comes through windows probably doesn’t pose a great risk to most people unless they spend long periods of time close to a window that gets direct sunlight.

  • Typical car, home, and office windows block most UVB rays, but they block fewer UVA rays.
  • Tinted windows help block more UVA rays, but this depends on the type of tinting. (If you do have your car windows tinted, check local laws, as some states regulate this.)
  • There are solar shades, blinds, and coverings that you can buy and use in your car, home, or workplace. Many are made with ultraviolet protection factor (UPF) fabric to help limit exposure.

Does getting a tan or having darker skin protect from UV rays?

Anyone who spends time outdoors in the sun is at risk for skin damage from UV radiation. People with light skin are much more likely to have their skin damaged by UV rays (and to get skin cancer), but darker-skinned people, including people of any ethnicity, can also be affected.

For some people with certain skin tones, their skin tans when it absorbs UV rays. The tan color is caused by an increase in the activity and number of melanocytes. Melanocytes are the cells that make a brown pigment called melanin. Melanin helps block out damaging UV rays up to a point, which is why people with naturally darker skin are less likely to get a sunburn.

People with lighter or more pale skin have less melanin. They are more likely to burn from UV rays rather than tan. How quickly a person’s skin can redden when exposed to UV rays can vary, but usually, the fairer or lighter the skin, the quicker it will burn.

Sunburns can greatly increase your risk of skin cancer, including basal and squamous cell cancers and melanoma. But even for people who don’t usually get sunburned, UV exposure increase the risk of skin cancer.

What other factors affect my risk of damage from UV rays?

In addition to skin tone, other factors can also affect your risk of damage from UV rays. You need to be especially careful in the sun if you:

  • Have had skin cancer in the past
  • Have a family history of skin cancer, especially melanoma
  • Have many moles, irregular moles, or large moles
  • Have freckles and burn before tanning
  • Have fair skin, blue or green eyes, or blond, red, or light brown hair
  • Live or vacation at high altitudes (UV rays are stronger the higher up you are)
  • Live or vacation in tropical or subtropical climates
  • Work indoors all week and then get intense sun exposure on weekends
  • Spend a lot of time outdoors
  • Have certain autoimmune diseases, such as systemic lupus erythematosus (SLE or lupus)
  • Have certain inherited conditions that increase your risk of skin cancer, such as xeroderma pigmentosum (XP) or nevoid basal cell carcinoma syndrome (Gorlin syndrome).
  • Have a medical condition that weakens your immune system, such as infection with HIV (the virus that causes AIDS)
  • Have had an organ transplant
  • Take medicines that lower or suppress your immune system
  • Take medicines that make your skin more sensitive to sunlight

No matter how sensitive your skin is to the sun, it’s important to know how to protect yourself from UV rays. Ask your doctor, nurse, or pharmacist if you are taking any medicines that could increase your sensitivity to sunlight.

The American Cancer Society medical and editorial content team

Our team is made up of doctors and oncology certified nurses with deep knowledge of cancer care as well as editors and translators with extensive experience in medical writing.

Christensen SR, Wilson LD, Leffell DJ. Section 9: Cancers of the Skin. In: DeVita VT, Lawrence TS, Rosenberg SA, eds. DeVita, Hellman, and Rosenberg’s Cancer: Principles and Practice of Oncology. 12th ed. Philadelphia, Pa: Lippincott Williams & Wilkins; 2022.

National Cancer Institute. Genetics of Skin Cancer (PDQ®)–Health Professional Version. 2023. Accessed at on June 26, 2024.

US Food and Drug Administration (FDA). UV Index Overview. 2023. Accessed at on June 26, 2024.

Young AR, Tewari A. Sunburn. 2022. UpToDate. Accessed at on June 26, 2024.

Last Revised: June 26, 2024

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