Merck Manual

Please confirm that you are a health care professional

honeypot link

Overview of Effects of Sunlight


Julia Benedetti

, MD, Harvard Medical School

Reviewed/Revised Oct 2023
Topic Resources

Ultraviolet (UV) radiation

The sun emits a wide range of electromagnetic radiation. Most of the dermatologic effects of sunlight are caused by UV radiation, which is divided into 3 bands: UVA (320 to 400 nm), UVB (280 to 320 nm), and UVC (100 to 280 nm). Because the atmosphere filters the radiation, only UVA and UVB reach the earth’s surface. The character and amount of sunburn-producing rays (primarily wavelengths < 320 nm) reaching the earth's surface vary greatly with the following factors:

  • Atmospheric and surface conditions

  • Latitude

  • Season

  • Time of day

  • Altitude

  • Ozone layer

Exposure of skin to sunlight also depends on multiple lifestyle factors (eg, clothing, occupation, recreational activities).

Sunburn-producing rays are filtered out by glass and to a great extent by heavy clouds, smoke, and smog; however, they may still pass through light clouds, fog, or 30 cm of clear water, potentially causing severe burns. Snow, sand, and water enhance exposure by reflecting the rays. Exposure is increased at low latitudes (nearer the equator), in the summer, and during midday (10 AM to 4 PM) because sunlight passes through the atmosphere more directly (ie, at less of an angle) in these settings. Exposure is also increased at high altitudes primarily because of a thinner atmosphere. Stratospheric ozone, which filters out UV radiation, especially shorter wavelengths, is depleted by man-made chlorofluorocarbons (eg, in refrigerants and aerosols). A decreased ozone layer increases the amount of UVA and UVB reaching the earth's surface.

Pathophysiology of Sun Exposure

As a protective response after exposure to sunlight, the epidermis thickens, and melanocytes produce the pigment melanin at an increased rate, causing what is commonly referred to as a "tan." Tanning provides some natural protection against UV radiation but otherwise has no health benefits.

People differ greatly in their sensitivity and response to sunlight, based mainly on the amount of melanin in the skin. The skin has been classified into 6 types (I to VI) in order of decreasing susceptibility to sun injury. Classification is based on the interrelated variables of skin color, UV sensitivity, and response to sun exposure (see table ). Skin type I is white to very lightly pigmented, very sensitive to UV light, has no immediate pigment darkening, always burns easily, and never tans. Skin type VI is dark brown or black, is most protected from UV light, and is a deep, dark, black-brown color with or without sun exposure. However, dark-skinned people are not immune to the effects of the sun, and darkly pigmented skin can develop sun damage with strong or prolonged exposure. Long-term effects of UV exposure in darker-skinned people are the same as those in lighter-skinned people but are often delayed and less severe because the melanin in their skin provides built-in UV protection.

People with blond or red hair are especially susceptible to the acute and chronic effects of UV radiation. Uneven melanocyte activation occurs in many of these fair-haired people and results in freckling.

There is no skin pigmentation in people with albinism Albinism Oculocutaneous albinism is an inherited defect in melanin formation that causes diffuse hypopigmentation of the skin, hair, and eyes. Ocular albinism affects the eyes and usually not the skin... read more Albinism because of a defect in melanin metabolism. Patchy areas of depigmentation are present in patients with vitiligo Vitiligo Vitiligo is a loss of skin melanocytes that causes areas of skin depigmentation of varying sizes. Cause is unknown, but genetic and autoimmune factors are likely. Diagnosis is usually clear... read more Vitiligo because of immunologic destruction of melanocytes. These and any other group of people who are unable to produce melanin at a rapid and complete rate are especially susceptible to sun damage.


Prevention of Effects of Sun Exposure

Avoiding the sun, wearing protective clothing, and applying sunscreen help minimize UV exposure. Two oral supplements—Polypodium leucotomos and nicotinamide—can also provide some protection from sun damage (eg, skin thickening, wrinkling).

Avoiding the sun

Simple precautions help prevent sunburn and the chronic effects of sunlight. These precautions are recommended for people of all skin types, particularly those who are fair skinned and burn easily. Exposure to bright midday sun and other high-UV environments (see Ultraviolet [UV] radiation Ultraviolet (UV) radiation The skin may respond to sunlight with chronic (eg, dermatoheliosis [photoaging], actinic keratosis) or acute (eg, photosensitivity, sunburn) changes. The sun emits a wide range of electromagnetic... read more ) should be minimized (30 minutes or less), even for people with dark skin. In temperate zones, UV ray intensity is less before 10 AM and after 4 PM because more sunburn-producing wavelengths are filtered out. Fog and clouds do not reduce risk significantly, and risk is increased at high altitudes and low latitudes (eg, at the equator).

Although sun exposure helps generate vitamin D, many experts recommend maintaining adequate vitamin D levels by consuming supplements if needed rather than through excessive sunlight exposure.

Protective clothing

Skin exposure to UV radiation can be minimized through the use of protective coverings such as hats, shirts, pants, and sunglasses. Fabrics with a tight weave block the sun better than fabrics with a loose weave. Special clothing that provides high sun protection is commercially available. This type of clothing is labeled with ultraviolet protection factor (UPF) followed by a number that indicates the level of protection (similar to sunscreen labeling). Broad-brimmed hats help protect the face, ears, and neck, but these areas still need supplemental protection with a topical sunscreen. Regular use of UV-protective, wrap-around sunglasses helps shield the eyes and eyelids.


Sunscreens help protect the skin from sunburn and chronic sun damage by absorbing or reflecting the sun's UV rays. Older sunscreens tended to filter only UVB light, but most newer sunscreens now effectively filter UVA light as well and are labeled "broad spectrum." In the United States, the Food and Drug Administration (FDA) rates sunscreens by sun protection factor (SPF): the higher the number, the greater the protection. The SPF only quantifies the protection against UVB exposure; there is no scale in the United States for UVA protection, but formal recommendations may be forthcoming. People should typically use a broad-spectrum sunscreen with an SPF rating of 30 or higher.

Sunscreens are available in a wide variety of formulations, including creams, gels, foams, sprays, powders, and sticks. Sunscreen ingredients function by absorbing and/or reflecting light. Self-tanning products do not provide significant protection from UV exposure.

Chemical sunscreens include ingredients that absorb UV radiation. Cinnamates, salicylates, and para-aminobenzoic acid (PABA) derivatives provide UVB protection. Benzophenones are commonly used to provide UVB and shortwave UVA protection. Avobenzone and ecamsule filter in the UVA range and may be added to provide further UVA protection.

Physical blocking (mineral sunscreens) reflect or scatter light and contain the ingredients zinc oxide and titanium dioxide, which physically reflect both UVB and UVA rays. Although formulations of these products were previously very white and pasty when applied, micronization and nanotechnology have allowed them to form a more transparent layer while providing broad-spectrum protection.

All chemical sunscreen ingredients are thought to be systemically absorbed to some degree. Although most ingredients have minimal adverse effects, some have potential risk, and others are currently being investigated. For people concerned about systemic absorption, mineral sunscreens that have not been micronized may be preferred, because their molecules are too large to be absorbed through the skin.

Sunscreen failure is common and usually results from too-late application (sunscreens should optimally be applied 30 minutes before exposure), failure to reapply after swimming or sweating, failure to apply every 2 to 3 hours during sun exposure, or insufficient application of the product. One ounce (about 30 mL) should be used to cover the entire body surface of an average-sized person; most people apply less than half the recommended amount.

Protective supplements

Polypodium leucotomos (a natural tropical fern extract) (1 Prevention reference The skin may respond to sunlight with chronic (eg, dermatoheliosis [photoaging], actinic keratosis) or acute (eg, photosensitivity, sunburn) changes. The sun emits a wide range of electromagnetic... read more ) and nicotinamide are oral supplements that provide some protection against damaging effects of sunlight but should not be considered a replacement for other methods of sun protection. Caution should be used with higher doses of nicotinamide because they may cause liver damage and elevate blood glucose levels.

Prevention reference

  • 1. Nestor MS, Berman B, Swenson N: Safety and efficacy of oral Polypodium leucotomos extract in healthy adult subjects. J Clin Aesthet Dermatol 8(2):19–23, 2015. PMID: 25741399

Drugs Mentioned In This Article

Drug Name Select Trade
Calcidol, Calciferol, D3 Vitamin, DECARA, Deltalin, Dialyvite Vitamin D, Dialyvite Vitamin D3, Drisdol, D-Vita, Enfamil D-Vi-Sol, Ergo D, Fiber with Vitamin D3 Gummies Gluten-Free, Happy Sunshine Vitamin D3, MAXIMUM D3, PureMark Naturals Vitamin D, Replesta, Replesta Children's, Super Happy SUNSHINE Vitamin D3, Thera-D 2000, Thera-D 4000, Thera-D Rapid Repletion, THERA-D SPORT, UpSpring Baby Vitamin D, UpSpring Baby Vitamin D3, YumVs, YumVs Kids ZERO, YumVs ZERO
Aquaphor 3 IN 1 Diaper Rash, Aquaphor Baby Fast Relief Diaper Rash, Aquaphor Fast Relief Diaper Rash, Balmex, Boudreaux Butt Paste, Boudreauxs Rask, Carlesta, Coppertone, COZIMA, DermacinRx Zinctral, Desitin, Desitin Maximum Strength, Desitin Rapid Relief, Diaper Rash , Dr. Smith Adult Barrier, Dr. Smith's, Dr. Smith's Diaper Rash, Dr. Smith's Rash + Skin, DynaShield, Eucerin Baby Sunscreen Sensitive Mineral , Eucerin Sunscreen Sensitive Mineral, Flanders Buttocks , Medi-Paste, Novana Protect, PanOxyl AM, Triple Paste, Triple Paste Adult Incontinence, Z-Bum
NOTE: This is the Professional Version. CONSUMERS: View Consumer Version
quiz link

Test your knowledge

Take a Quiz!