Sunburn is a condition characterized by erythema and oedema resulting from excessive exposure to the sun’s rays, specifically the ultraviolet (UV) radiation emitted by the sun. This condition can also occur from exposure to other UV light sources, such as those found in solariums or tanning salons. At the cellular level, sunburn is associated with microscopic changes in the skin, including the formation of UV-induced sunburn cells and a reduction in Langerhans cells and mast cells, both of which play essential roles in the body’s immune defence system.

To understand the causes of sunburn, it is essential to examine some basic principles of the electromagnetic (light) spectrum. This spectrum is divided according to wavelength into ultraviolet (less than 400 nm), visible (400–760 nm), and infrared (greater than 760 nm) light. The ultraviolet (UV) spectrum is further divided into three broad areas: Ultraviolet A (UV-A) ranges from 320–400 nm, Ultraviolet B (UV-B) ranges from 290–320 nm, and Ultraviolet C (UV-C) is less than 290 nm. UV-C radiation is filtered out or absorbed in the outer atmosphere, so it does not pose a problem to humans. The primary causes of sunburn are UV-A and UV-B radiation, and the skin reacts differently to each waveband.

UV-A radiation, while less potent than UV-B, reaches the earth’s surface most significantly, about 90% at midday. It penetrates the middle skin layer (dermis) and subcutaneous fat, causing damage where new skin cells are created. Long-term exposure to UV-A causes injury to the dermis, resulting in aging skin. On the other hand, UV-B radiation is much more potent at causing erythema, with about 90% absorbed by the surface skin layer (epidermis). The epidermis responds by releasing chemicals that cause the reddening and swelling characteristic of early sunburn signs. Repeated exposure to UV-B causes injury to the epidermis, leading to aging skin.

Skin phototyping categorizes individuals into one of six groups based on their baseline skin color and their tendency to tan and burn when exposed to UV radiation. 

Skin phototype I includes individuals with pale white skin, blue or hazel eyes, and blond or red hair. They always burn and do not tan, with a minimal erythema dose (MED) ranging from 15 to 30 mJ/cm2. 

Skin phototype II consists of people with fair skin and blue eyes. They burn easily and tan poorly, with a MED between 25 and 40 mJ/cm2. 

Skin phototype III encompasses those with darker white skin. They tan after an initial burn, with a MED ranging from 30 to 50 mJ/cm2. 

Skin phototype IV includes individuals with light brown skin. They burn minimally and tan easily, with a MED between 40 and 60 mJ/cm2.

Skin phototype V is characterized by brown skin. These individuals rarely burn and tan darkly and easily, with a MED ranging from 60 to 90 mJ/cm2. 

Finally, skin phototype VI comprises those with dark brown or black skin. They never burn and always tan darkly, with a MED between 90 and 150 mJ/cm2.

People with type I skin phototyping, characterized by pale white skin, blue or hazel eyes, and blond or red hair, are at a much greater risk of sunburn than those with type VI skin phototyping, characterized by dark brown or black skin. The minimal erythema dose (MED), the amount of UV radiation needed to produce erythema at an exposed site, is significantly lower in people with a low skin phototype grading. For instance, fifteen minutes of midday sun exposure may cause sunburn in a person with pale skin, while a darker-skinned individual may tolerate hours of exposure.

Other factors increasing the incidence of sunburn include regions closer to the equator, areas at high altitudes where UV radiation increases by 4% for every 300m elevation gain, skin exposure between 10 am and 2 pm when 65% of UV radiation reaches the earth, clear skies, and environmental reflection where UV radiation is 80% reflected by snow and ice.

The signs and symptoms of sunburn vary according to the skin phototype and length of UV radiation exposure. Typically, signs and symptoms occur 2-6 hours after exposure and peak at 12-24 hours. These may include erythema (redness), oedema (swelling), tenderness and irritation, skin that feels hot to touch, pain, blistering in severe cases, and chills and fever in severe cases. In extreme cases, severe sunburn can result in second-degree burns, dehydration, electrolyte imbalances, secondary infection, shock, or even death.

The primary treatment of sunburn aims to provide relief from the discomfort it causes using analgesics (painkillers), cool baths, aloe vera lotions, and moisturizers. However, prevention is the best approach to managing sunburn. Effective sun protection includes avoiding sun exposure, especially between 10 am to 2 pm, wearing protective clothing such as wide-brimmed hats, and applying and reapplying sunscreen with a Sun Protection Factor (SPF) of 50+. Additionally, an oral food supplement containing Polypodium leucotomas may offer additional photoprotection and reduce the severity of sunburn.

If sunburn is anticipated, taking immediate measures can lessen its severity. This includes taking two aspirin immediately and then every four hours and applying a topical steroid to the exposed areas twice daily for two or three days.

The long-term consequences of overexposure to the sun or other UV radiation sources are significant. Even one blistering sunburn can at least double the likelihood of developing skin cancer later in life. Other long-term effects include premature skin aging and wrinkling, the appearance of brown spots and freckles (lentigines), the development of premalignant lesions (actinic keratoses), and the development of skin cancers such as melanoma, basal cell carcinoma, and squamous cell carcinoma.

Understanding the risks and effects of sunburn, alongside taking preventative measures, can significantly reduce the potential for immediate discomfort and long-term damage. Through education and diligent sun protection practices, the harmful impacts of sunburn can be effectively managed and minimized.