In the short term, UV radiation exposure causes sunburn, and in the long-term is a strong causal risk factor for melanoma and keratinocyte skin cancers (basal cell and squamous cell carcinomas).

Keratinocyte skin cancers (also known as non-melanoma skin cancers) are the most common group of human cancers observed in many countries. The global incidence of keratinocyte skin cancers is estimated to be between 2-3 million each year.^{5 5}Cancers attributable to UV radiation. Arnold et al. 2018; WHO: Ultraviolet (UV) radiation and skin cancer. 2017 X

Exposure to UV radiation over long periods can also lead to damage to the eyes, including:^{6 6}Climatic droplet keratopathy and pterygium. Taylor H. 1981; Pterygium and ultraviolet radiation: a positive correlation. Moran D, Hollows F. 1984;
Pterygium: The curse of the Australian sun lover. Roberts T, Coroneo M. 1999; Pterygium as an early indicator of ultraviolet insolation: A hypothesis. Coroneo M. 1993; Sunlight exposure and risk of lens opacities in a population-based study. West et al. 1998
The biological effects of UVB on the eye. Taylor H. 1989; Incidence of ocular melanoma in Australia from 1990 to 1998. Vajdic et al. 2003
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• cataracts (cloudiness of the lens)
• cancer of the conjunctiva (the membrane covering the white part of the eye)
• pterygium (an overgrowth of the conjunctiva on to the cornea)
• solar keratopathy (cloudiness of the cornea)
• skin cancer of the eyelids and around the eyes
• ocular melanoma.

Chronic overexposure to UV radiation (particularly UVB) can result in immunosuppression, with UVB-induced DNA damage a major trigger.^{7 7}Mechanisms of UV-induced immunosuppression. Schwarz T. 2005; An action spectrum for ultraviolet radiation-induced immunosuppression in humans. Damian et al. 2011X

Most of the visible signs of ageing are the result of damage to the skin caused by exposure to UV. It is estimated that as much as 80% of premature facial aging, such as wrinkles and fine lines, is due to sun exposure.^{8 8}What is photoaged skin? Leyden J. 2001; Understanding premature skin aging. Uitto J. 1997X

However, some UV exposure can be beneficial. Most vitamin D is produced in the skin by exposure to the sun’s UVB.^{9 9}Vitamin D fortification in the United States and Canada: Current status and data needs. Calvo MS, Whiting SJ, Barton CN. 2004X Low levels are also present in some foods. Vitamin D is stored in fat and muscle and slowly released.^{10 10}The distribution and storage of vitamin D and its metabolites in human tissues. Mawer et al. 1972X Production of vitamin D from UV exposure to the skin is influenced by a number of factors including age, skin colour, latitude, altitude, weather, ground surface properties, season and time of day, making it difficult to provide general advice to suit all situations. 

Typically, during summer in locations where UV levels are consistently three and above, a few minutes of mid-morning or mid-afternoon sun exposure to arms and hands (or equivalent area) on most days of the week should be sufficient to maintain adequate vitamin D levels. Given the considerable evidence showing UV to be carcinogenic, extended and deliberate sun exposure without any form of sun protection when the UV Index is three and above is not recommended, even for those diagnosed with vitamin D deficiency. It is recommended that people who may be at risk of vitamin D deficiency discuss their vitamin D requirements with their medical practitioner to determine if vitamin D supplementation rather than sun exposure is appropriate.^{11 11}Position statement – Sun exposure and vitamin D – risks and benefits. Australian and New Zealand Bone and Mineral Society, the Australasian College of Dermatologists, Cancer Council Australia, Endocrine Society of Australia, Osteoporosis Australia, 2016X

UV radiation levels vary depending on latitude, altitude, location, time of year, time of day and cloud coverage and various environmental factors. Most of the world’s population live in climates where the UV peaks in the extreme range (UV Index 11+) during summer months.^{12 12}Where on Earth has the highest UV? Liley J.B. and McKenzie R.L. 2005X

The incidence of melanoma varies with geographic location, but also within any location due to the different impact on individual people based on their sensitivity and response to the same level of exposure. 

Worldwide incidence of melanoma has steadily increased over the last several decades. Annual incidence has risen as rapidly as 4–6% in many fair-skinned populations in regions such as North America, Northern Europe, Australia, and New Zealand.^{13 13}Cutaneous Melanoma: Etiology and Therapy, Chapter 1 Epidemiology of Melanoma. Matthews et al. 2007X

Australia and New Zealand have the highest estimated global age-standardised (ASR) rate (per 100,000) of melanoma (35.8), followed by Western Europe (18.9), Northern Europe (17.8) and North America (16.1).^{14 14}Global Cancer Statistics 2020: GLOBOCAN estimates of incidence and mortality worldwide for 36 cancers in 185 countries, Melanoma of Skin Fact Sheet. Sung et al. 2020X

Solar emissions include visible light, infrared radiation (heat) and ultraviolet (UV) radiation. Just as visible light consists of different colours that become apparent in a rainbow, based on their wavelengths, depth of its penetration into skin/tissue and consequent health impact the UV radiation spectrum is divided into three regions called UVA, UVB and UVC. 

• UVA (about 95% of UV radiation) – too much UVA exposure causes roughening, dryness, blotchiness, wrinkling and sagging of the skin. High doses of UVA radiation can also cause sunburn, DNA damage in the skin, and skin cancer.^{15 15}What is photoaged skin? Leyden J. 2001X
• UVB (about 5% of UV radiation) – UVB is more dangerous than UVA with much of it filtered by the atmosphere. UVB is the major cause of skin cancers, sunburning and cataracts.^{16 16}Volume 55: Solar and ultraviolet radiation. WHO/IARC, 1997; Vol. 100D: Solar and ultraviolet radiation. IARC 2012.X

UVC is the most damaging type of UV radiation and is extremely dangerous. Fortunately, all UVC is absorbed by oxygen and ozone in the stratosphere and never reaches the Earth’s surface.

Factors affecting UV radiation levels include:

• the height of the sun (the higher the sun is in the sky, the higher the UV radiation level)
• distance from the equator (the closer to the equator, the higher the UV radiation level)
• the amount of cloud cover (on average, clouds reduce clear sky UV by about 30%)
• altitude (UV increases by about 5% per kilometer)
• UV-reflective surfaces
• pollution levels (clear, open skies allow more UV to pass through)

Approximately 60% of the day’s total carcinogenic radiation is received between 10am and 2pm (local standard time).^{17 17}Ultraviolet radiation and skin cancer in man. Urbach F. 1980X

Skin colour has long been established as a risk factor for skin cancer. The risk of melanoma, the most serious form of skin cancer, and keratinocyte skin cancers is much higher for individuals with red or fair hair, light-coloured eyes and fair skin that freckles or burns easily.^{18 18}Who gets skin cancer: Individual risk factors. Elwood JM. 2004; Cutaneous malignant melanoma. In: Cancer epidemiology and prevention. Armstrong BK, English DR. 1996.
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However all skin types can be damaged by over-exposure to UV radiation. Darker skin has more protective melanin pigment, and the incidence of skin cancer is lower in dark skinned people. People with skin types that are less likely to burn can still develop skin cancer, and unfortunately, they are often detected at a later, more dangerous stage.^{19 19}Skin cancer in skin of color. Gloster Jr HM, Neal K. 2006X 

It is recommended babies under 12 months are not exposed to direct sun when the UV Index is three and above. A baby’s skin is thin, extremely sensitive and can burn easily. The more sun exposure during childhood, the greater the risk of skin cancer in later years. Young people are particularly vulnerable and sun protection should be a priority as adolescence and childhood are critical periods during which exposure to UV radiation is more likely to contribute to skin cancer in later life.^{20 20}Season of birth and other perinatal risk factors for melanoma. Crump et al. 2014;
Thickness and echogenicity of the skin in children as assessed by 20-MHz ultrasound. Seidenari et al. 2000; How sun exposure causes skin cancer: An epidemiological perspective. In: Prevention of Skin Cancer. Armstrong BK. 2004;
Artificial ultraviolet sources and skin cancers: Rationale for restricting access to sunbed use before 18 years of age. Autier P, Boyle P. 2008X

UV radiation is an occupational health and safety hazard for people who work outdoors. Outdoor workers, both full-time and part-time, have a higher than average risk of basal cell carcinoma (BCC) and squamous cell carcinoma (SCC) because they spend long periods of time working in the sun, all year round, over many years of working life.^{21 21}World Cancer Report: Cancer Research for Cancer Prevention. IARC.; Is occupational solar ultraviolet irradiation a relevant risk factor for basal cell carcinoma? A systematic review and meta-analysis of the epidemiological literature. Bauer A, Diepgen TL, Schmitt J. 2011;
Occupational ultraviolet light exposure increases the risk for the development of cutaneous squamous cell carcinoma: a systematic review and meta-analysis. Schmitt J, Seidler A, Diepgen TL, Bauer A. 2011X

We are still learning what impact climate change will have on stratospheric ozone recovery as well as skin cancer rates. UV-B radiation at low and mid latitudes is projected to increase in the second half of the 21st century due to reduced cloud cover resulting from climate change.^{22 22}Environmental effects of stratospheric ozone depletion, UV radiation, and interactions with climate change: UNEP Environmental Effects Assessment Panel, Update 2020. PNeale, R.E., Barnes, P.W., Robson, T.M. et al. 2021; Environmental Effects and Interactions of Stratospheric Ozone Depletion, UV Radiation, and Climate Change. 2018 Assessment Report. Environmental Effects Assessment Panel, UNEP 2018 X

In some geographic locations, climate change may lead to warmer and drier weather with more clear sky days that will encourage more people to spend more time outdoors, thereby increasing their UV exposure, while in other locations, the opposite effect may happen.^{23 23}The influence of climate change on skin cancer incidence – A review of the evidence. Parker, E.R. 2020 X

Most skin cancer can be prevented. Whenever possible check the daily UV Index for your location. Most weather forecasts will include UV Index levels. Some locations have specific UV apps to indicate changing UV levels across the day and recommended sun protection times. 

When UV Index levels are forecast to be three and above remember these five key steps to sun protection:

1. Wear cool, covering clothing that covers as much skin as possible.
2. Wear a wide-brimmed hat that shades the face, neck and ears.
3. Apply broad-spectrum sunscreen to any parts of the skin that can’t be covered with clothing. Sunscreen will not block all UV so it should only be used with other sun protection measures.
4. Wear UV protective sunglasses to help protect the eyes.
5. Use shade wherever possible. Even in the shade UV can still reach you so it is important to continue to use all forms of sun protection.