Ultraviolet radiation and human health

Author: Dr Dhawshini Ravindran, Core Medicine Trainee, Northamptonshire Healthcare, England. DermNet NZ Editor in Chief, A/Prof Amanda Oakley, Dermatologist, Hamilton, New Zealand. January 2018.

This article was supported by an educational grant from La Roche Posay, distributors of sun protection and skin care products in New Zealand and Australia. Sponsorship does not influence content. 

What is ultraviolet radiation?

Ultraviolet radiation (UVR) forms part of the electromagnetic spectrum — the energy emitted from the sun. UVR is categorised according to wavelength, which ranges from 100–400 nm.

  • UVR is involved in tanning, accelerated skin ageing, ocular damage and the development of skin cancer.
  • UVR also has beneficial effects due to the production of vitamin D in the skin and its immune modulating effects on inflammatory skin diseases.

Other sources of UV radiation

Artificial sources of UV including tanning booths and mercury vapour lighting; some halogen, fluorescent and incandescent lights; and some lasers.

Types of ultraviolet radiation


  • UV-A is long-wavelength UVR (315–400 nm).
  • It passes through the whole atmosphere and accounts for 95% of UV radiation reaching the Earth's surface.
  • It penetrates through the epidermis into the dermis.
  • UVA has lower energy than UVB and causes less damage per photon.
  • It contributes to sunburnskin ageing and skin cancer through oxidative injury.
  • It stimulates epidermal melanin oxidisation (immediate tanning) and production (delayed tanning), which protect the skin by absorbing UVR within the epidermis.


  • UV-B is medium-wavelength UVR (280–315 nm).
  • Most solar UVB is filtered by the atmosphere.
  • It penetrates into the epidermis but not into the dermis
  • UV-B has high energy and damages epidermal cell DNA.
  • It stimulates melanin production.
  • UV-B is the main cause of sunburn and some forms of skin cancer.


  • UV-C is short-wavelength UVR  (< 280 nm).
  • Solar UV-C is completely absorbed by the atmosphere and doesn’t reach the earth’s surface.
  • UV-C can be emitted by artificial sources of UVR, for example by welding arcs and mercury vapour lamps.
  • Although shorter wavelengths do not penetrate through the skin, UV-C has higher energy than UVB and injures the skin surface.
  • Exposure to UV-C can lead to severe but superficial sunburn.
Electromagnetic spectrum of visible and UV radiation and biologic effects on the skin

UV Index

The UV Index (UVI) is a measure of the strength of UVR causing erythema (sunburn). If UVI is > 10, it is extreme, and if it is < 3, it is low. 

Factors that result in greater exposure to ultraviolet radiation

The UVI is higher, that is, UV radiation is more intense closer to the equator compared to longer latitudes, as the sun is overhead so the distance to reach the earth is shorter.  UV levels are also greater:

  • During the summer months and during the middle of the day, when the angle of the sun is smallest
  • At higher altitude, when the atmosphere is thinner
  • On a clear day compared to a cloudy day or where the atmosphere is polluted
  • Around reflective surfaces such as snow, sand, grass, or water
  • In the Southern hemisphere compared to the Northern hemisphere, as sun-earth separation is less
    • Relative to the Northern Hemisphere, New Zealand is exposed to 40% higher peak UVI at commensurate latitudes
  • If the ozone layer is depleted.

The NIWA Lauder ozone measurements show seasonal and annual climatological variability. Ozone column amounts are measured in Dobson Units (DU), where 1 DU = 2.69 x 1016 molecules/cm2.

Ozone variation in 2016 in Lauder, New Zealand

What is the ozone layer?

Ozone is a trace gas that forms the ozone layer within the stratosphere, which is part of the atmosphere around the earth.

  • There are 3 molecules of ozone for every 10 million molecules of air.
  • Ozone is a highly reactive molecule that contains three oxygen atoms.
  • It absorbs solar UVR.
  • This heats the stratosphere.
  • Ozone also shields the Earth from the biologically harmful effects of UVR.
  • Reduced ozone leads to higher levels of ultraviolet radiation on earth’s surface.

The ozone hole and the Montreal Protocol

  • Ozone depleting substances (ODS) such as chlorofluorocarbons (CFCs, freons, and halons), once used in refrigerators, spray cans and fire extinguishers, have reduced the amount of ozone in the stratosphere.
  • When CFCs reach the upper atmosphere, they are exposed to UVR, which causes them to break down and form chlorine gas. Chlorine reacts with the oxygen atoms in ozone and rips apart the ozone molecule.
  • Over 50% of ozone in the Antarctic stratosphere has been depleted, leading to the formation of the springtime ‘ozone hole’. During summer, ozone-depleted air from the ozone hole can drift over to South America and New Zealand, resulting in higher levels of UVR than usual for the latitude.
  • Higher winter UV levels in Australia have been calculated for the 1990s compared with the 1970s, resulting in predictions that the incidence of UV-related skin cancer will rise. 

The ‘Montreal Protocol on Substances that Deplete the Ozone Layer’ was signed in 1987. It is an international environmental agreement to protect the ozone layer by reducing the production and usage of ozone depleting substances (ODS) such as CFCs. Since its initiation, atmospheric levels of ODS have significantly decreased and the stratospheric ozone is expected to fully recover by the year 2050.

How does ultraviolet radiation affect the skin?

  • UVA induces oxidative damage to the DNA in the presence of melanin.
  • UVB is independent of melanin and directly damages DNA.
  • UV-B induced release of pro-inflammatory cytokines causes sunburn, which is maximum about 8 hours after exposure. Peeling is accompanied by cellular apoptosis.
  • DNA damage also causes loss of structural integrity of the skin and photo-ageing.
  • Accumulation of unrepaired DNA, and UV-induced immunosuppression, increase the risk of mutations and the development of skin cancer.

Protective mechanisms

  • Tanning is due to a series of complex steps that are triggered by UV-induced damage to DNA and other components of the skin.
  • The thickness of the epidermis is increased in response to growth factors released by injured keratinocytes.

Chronically sun exposed skin of bald scalp, face, neck and hands has unique characteristics compared to skin that is not exposed to ultraviolet radiation.

  • Photo-aged skin is thinner than non-sun exposed skin in the same individual.
  • It is dryer. Over time, actinic keratoses appear in the most sun exposed sites. These are tender, red, scaly papules and plaques.
  • Sun damaged skin is less elastic than skin in other areas, and develops a dull yellowish hue due to solar elastosis.
  • Photoaged skin has more variable pigmentation than other sites due to freckles, solar lentigines, hypopigmented and scarred areas.
  • There are often prominent blood vessels (telangiectasia).

The role of ultraviolet radiation in the development of skin cancer

UVR is a major carcinogen. It damages the skin by producing reactive oxygen species (free radicals) that damage proteins, lipids, RNA and DNA. According to the two-hit model, the development of skin cancer depends on an individual’s genetic makeup (particularly MC1R signaling polymorphisms — which determine skin/hair colour and sun sensitivity) plus two factors.

  1. UVR causes tumour initiation through mutations in DNA.
  2. UVR is a tumour promotor and causes progressive tumour growth.  

 Cutaneous melanoma

  • The incidence of melanoma is high in fair skinned populations living close to the equator; and is higher in Australia and New Zealand compared to the UK and Canada where levels of UVR are lower.  
  • In older adults, melanoma mainly affects the head and neck and is associated with chronic exposure to UVR.
  • In younger people, melanoma affects the trunk and extremities, especially in people with many melanocytic naevi (moles). This is thought to be due to earlier exposure to UVR causing sunburns.

Non-melanoma skin cancer

Most BCCs and SCCs in sun-exposed skin carry ‘UV signature’ mutations (cytosine to tyrosine transitions at cyclobutane pyrimidine dimers) in tumour suppressor genes (PTCH1 and p53 respectively).  

Skin cancers are more common in people with certain inherited photosensitivity disorders. These include:

UV exposure also increases the risk of:

Skin cancers due to to exposure to ultraviolet radiation

How does ultraviolet radiation affect the eyes?

Eyes are partially protected from sunlight by brow ridges. Before reaching the retina, reflected UVR is filtered by the cornea, melanin in the iris, aqueous humour, and the lens. UVR causes oxidative damage to mitochondrial DNA in the macular part of the neural retina and the retinal pigment epithelium. UVR can also promote inflammatory cytokines and transcription factors within the eye.

Exposure to solar UVR is associated with:

  • Photokeratitis and photoconjunctivitis
  • Pterygium
  • Cataract, especially cortical cataract
  • Age-related macular degeneration
  • Ocular malignancies
    • Basal cell carcinoma: > 90% of eye lid malignancies
    • Squamous cell carcinoma: the most common malignancy affecting cornea and conjunctiva
    • Melanoma: the most common primary intraocular malignancy.

How does ultraviolet radiation affect the immune system?

Photons of UVR are absorbed by chromophores in the epidermis and dermis, triggering an immune response.

  • UVR promotes the innate immune response by generating antimicrobial peptides in the epidermis. These protect against pathogens and help with cell growth and repair.
  • UVR suppresses acquired immune responses via stimulation of T-regulatory cells, which play a crucial role in maintaining cutaneous homeostasis.
  • UVR, especially UVB, induces the production of platelet-activating factor, prostaglandin E2, histamine, and tumour necrosis factor-α. These cytokines generate a lifelong population of T suppressor cells and induce neutrophil-mediated inflammation.
  • UVB leads to the synthesis of vitamin D, which can also affect immune function.

Photosensitive skin conditions arising from failure of the normal immune response include:

Photosensitivity disorders

Other immune-related conditions associated with UV radiation


  • People who live at latitudes close to the equator with higher UVB radiation have an increased risk of developing atopic asthma.
  • Conversely, they have a lower risk of non-atopic asthma.


  • UVR supresses the acquired immune response to microbial infections.
  • It reduces the protective efficacy of vaccinations.
  • UV-related immunosuppression may reactivate some latent infections and convert asymptomatic infections to symptomatic infections.
  • It can also boost the oncogenic properties of microbes, such as human papillomavirus.

Beneficial effects of ultraviolet radiation

The beneficial effects of UVR include:

  • Synthesis of vitamin D2 and D3 in the skin.
  • Reduction in cardiac mortality. This may be due to vitamin D synthesis or to lower blood pressure and serum cholesterol.
  • Protection against some auto-immune diseases such as multiple sclerosis, type 1 diabetes mellitus, rheumatoid arthritis.
  • Reduction in risk of infections such as tuberculosis (including cutaneous tuberculosis) and viral respiratory tract infections.
  • Improved wellbeing and reduced risk of depression. One theory relates this to increased production and expression of beta-endorphin in keratinocytes.
  • Maintenance of healthy circadian cycles by suppression of melatonin produced by the pineal gland, reducing the risk of seasonal affective disorder (SAD).


Related Information


  • NIWA Taihoro Nukurangi. UV and ozone. Available from: https://www.niwa.co.nz/our-services/online-services/uv-ozone. [Accessed 12/11/17)
  • Guus J M Velders, Stephen O Anderson, John S Daniel, David W Fahey, Mack McFarland. The importance of the Montreal Protocol in protecting climate. PNAS. 2007; 104 (12): 4814–4819, doi: 10.1073/pnas.0610328104. Available from: http://www.pnas.org/content/104/12/4814.full (accessed 10/11/2017).
  • United States Environmental Protection Agency. International Treaties and Cooperation. Available from: https://www.epa.gov/ozone-layer-protection/international-treaties-and-cooperation (accessed 10/11/2017)
  • Olaf Morgenstern, M Dameris, Kleareti Tourpali,… Alkiviadis F Bais. Skin Cancer Risks Avoided by the Montreal Protocol-Worldwide Modeling Integrating Coupled Climate-Chemistry...Photochemistry and Photobiology. 2013; 89 (1):234-246. Available from: DOI:10.1111/j.1751-1097.2012.01223.x. Journal.
  • NASA. The Antarctic Ozone hole. Available from: https://www.nas.nasa.gov/About/Education/Ozone/antarctic.html (accessed 10/11/2017)
  • Fifty years of changes in UV Index and implications for skin cancer in Australia. Int J Biometerorol. 2011; 56 (4):727-735. Available from: DOI 10.1007/s00484-011-0474-x [Accessed 10/11/2017]. PubMed.
  • R M Lucas, M Norval, R E Neale, A R Young, F R de Gruijil, Y Takizawa, J C Van der Leun. The consequences for human health of stratospheric ozone depletion in association with other environmental factors. Photochemical & Photobiological Sciences. 2015; 14 (1): 53-87. Available from: DOI: 10.1039/c4pp90033b [Accessed 10/11/2017]. Journal.
  • M F Holick. Bilogical Effects of Sunlight, Ultraviolet Radiation, Visible Light, Infrared Radiation and Vitamin D for health. Anticancer Research. 2016; 36 (3): 1345-1356. Available from: https://www.ncbi.nlm.nih.gov/pubmed/26977036 [Accessed 12/11/2017]
  • André Passaglia Schucha, Natália Cestari Morenob, Natielen Jacques Schuchc, Carlos Frederico Martins Menckb, Camila Carrião Machado Garciad. Sunlight damage to cellular DNA: Focus on oxidatively generated lesions. Free Radical Biology and Medicine. 2017; 107:110-124. Available from: http://www.sciencedirect.com/science/article/pii/S0891584917300382?via%3Dihub [Accessed 12/11/2017]
  • Lauren Pinault, Tracey Bushnik, Vitali Fioletov, Cheryl E. Peters, Will D. King, Michael Tjepkema. The risk of melanoma associated with ambient summer ultraviolet radiation. Health Reports. 2017; 28, (5) :3-11. Available from: https://www.ncbi.nlm.nih.gov/pubmed/28513818 [accessed 12/11/17]
  • D’Orazio J, Jarrett S, Amaro-Ortiz A, Scott T. UV Radiation and the Skin. International Journal of Molecular Sciences. 2013;14(6):12222-12248. doi:10.3390/ijms140612222. Journal.

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