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From fundamental science to product: a bottom-up approach to sunscreen development

Posted on 5. April, 2018.

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Ultraviolet (UV) radiation from the Sun is categorised as UVC (100–280 nm), UVB (280–315 nm) and UVA (315–400 nm). Stratospheric ozone prevents any significant amount of highly destructive UVC radiation from reaching the Earth’s surface, as well as absorbing a significant portion of UVB radiation.

The UV radiation at the Earth’s surface is, therefore, composed mostly of UVB and UVA radiation, the combined total of these accounting for approximately 3.4% of the total atmosphere attenuated solar spectrum (cf. 8% before the atmosphere)2–4. The UV radiation provided by the Sun plays a crucial role in sustaining life on Earth. When UV photons are absorbed by key chromophores in biological systems, these light absorbing molecules are left in excited states with enough energy to undergo chemical reactions. Such photoinduced chemistry, otherwise termed photochemistry, is the trigger for photosynthesis, for example, the unique biological process which converts the energy from sunlight into the biochemical energy necessary to sustain life on Earth. In humans, one of the most important UV-induced processes is the production of vitamin D, which is best known for its ability to protect against musculoskeletal disorders but has also been found to provide protection against infectious, autoimmune and cardiovascular diseases.

The extensive benefits of UV radiation to life are heavily counterbalanced by the serious consequences of excess exposure to this type of radiation. In plants, excess UV radiation disrupts photosynthetic processes, as well as reducing carbon dioxide fixation and oxygen evolution. For humans, it is now firmly established that overexposure to UV radiation is related to erythema (sunburn), skin aging and carcinogenesis – namely, it is related to melanoma, one of the most aggressive human cancers. UVB and UVA interact differently with the human skin, and their effects can also be quite distinct. UVB radiation is readily absorbed by DNA and can thus cause alterations to the genetic sequence which, if unrepaired by excision repair pathways, may eventually lead to cancer. In comparison, UVA has been extensively reported to produce harmful levels of reactive oxygen species (ROS) in the skin, which is a major factor contributing to both carcinogenesis and skin ageing via oxidative stress pathways. The human body defends itself against such radiative stress by producing melanin pigments which absorb harmful radiation before it reaches DNA. The production of melanin – melanogenesis or, as it is more commonly known, tanning – is triggered by exposure to UV radiation. However, this is a delayed response and it may take 3–5 days for any significant protection to be afforded by the extra melanin produced. Any photodamage occurring before the skin is sufficiently protected may have a catastrophic effect on the skin.

Read the full article in Science Progress, Volume 101, Number 1, March 2018, pp. 8-31.

DOI: https://doi.org/10.3184/003685018X15166183479666

Authors: Natércia d.N. Rodrigues and Vasilios G. Stavros
University of Warwick, UK
E-mail: n.das-neves-rodrigues@warwick.ac.uk and v.stavros@warwick.ac.uk

Keywords: sunscreens, photophysics, photochemistry, photodynamics, ultrafast laser spectroscopy, time-resolved, ultraviolet, photoprotection

Image: Absorbance spectra of alcohol extractions of two commercial sunscreens with different advertised SPF values: 15/20 (black line); 30 (red line); and 50 (blue line). Each plot also shows some of the active ingredients responsible for the observed absorbance for each respective sunscreen.