Understanding UV Index and the rationale for using sunscreen indoors

The UV Index (UVI) is a valuable tool for assessing the strength of ultraviolet (UV) radiation from the sun at any given location and time. The UVI values are determined using the STAR (System for Transfer of Atmospheric Radiation) model. This model takes into account various atmospheric conditions to estimate UV radiation levels. The values provided reflect typical conditions for each location and serve as reference points. Actual UV Index readings can vary due to local factors, such as temporary changes in ozone levels and other atmospheric conditions. The values range from 0 to 11+, serving as a standardized guide for sun protection measures. This helps us understand the potential for skin damage based on UV exposure levels. They are specified for the 21st of each month across different regions. Higher UVI values indicate a greater risk of harm, particularly concerning sunburn, DNA damage, premature skin aging and hyperpigmentation [1][2].
 
HIGHEST AND LOWEST UV INDEX VALUES
Highest UV Index
The highest recorded UV Index values can reach 12 or more, especially in regions near the equator, high-altitude areas, and places with low ozone levels. The Atacama Desert in Chile has documented peaks as high as 20, highlighting the extreme UV exposure possible in certain environments [2]. 
Lowest UV Index
The lowest values are typically observed at night or during winter months in polar regions, where solar angles are significantly reduced, often resulting in readings close to zero [2][3].
 
GEOGRAPHIC INFLUENCES ON UV LEVELS
UV exposure varies widely across different geographical regions and withing the regions:
 
▌Europe: Generally experiences moderate UV levels due to higher latitudes and frequent cloud cover [4].
▌Asia: Significant variability; tropical areas encounter high UV levels while northern regions have lower indices [2].
▌Australia: Known for high UV exposure, particularly during summer months, due to its proximity to the equator and clearer skies.
▌USA: Southern states typically report higher UV indices compared to their northern counterparts.
▌Latin America: High UV indices are prevalent near the equator, while southern regions like Argentina experience lower values [2][3].
 
▌Altitude: Higher altitudes receive more intense UV radiation due to a thinner atmosphere [2].
▌Reflection: Beaches can experience increased UV levels due to sunlight reflecting off water and sand [3].
▌Northern vs. Southern hemisphere: The Southern hemisphere generally has higher UV levels attributed to less atmospheric pollution and ozone depletion [2].
▌Equatorial regions: These areas maintain consistently high UV indices throughout the year due to direct sunlight [2][3].

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INDOOR vs OUTDOOR UV EXPOSURE
The UV Index indoors is significantly lower than outdoor levels on a sunny day. This is primarily due to the filtering effect of window glass, which blocks most UVB radiation. On a clear day, outdoor UV levels can reach up to 8,000 µW/cm², while indoor levels near a window may be as low as 250 µW/cm², dropping further with distance from the window.

The indoor UVI reduction is primarily due to the filtering effect of glass windows, which block most UVB (320–400 nm) radiation while allowing some UVA (320–400 nm) rays to penetrate and can still contribute to premature skin aging, hyperpigmentation and DNA damage. Blue Light (400–495 nm): Part of visible light spectrum; penetrates glass easily. High energy Visible Light is responsible for 50% of the free radical activity [5] and like UV radiations contributes to premature skin aging, hyperpigmentation and  DNA damage. Factors influencing indoor UV exposure include window size, orientation, and surrounding obstructions like trees.
 Direct and indirect exposure
▌Direct exposure occurs when sunlight directly enters through windows.
▌Indirect (Diffuse) exposure results from sunlight scattering off surfaces or atmospheric particles. While diffuse exposure is reduced by walls and roofs, it can still penetrate through windows [3].
 
Factors affecting indoor exposure
1. Window glass: Standard glass blocks most UVB but allows some UVA and High energy Visible Light  through.
2. Sky view: More visible sky from indoors increases diffuse UV exposure.
3. Distance from windows: The intensity of UV radiation decreases with distance from windows due to the inverse square law [3].
4. Window orientation and size: Larger windows facing south (in the Northern Hemisphere) or north (in the Southern Hemisphere) allow more sunlight into indoor spaces [3].
5. Scattering (indirect – diffuse exposure)

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CHANGING UVI OVER TIME
There is scientific evidence indicating that the UV Index (UVI) is influenced by various environmental factors, including changes in ozone levels and climate conditions, which can affect UV radiation exposure over time. 
 
1. UV radiation: A study by Fountoulakis et al. (2020) analyzed long-term changes in UV-B radiation and found that variations in UV levels are primarily driven by changes in aerosols and total ozone, with significant regional differences observed. The study indicates that while some areas have experienced increases in UV-B irradiance, others have shown decreases, particularly during summer months in polar regions due to improvements in ozone levels [6].
 
2. Impact of ozone depletion: Research has shown that the decline of stratospheric ozone has historically led to increased UV radiation at certain wavelengths. For instance, a study by Bais et al. (2011) projected that UV irradiance would likely return to its 1980 levels by the early 21st century at northern mid-latitudes and high latitudes, suggesting ozone recovery influences UV radiation levels [7].While standard windows block most harmful UVB rays, damaging UVA and blue light (or HEVIS) can still penetrate indoors, affecting skin´s beauty and health. Awareness of these factors and UV Index enables you to take appropriate protective measures against harmful effects of sunlight even indoors while considering the benefits of controlled exposure for vitamin D synthesis [3].
 
Take care
Anne-Marie
 
References
[1] Federal Office for Radiation Protection (BfS). (n.d.). What is the UV Index? Retrieved December 7, 2024, from bfs.de/EN/topics/opt/uv/index/introduction/introduction_node.html
[2] Fioletov V, Kerr JB, Fergusson A. The UV index: definition, distribution, and factors affecting it. Can J Public Health. 2010;101(4):I5-9. doi: 10.1007/BF03405303.
[3] Heckman CJ, Liang K, Riley M. Awareness and impact of the UV index: A systematic review of international research. Prev Med. 2019;123:71-83. doi: 10.1016/j.ypmed.2019.03.004.
[4] World Health Organization. (n.d.). Radiation: The UV index. Retrieved December 7, 2024, from who.int/news-room/questions-and-answers/item/radiation-the-ultraviolet-(uv)-index
[5] Albrecht S et al. Effects on detection of radical formation in skin due to solar irradiation measured by EPR spectroscopy. Methods. 2016;109:44-54.
[6] Fountoulakis I et al. Long-term changes in UV-B radiation. Atmos Chem Phys. 2020;20(5):3075-3091.
[7] Bais AF et al. Projections of UV radiation changes in the 21st century: impact of ozone recovery and cloud effects. Atmos Chem Phys. 2011;11(20):7533-7545. doi: 10.5194/acp-11-7533-2011
[8] Eleftheratos K et al. Ozone, DNA-active UV radiation, and cloud changes due to enhanced greenhouse gas concentrations. Atmos Chem Phys. 2022;22:12827–12855. doi: 10.5194/acp-22-12827-2022

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