The positive and negative effects of sunlight exposure

The positive and negative effects of sunlight exposure

Highlights:

• Ultraviolet radiation is classified into three subtypes, UVA, UVB, and UVC. The first two reach earth, while the ozone layer scatters the third
• Sunlight exposure carries many health benefits, like vitamin D synthesis, reduced risk of metabolic syndrome, or maintenance of circadian rhythm
• Prolonged, unprotected sun exposure can cause photoaging, cancer, ocular damage, and immunosuppression
• Photoprotection comes from primary sources like sunscreens and secondary sources like antioxidants

Introduction

Over the past several decades, humans started spending more time indoors and less outdoors. This has led to a decreased exposure to sunlight. Research indicates that reduced sunlight exposure increases the risk of hypertension, type 2 diabetes mellitus, and thromboembolism. Also, reduced sun exposure has decreased vitamin D levels in the general population. Several studies have linked decreased serum vitamin D levels with increased morbidity and mortality from several diseases. However, increased unprotected sun exposure is linked to adverse outcomes, like skin cancer. In addition, it causes accelerated skin aging, damaged eyes, and a compromised immune system.

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Understanding the ultraviolet (UV) radiation

Based on the wavelength (distance between two successive crests or troughs), UV radiation is classified into three types, including (1-3):

Type

Explanation

UVA (320–400nm)

This is the most abundant type of UV radiation. Due to its longer wavelength, it could penetrate deeper into the skin. UVA can penetrate the epidermis and dermis layers, producing free radicals that damage the DNA. This kind of radiation is potentially carcinogenic. It is present throughout the day and can penetrate glass windows, affecting photosensitive individuals indoors. UVA radiation is divided into UVA I (340 to 400 nm) and UVA II (320 to 340nm).

UVB (290–320 nm)

UVB is also known as burn rays because they cause sunburn. Exposure to UVB causes erythema and skin burns. It can penetrate the stratum corneum and reach the epidermis. Usually, sunscreens are made to protect against this kind of rays. UVB potentially has carcinogenic effects, causing breaks in the DNA strands. However, body exposure to UVB is less than UVA. Most windows can block it.

UVC (200–290 nm)

UVC is the most dangerous type of all UV rays. Fortunately, UVC is cleared by the ozone layer. UVC rays do not penetrate the stratum corneum. UVC is used as a disinfect for a range of bacteria and viruses.

So, does this mean that the entirety of UV rays is bad? The answer is no. For example, exposure to UVB is what helps the body make vitamin D (4). The literature recommends direct unprotected sun exposure of 15-30 minutes per session to get sufficient levels of vitamin D (5, 6). The latter is done twice to thrice per week, with the optimal time for sun exposure being midday (6).

How to keep the skin protected from UV rays?

Photoprotection involves both primary (like sunscreens that either scatter the light or absorb it) and secondary (like antioxidants (such as vitamins C and E) and DNA repair enzymes that limit skin damage) protective factors (3).

Sunscreens are meant to protect against UV rays. They are divided into physical and chemical sunscreens. The former reflects and scatters light just like clothing, while the latter absorbs UV rays (3).  Sun protection factor (SPF) measures how much UV radiation is required to produce sunburn on protected skin (7, 8). In other words, it is the ratio of UV radiation required to burn protected skin (with sunscreen) to that required to burn the same unprotected skin (without sunscreen), with all other factors being constant. A sunscreen with SPF 15 blocks 93% of UVB radiation (2). The Centers for Disease Control and Prevention recommends using a sunblock with SPF-15 or higher (9). No sunscreen can stop 100% of UV rays. It is important to note that the higher the SPF, the more time skin is protected. For example, a sunscreen with SPF 15 can protect against harmful UV rays for up to 150 minutes (15 times longer than unprotected skin).

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What are the negative implications of unprotected sun exposure?

Just as sun exposure is needed to maintain general health, prolonged exposure could potentially put people at risk of many diseases. Usually, these effects result from UV rays penetrating the skin (10). According to the literature, prolonged, unprotected sun exposure could cause:

Negative outcome

Explanation

Photoaging

Just like all organs, the skin undergoes deleterious effects due to aging. But unlike other organs, the skin is exposed to many additional factors that might accelerate the aging process, such as chronic, unprotected sun exposure (11). UVA radiation causes photoaging that is additive to the effect of normal aging (11, 12). This causes skin areas exposed to the sun to display signs of more rapid aging (11). The aging process compromises skin barrier efficiency and thermoregulation capacity. Also, it affects the skin’s immune surveillance functionality and sensory role (11). Visually, photoaging appears in the form of premature hyperpigmentation (skin damage) and wrinkles (skin aging) (12).

Ocular damage

Long-term, unprotected exposure of the eyes to sunlight has negative consequences, like cataracts, malignancies, and other conditions (13). Some of the sun-induced eye damage is acute, such as photokeratitis (14). On the other hand, chronic exposure causes more serious conditions, like uveal melanoma and age-related macular degeneration (mainly due to blue light).

Skin cancer

Prolonged, unprotected exposure to the sun causes skin cancer. Examples of the condition include melanoma, basal cell, and squamous cell carcinomas (5). The latter two are keratinocyte cancers, representing the majority of skin cancers. But melanoma is the leading cause of death (5). Sunburns are the main drivers of these cancers. In this context, the literature indicated that individuals with a higher number of sunburns are at a greater risk of developing these cancers than their counterparts without sunburns.

Immunosuppression

UV-induced immunosuppression is one of the driving mechanisms behind skin cancer (2). This is because a weakened immune system cannot remove damaged skin cells before turning malignant.

The bright side of the sun

The sun plays an essential role in promoting our overall health. In addition to vitamin D, the literature mentions many health benefits from sun exposure, like treatment of skin diseases such as psoriasis and management of depression (2). The benefits of sun exposure include:

Positive outcome

Explanation

Vitamin D synthesis  

Sun exposure is essential for synthesizing vitamin D (2). According to the literature, the prevalence of vitamin D deficiency amounts to up to 1 billion individuals worldwide (15). Vitamin D supports calcium absorption, which is vital for bone health. This is important for children to prevent rickets and for the elderly to reduce the risk of osteoporosis (2). In addition, vitamin D plays an important role in promoting longevity. In this context, research has shown that the mortality rate in those with vitamin D deficiency is almost doubled compared to those with normal levels (16-19). Also, having sufficient plasma vitamin D levels has been suggested to reduce the risk of cancers like breast and colorectal carcinoma (5, 10).

Promote circadian rhythms

Sun exposure is important in regulating the physical, mental, and behavioral changes throughout the 24-hour cycle. Sun exposure influences the circadian rhythm by acutely suppressing melatonin upon light exposure and shifting the circadian phase (20). Sunlight exposure also influences mood and affects the availability of neurotransmitters, like serotonin, throughout the day. This is why light therapy is suggested as a treatment modality in psychiatric conditions like seasonal affective disorder (20). Preclinical research indicates that a disturbed circadian rhythm could shorten the lifespan by up to 20% (21).

Cardiovascular diseases (CVDs)

Research has suggested an inverse relationship between sunlight exposure, blood pressure, and CVDs. In this context, sunlight exposure has been shown to promote nitric oxide, reducing arterial resistance (5, 19, 22). UVA exposure lowered blood pressure, independent of temperature and serum vitamin D levels. Further investigation is needed to verify such results (19).

Metabolic syndrome

The literature suggests that sunlight exposure reduces the risk of metabolic syndrome. In this context, evidence suggests that exposure to UV rays decreases the risk of obesity and type 2 diabetes (5, 22). Other studies have linked these beneficial outcomes to vitamin D levels, where an inverse relationship exists between vitamin D plasma levels and the said diseases. Others have highlighted that this effect is independent of vitamin D (22).

Neurological disorders

Studies have linked sunlight exposure with vitamin D and neurological disorders, like cognitive decline and autism (5). In this context, research has shown that individuals with insufficient vitamin D levels were at greater risk of cognitive decline (22).

Respiratory diseases

Through vitamin D, sunlight exposure supports the health of the respiratory system. In this context, evidence suggests that lower serum vitamin D levels are linked to higher asthma and infection rates (5).

Autoimmune diseases

Multiple studies have highlighted that lower sun exposure has been linked to autoimmune disorders like multiple sclerosis (22). This was independent of vitamin D status.

Liver disorders

Recent studies suggest that exposure to UV rays reduces the risk of non-alcoholic fatty liver (22). Evidence highlights that this occurs through vitamin D independent and dependent pathways.

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Tips to maximize the benefit and minimize the harm from sun exposure

You could give your clients a few tips to help them safely enjoy their outdoor time. These include:

• Explain unprotected sun exposure is needed to get sufficient vitamin D levels. This is especially important for the elderly and children.
• Recommend that your clients have 15-30 minutes (depending on region) of unprotected sun exposure twice to three times a week. The optimal time would be around midday.
• Mention that light-skinned people are at a greater risk of sunburns. Therefore, they should use sunscreens.
• Explain to your clients that UV rays penetrate the clouds, so they should apply sunscreen even on cloudy days.
• Highlight the importance of using a broad-spectrum, water-resistant sunscreen to protect against the harmful effects of UV rays.
• Recommend that your clients use sunscreen with an SPF of at least 15 or higher (higher is better) to reduce the risk of skin cancer and early skin aging. Mention that no evidence supports that sunscreens with SPF of more than 50 provide any value.
• Explain to your clients that sunscreen should be applied 15 minutes before sun exposure. Highlight that applying sunscreen between 10 AM and 2 PM is crucial.
• Mention that the optimal way to apply sunscreen involves 2 mg of product per 1 cm², equivalent to 30 mL/ body application.
• Highlight the importance of reapplying the sunscreen every two hours and after sweating or swimming.
• Explain to your clients that they might suffer from irritant contact dermatitis, especially if they have sensitive skin or a history of eczema. If they start noticing irritation in areas where sunscreen is applied, ask them to check with their healthcare professional for alternative options.
• Recommend that your clients wear protective sunglasses when they are out in the sun.
• Highlight that prolonged, unprotected sun exposure could potentially increase their risk of skin cancer, premature skin aging, and weakened immunity.
• Educate your clients about the importance of sufficient sun exposure and how it can promote longevity.

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Sun exposure is best when done in moderation

By now, we have established the benefits and risks associated with exposure to sunlight. When done in moderation and in alignment with the abovementioned suggestions, sunlight carries multitudes of health benefits. In addition, it could promote longevity and reduce the risk of multiple diseases.

References

1.            Jindal AK, Gupta A, Vinay K, Bishnoi A. Sun Exposure in Children: Balancing the Benefits and Harms. Indian Dermatology Online Journal. 2020;11(1).

2.            Dale Wilson B, Moon S, Armstrong F. Comprehensive review of ultraviolet radiation and the current status on sunscreens. J Clin Aesthet Dermatol. 2012;5(9):18-23.

3.            Gabros S, Nessel TA, Zito PM. Sunscreens and photoprotection. 2019.

4.            Chandra P, Wolfenden LL, Ziegler TR, Tian J, Luo M, Stecenko AA, et al. Treatment of vitamin D deficiency with UV light in patients with malabsorption syndromes: a case series. Photodermatol Photoimmunol Photomed. 2007;23(5):179-85.

5.            Alfredsson L, Armstrong BK, Butterfield DA, Chowdhury R, de Gruijl FR, Feelisch M, et al. Insufficient Sun Exposure Has Become a Real Public Health Problem. Int J Environ Res Public Health. 2020;17(14).

6.            Lee YM, Kim SA, Lee DH. Can Current Recommendations on Sun Exposure Sufficiently Increase Serum Vitamin D Level?: One-Month Randomized Clinical Trial. J Korean Med Sci. 2020;35(8):e50.

7.            Latha MS, Martis J, Shobha V, Sham Shinde R, Bangera S, Krishnankutty B, et al. Sunscreening agents: a review. J Clin Aesthet Dermatol. 2013;6(1):16-26.

8.            Geoffrey K, Mwangi AN, Maru SM. Sunscreen products: Rationale for use, formulation development and regulatory considerations. Saudi Pharm J. 2019;27(7):1009-18.

9.            Sun Exposure | Travelers' Health | CDC Cdc.gov: Centers for Disease Control and Prevention; 2022 [updated December 13, 2018; cited 2022 03-08]. Available from: https://wwwnc.cdc.gov/travel/page/sun-exposure.

10.          Queirós CS, Freitas JP. Sun Exposure: Beyond the Risks. Dermatol Pract Concept. 2019;9(4):249-52.

11.          Rittié L, Fisher GJ. Natural and sun-induced aging of human skin. Cold Spring Harb Perspect Med. 2015;5(1):a015370.

12.          Kimlin MG, Guo Y. Assessing the impacts of lifetime sun exposure on skin damage and skin aging using a non-invasive method. Sci Total Environ. 2012;425:35-41.

13.          Backes C, Religi A, Moccozet L, Behar-Cohen F, Vuilleumier L, Bulliard JL, et al. Sun exposure to the eyes: predicted UV protection effectiveness of various sunglasses. J Expo Sci Environ Epidemiol. 2019;29(6):753-64.

14.          Behar-Cohen F, Baillet G, de Ayguavives T, Garcia PO, Krutmann J, Peña-García P, et al. Ultraviolet damage to the eye revisited: eye-sun protection factor (E-SPF®), a new ultraviolet protection label for eyewear. Clin Ophthalmol. 2014;8:87-104.

15.          Sizar O, Khare S, Goyal A, Bansal P, Givler A. Vitamin D deficiency.  StatPearls [Internet]: StatPearls Publishing; 2021.

16.          Gröber U, Reichrath J, Holick MF. Live longer with vitamin D? Nutrients. 2015;7(3):1871-80.

17.          Milman S, Schulder-Katz M, Deluty J, Zimmerman ME, Crandall JP, Barzilai N, et al. Individuals with exceptional longevity manifest a delayed association between vitamin D insufficiency and cognitive impairment. J Am Geriatr Soc. 2014;62(1):153-8.

18.          Zeng J, Li T, Sun B, Miao X, Wang L, Ma LC, et al. Change of vitamin D status and all-cause mortality among Chinese older adults: a population-based cohort study. BMC Geriatr. 2022;22(1):245.

19.          Lindqvist PG, Epstein E, Landin-Olsson M. Sun Exposure - Hazards and Benefits. Anticancer Research. 2022;42(4):1671.

20.          Blume C, Garbazza C, Spitschan M. Effects of light on human circadian rhythms, sleep and mood. Somnologie (Berl). 2019;23(3):147-56.

21.          Acosta-Rodríguez VA, Rijo-Ferreira F, Green CB, Takahashi JS. Importance of circadian timing for aging and longevity. Nature Communications. 2021;12(1):2862.

22.          Hoel DG, Berwick M, de Gruijl FR, Holick MF. The risks and benefits of sun exposure 2016. Dermatoendocrinol. 2016;8(1):e1248325.

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