Most people's skin darkern when exposed to UV light giving them natural shelter when necessary, which is the physiological purpose of sun tanning.
Dark-skinned people, who produce more skin-protecting melanin, have a greater protection against sunburn and development of melonoma (potentialy deadly form of skin cancer), however they also have greater risk of vitamin D deficiency for the same reason .
We divide UV rays into: UVA rays (those going deeper into the skin causing skin aging and wrinkles) and UVB (causes sunburn, but also allows skin vitamin D production from 7-dehydrocholesterol) .
Too much UV rays can have a harmful effect on skin, increasing the risk of getting cancer and accelerating skin aging.
Using sunscreens might be a good idea to protect against excessive sunlight and mitigate the risk of sunburns.
Sunscreen by it's definition is a lotion, spray, gel or other topical product that absorbs or reflects the sun's ultraviolet (UV) radiation.
Depending of the mechanism of action they come as a physical sunscreens (inorganic, those which reflect the sunlight i.e. containing zinc oxide or titanium dioxide) and those which absorb the UV light (organic – not in a sense that they are natural, rather that from chemical viewpoint are carbon based) which allow the UV energy to dissipate in a more safe way .
The chemical filters (organic in a chemical sense) can contain active ingredients such as: oxybenzone, avobenzone, octisalate, ococrylene, homosalate and octinoxate .
Proofs of work
According to the studies suscreen can prevent melanoma and squamous cell carcinoma so it is reasonable to use them on your summer holiday as they have been proven to work .
Some of the sunscreens only block UVB, but actually an excess of UVA radiation increases the risk to melanoma nad photodermatitis .
Therefore it is adviced to use broad-spectrum (UVA and UVB) sunscreens to address those concerns.
It is known that exposure to UVB radiation is a source for more than 90% of vitamin D production in most of the individuals.
Some studies have been shown that typical use of sunscreen does not lead to vitamin D deficiency, however extensive usage does .
Therefore, prior to holidays it might be worth checking the level of 25-hydroxyvitamin D in your blood, and then decide how much protection you need.
Chemical dissruptors and allergies
Some of the active ingredients in sunscreens act like a hormonal dissruptors (can mimimic human hormones, particulary estrogen) or cause skin allergies.
The table below (source: Environmental Working Group)  outlines human exposure and toxicity information for nine FDA-approved sunscreen chemicals.
Now, armed with magnifying glass, you can screen your sunscreen's ingredients list and spot the substances which can bring more harm than good (particulary oxybenzone and octinoxate).
|Chemical||EWG Hazard Score||Use in U.S. sunscreens||Skin Penetratio||Hormone disruption||Skin Allergy||References|
|UV filters with higher toxicity concerns|
|Oxybenzone||8||Widespread||Detected in nearly every American; found in mother’s milk; 1-to-9% skin penetration in lab studies||Acts like estrogen in the body; alters sperm production in animals; associated with endometriosis in women||Relatively high rates of skin allergy||Janjua 2004, Janjua 2008, Sarveiya 2004, Gonzalez 2006, Rodriguez 2006, Krause 2012|
|Octinoxate (Octylmethoxycinnamate)||6||Widespread||Found in mothers’ milk; less than 1% skin penetration in human and laboratory studies||Acts like estrogen in the body; alters sperm production in animals; associated with endometriosis in women||Moderate rates of skin allergy||Krause 2012, Sarveiya 2004, Rodriguez, 2006, Klinubol 2008|
|UV filters with moderate toxicity concerns|
|Homosalate||4||Widespread||Found in mothers’ milk; skin penetration less than 1% in human and laboratory studies||Disrupts estrogen, androgen and progesterone||Krause 2012, Sarveiya 2004, SCCNFP 2006|
|Octisalate||3||Widespread; stabilizes avobenzone||Skin penetration in lab studies||Rarely reported skin allergy||Walters 1997, Shaw 2006 Singh 2007|
|Octocrylene||3||Widespread||Found in mothers’ milk; skin penetration in lab studies||Relatively high rates of skin allergy||Krause 2012, Bryden 2006, Hayden 2005|
|UV filters with lower toxicity concerns|
|Titanium Dioxide||2 (topical use), 6 (powder or spray)||Widespread||No finding of skin penetration||No evidence of hormone disruption||None||Gamer 2006, Nohynek 2007, Wu 2009, Sadrieh 2010, Takeda 2009, Shimizu 2009, Park 2009, IARC 2006b|
|Zinc Oxide||2 (topical use), 4 (powder or spray)||Widespread; excellent UVA protection||Less than 0.01% skin penetration in human volunteers||No evidence of hormone disruption||None||Gulson 2012, Sayes 2007, Nohynek 2007, SCCS 2012|
|Avobenzone||2||Widespread; best UVA protection of chemical filters||Very limited skin penetration||No evidence of hormone disruption||Breakdown product causes relatively high rates of skin allergy||Klinubol 2008, Bryden 2006, Hayden 2005, Montenegro 2008, Nash 2014|
|Mexoryl SX||2||Uncommon; pending FDA approval; offers good, stable UVA protection||Less than 0.16% penetrated the skin of human volunteers||No evidence of hormone disruption||Skin allergy is rare||Benech-Kieffer 2003, Fourtanier2008|
|6 other ingredients approved in the U.S. are rarely used in sunscreens: benzophenone-4, benzophenone-8, menthyl anthranilate, PABA, Padimate O, and trolamine salicylate|
Mineral vs Chemical (organic)
The mineral based (inorganic) sunscreens tend to do better in the comparison of active ingredients.
Sunscreens based on zinc oxide (ZnO) or titanium dioxide (TiO2) (yes, those one which left the white marks on your skin).
They seems to be stable in sunlight, offer broad-spectrum protection (UVA/UVB) and don't often contain potentially harmful additives.
Morover, the current weight of evidence suggest that TiO2 and ZnO do not reach viable skin cells, instead remaining on the outer layer of skin that is composed of non-viable, keratinized cells.
Growing body of evidence suggest that organic (carbon based) sunscreens can have several harmful effects, therefore benefit/risk ratio suggest topical usage of inorganic one (TiO2 and ZnO) over the organic one [4,5].
Sunscreens are normally rated and labeled with a sun protection factor (SPF) that measure the fraction of sunburn-producing UV rays that reach the skin.
It is crucial, that the difference between SPF-15, 30, 45, 60 etc. is neglible (2% or less) and sunscreens with SPF 15 block approximately 94% of all UV rays. The SPF-30 will block aproximately 96% and so on so forth.
Other UV shelters
You might consider other ways than sunscreens to protect your skin such as :
- wearing shirts, hats, shorts and pants can reduce the negative impact of UV rays by 27%
- trying sunbathing in early morning and late afternoon rather than in the middle of the day (10 am – 4 pm)
- when your skin turn pink, its best to cover your body up or get some shade to cool down
- try to find some shade under a palm/tree on the beach. This is especially important for children
- protect your eyes using sunglasses with UVA and UVB filter
- tomato derived product can protect you against sunburns, one study has shown that 40g tomato paste with 10g olive oil, taken for 10 weeks decreased risk of sunburn by 40% - you might then consider a tomato diet prior your summer holidays 
In conclusion if your severely vitamin D deficient, using sunscreen might not be a good idea, however when going on longer sunny holiday you can use sunscreen with SPF-15, UVA and UVB protection and never ever overpay for the creams with SPF > 15 as the bottles can be 2-times smaller and even more expensive (at the same have failed to show that are more effective that SPF-15).
In terms of health hazard of sunscreens, you should go for inorganic forms containing zinc oxide or titanium dioxide as they are more safer choice than organic forms (act like a endocrinal dissruptors and can cause skin allergies).
- https://www.youtube.com/watch?v=bvG4sy_YfXM as seen on 28/08/2016 Sunscreen works, if you use it right. Aaron E. Carroll
- https://en.wikipedia.org/wiki/Sunscreen as seen on 28/08/2016
- Norval M., Wulf H.C., Does chronic sunscreen use reduce vitamin D production to insufficient levels. Br J Dematol 2009 Oct; 161(4): 732-6
- http://www.ewg.org/sunscreen/report/the-trouble-with-sunscreen-chemicals/ as seen on 28/08/2016
- Schlimpf M., Schimd P., Durrer S., Conscience M., Maerel K., Henseler M., Gruetter M., Herzog I., Reolon S., Ceccatelli R., Faass O, Struz E., Jarry H., Wuttke W., Lichtensteiger W. Endocrine activity and developmental toxicity of cosmetic UV filters an update. Toxicology 2004 Dec 1; 205(1-2): 113-22
- https://en.wikipedia.org/wiki/Melanin as seen on 28/08/2016
- Stahl W., Heinrich U., Wiseman S., Eichler O., Sies H., Tronnier H., Dietary tomato paste protects against ultraviolet light-induced erythema in humans. J Nutr. 2001 May; 131(5) 1449-1451