Solar Keratoes (sun spots): large studies have shown that regular use of sunscreen does prevent the development of solar keratoses.
Squamous Cell Carcinoma (SCC): regular use of sunscreen does reduce the risk of developing SCC. This effect occurs even if sunscreen use starts later in life.
Basal Cell Carcinoma (BCC): Childhood sun exposure is the most important risk factor for BCC. Sunscreen does reduce BCC development but studies have only been fairly short (4-5 years) and longer studies are required as there is a very long latent period with BCC after sun-exposure in childhood.
Melanoma: Some reports may show an increased risk of melanoma but a recent meta-analysis shows both no increased and no reduced risk with sunscreen use. But circumstantial evidence including evidence from solaria use and childhood sun-exposure and sunburn does seem to indicate that exposure to UV rays is a risk factor for melanoma. The amount of naevi on the skin is a strong marker for melanoma and sunscreen in kids has been shown to reduce the development of naevi and may therefore reduce the incidence of melanoma.
So the answer is YES: sunscreen reduces skin cancer.

What harm can sunscreen do?

Nanoparticles: There is some discussion whether nanoparticles in nanonised Zinc and Titanium are a health risk. Zinc and Titanium have been used in Australia for 15 years and longer overseas and there is currently no evidence for risk but there certainly is a large benefit.
Chemical sunscreen: There has been some discussion about the effects of absorbed chemicals that are used in sunscreen - there is no evidence of any harmful effects - sunscreens have been used for over 50yrs. Theoretically, some compounds can penetrate the skin but this would only occur in very small amounts which would be metabolised quickly and excreted. Some people are concerned about the oestrogenic effects of some of these chemicals especially in young growing males - there have been some in vitro and animal studies showing an effect but this is thought to be very insignificant in humans compared to other sources of oestrogenic effects (eg soya milk).
Vitamin D: This is now known to prevent lots of diseases and the sun producing UVB through the skin surface makes Vitamin D. Studies in high risk individuals have shown that using sunscreen does not cause Vitamin D deficiency nor osteoporosis. The bottom line is that skin cancer avoidance strategies and Vitamin D nutrition are not contradictory.
The greatest harm a sunscreen has is the harm of not using it. There is no evidence it is linked to any disease.
Sunscreen failures are not due to the sunscreen itself, generally, but are due to not using enough, not reapplying often enough or not reapplying after swimming.

Which sunscreen is best?

The answer to this is essentially the sunscreen the patient/customer is likely to use correctly. Public education is important. Sun protection by adolescents has declined over the last 10 years due to lack of immediacy. In a 2005 and 2004 a study in summer over a single weekend showed that 18 % of adults reported getting sunburnt and 25 % adolescents reported getting sunburnt.

The correct amount of sunscreen to use is 2 mg/cm² which equates to 30-40ml to cover an adults body. This means that if one buys 125ml bottle the entire bottle should be used for 1 day on the beach per person. Approximately 5ml or 1 teaspoon should be used on the face and neck. Most people do not use enough sunscreen, using only approx. 0.5-1.3mg per cm². This dramatically reduces the effectiveness of the sunscreen.

SPF30+ broad spectrum is best,t but essentially any product the patient will use is good eg. young males may prefer an alcohol spray or older females may prefer a sunscreen with anti-ageing benefits and a moisturiser.

What's new in sunscreen?

Dual protection anti UVA and UVB which means that a single chemical can do what multiple chemicals previously did.
Nicotinamide/niacin/vitamin B3. This is added to newer sunscreen. This helps to reduce: damage caused by sun, improves skin barrier properties (helps sensitive skin), helps oiliness, helps appearance of skin.

Consumers cannot be told the real SPF of sunscreen but doctor's can and therefore can recommend the higher SPF.

Ref: Dr Kerryn Greive, Ego Pharmaceuticals, Lecture, Skin Cancer Conference, Brisbane, July 30th-August 2nd, 2009

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