Choosing the best sunscreen can be tricky as there are many products on the market to chose from. Many people also wonder what different SPF values translate to in terms of sun protection, whether adding zinc is a good idea and what broad spectrum actually means.
To help answer these questions it is best to go back to the basics with sun protection. Most people are aware it is the ultraviolet radiation (UV) from the sun that causes skin damage. UVA rays deeply penetrate the skin and are responsible for photo-ageing in the form of skin wrinkling and solar lentigo (age spot) formation. UVB rays cause the skin to become red from sunburn.
A sunscreen's sun protection factor or SPF measures its effectiveness against UVB radiation. Broad-spectrum protection refers to the sunscreen's ability to protect against UVA + UVB radiation. The Australian and New Zealand standard for sunscreens (AU/NZ 2604:2012) is compulsory in Australia but voluntary in New Zealand. This means sunscreens that don't meet any international standard are still allowed to be sold in New Zealand, which is quite concerning!
A recent Consumer New Zealand independent test of 20 common sunscreens sold in New Zealand found several had SPF values of 5 or less but were marketed as having SPF values of 15 or more. This is a case of buyer beware and adds weight to the argument that the standard for sunscreens be mandatory in New Zealand instead of just voluntary. Access the Consumer New Zealand article here.
Sunscreens can be divided into two main groups: physical blockers + chemical absorbers. Physical blockers include zinc oxide and titanium dioxide which work by reflecting both UVA + UVB radiation. They are very minimally absorbed so potentially better for sensitive skin. One drawback with physical blockers is that higher concentrations don't rub into skin well and can leave white marks. Chemical absorbers include octocrylene + oxybenzone and work by absorbing UVA + UVB radiation.
The best place to start when choosing a sunscreen is to consider what skin type you have. If it is very light and mainly burns without sunscreen then a high value SPF such as 50+ with broad spectrum cover is recommended. If you have skin that only tans or is quite dark then a sunscreen with SPF 15+ may be quite satisfactory. If you have sensitive skin avoid sunscreen with a preservative called metholisothiazolinone (MIT) which can be responsible for skin irritation and allergic reactions. Adding in a physical blocker like zinc oxide in a low percentage eg. 10% to a chemical absorber sunscreen is likely to give you the best of both worlds but not make your skin look too white.
Also consider what you need the sunscreen for. Are you heading for a day on the beach where you'll be in the water, a round of golf where you be sweating a bit or are you stuck in the office and only going to see the sun on your lunch-break? If you are predominantly outside and active with potential for sweating or in the water then consider a sunscreen with water resistance also. It is recommended that sunscreen be reapplied every two hours and because sunscreen is often not applied in all areas exposed to the sun or applied thickly enough by some people it is important to remember other methods of sun protection also such as wearing a hat and long-sleeved shirt as well as sunglasses.
See Dermnet for another great resource on how to choose a sunscreen and further information about getting enough vitamin D.