A good quality sunscreen product should offer both UVB and UVA protection. Sunscreen ingredients can be organic chemical agents (e.g. oxybenzone, anisotriazine) that absorb UV radiation or inorganic agents (e.g. titanium dioxide) that both absorb and scatter UV radiation.

The sun protection factor (SPF) describes how well the product protects against UVB damage, based on the time it takes unprotected skin to start to turn red. For example, if this takes 10 minutes without protection, then using an SPF 30 protects the skin for 30 times longer, so 300 minutes or five hours.

UVA protection is more difficult to describe because there is no early warning of UVA damage like the burning caused by UVB. The star rating indicates how well the product protects against UVA on a scale of one to five. A four- or five-star rating is ideal.

Sunscreen products must be applied in sufficient quantity to provide the level of protection indicated by the SPF and star rating. As a rule of thumb, about six full teaspoons of suncream are needed for one adult body. Two teaspoons of sunscreen are enough for the head, arms and neck. To maintain the level of protection, sunscreen should be re-applied every two hours to replace product lost by washing and/or swimming, rubbing or sweating.

Myths and misconceptions about sunscreens can prevent effective use of the products. The following points should be noted:

• There is no evidence that commonly used sunscreen agents are carcinogenic, but there is compelling evidence that UV radiation is carcinogenic
• Allergy to sunscreen ingredients is rare
• Sunburn is usually the result of poor application rather than a reaction to the product
• Two applications of SPF 15 are not equivalent to one application of SPF 30
• It is still possible to get sunburnt on a cloudy day because clouds scatter UV light rather than absorbing it.

One good way to assess the level of UV radiation is to check the UV index for the current location. The UV index ranges from one to 11+. When the index is three or more, there is enough UV to damage skin and therefore sunscreens are required. A number of apps that provide the UV index are available for smartphones.