Applying sunscreen is the best way to protect against skin cancer as it works either by absorbing harmful ultraviolet (UV) rays or scattering them.

However, it will fail to protect you adequately if it is applied inappropriately. Often, it is applied too thinly or not frequently enough.

Many Singaporeans are fixated on the Sun Protection Factor (SPF) of sunscreen, thinking the higher it is the better, said Associate Professor Andrew Tan from the Nanyang Technological University's School of Biological Sciences.

"They think using SPF100 sunscreen means it will last the entire day but no, you must always reapply for it to be effective," he said.

Generally, applying sunscreen once every two hours when one is outdoors is encouraged.

About 2mg is needed for each square centimetre of skin, which amounts to a blob the size of a 20-cent coin for the face alone.

Prof Tan said that SPF15 and SPF30 sunscreens already protect against 93 to 97 per cent of UVB rays. An SPF level that is any higher provides little improvement in protection.

SPF is a measure of how much longer you can stay in the sun before your skin burns. If you usually get burned in 10 minutes, using an SPF15 sunscreen would mean your skin will start to burn only after 150 minutes.

There are two types of sunscreen in the market: physical blockers and chemical blockers.

Physical blockers, which tend to contain titanium dioxide or zinc oxide, protect by scattering UV rays before they penetrate the skin. They do not get absorbed by the skin and generally provide broad-spectrum protection from both UVA and UVB rays, said Prof Tan.

Chemical-based blockers work by absorbing the UV rays that hit the skin. Fewer of such blockers protect against both UVA and UVB, he said.

UVA rays penetrate deeper into the skin, and are known to age skin cells and can cause wrinkles. UVB rays, on the other hand, are behind sunburn. Both cause the development of skin cancers.

Prof Tan said one should apply sunscreen on the exposed parts of the body before leaving home every day and reapply every two hours if under the sun for extended periods of time.

"As UVA rays can penetrate clothes, people could also consider applying sunscreen under their clothes if they are going to be outdoors for hours," he said.