UV radiation is the major cause of skin cancer but many of us rely on heat and humidity as a guide to sun protection, according to a survey by Cancer Council Victoria.
That's because UV rays can't be detected by human senses, says SunSmart Manager Heather Walker.
"It's not like sunlight that we can see, or the sun's heat, which we can feel," Ms Walker said.
"As a result, we usually don't realise how strong the rays are until the damage has already been done."
The survey found 22 per cent of young Victorian adults wrongly selected temperature as the most useful measure to calculate sunburn risk for the day.
Just 61 per cent correctly identified UV level as the best measure to determine sunburn risk.
A new app that makes UV rays visible to the human eye has been launched to help people protect their skin from sunburn this summer.
The SunSmart app - seeUV- developed at Deakin University, uses augmented reality technology and real time data from the bureau of meteorology to provide a visual image of hidden UV and alert users when it's at dangerous levels.
A UV Index reading of 3 to 5 signifies moderate risk of harm from unprotected sun exposure. At this level people are advised to stay in shade during the middle of the day when the sun is strongest.
The seeUV app also has a selfie mode that uses the same augmented reality technology to generate the long-term consequences of UV damage on a person's skin, such as premature ageing, wrinkles and sunspots.
© AAP 2017