SPF explained by the inventor of the UV Index
Ever wondered what SPF means? As co-inventor of the UV Index Tom McElroy explains, it’s not just an arbitrary number.
In order to understand SPF, we have to first explain UV rays
10 % of light from the sun is UV rays
which are harmful to our skin in high doses.
Sunscreen limits our skin’s exposure to UV rays
increasing the amount of time we can safely spend in the sun
SPF (Sun protection factor)
Measures the fraction of UV rays that will pass through
The sunscreen and reach your skin
For example SPF 30 means that only about 1/30th of the UV rays
will actually penetrate through to your skin.
If 10 minutes in the sun would give you a sunburn
SPF 30 sunscreen could theoretically increase that threshold to 300 minutes.
But sunscreen typically wears off after 2-3 hours (regardless of SPF)
which is why it’s important to reapply sunscreen as needed.
On that note SPF above 50 doesn’t give your skin better protection
It’s just advertising
SPF is not just an arbitrary number
It’s a great guide for staying sun-safe all year round.
About Tom McElroy
Tom McElroy is a professor at the Lassonde School of Engineering in Toronto, Ontario. As co-inventor of the UV Index, his work has been highlighted in publications such as the Toronto Star. In addition to the UV Index, McElroy also developed the Brewer Ozone Spectrophotometer, an instrument that accurately measures the amount of ozone in the atmosphere. Professor McElroy received a United Nations Innovators Award for his pioneering work.