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.

Transcript below.


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.

Show your support

Clapping shows how much you appreciated Lassonde School of Engineering’s story.