How high an SPF should you wear?

The average person can be in the sun for about 10 minutes before they start to burn. The SPF increases the amount of time you can be in the sun before you burn.



So haul out your maths and let’s calculate that length of time by multiplying the number on the bottle by the 10 minute average.


If you wear a sun protection factor 30 and you hit the beach, how long have you got?

30 x the 10 minute average = 300 minutes before you are likely to burn.

A 15 SPF?

15 x 10mins = 150 minutes before you burn (See. You rock at maths.)

There is a catch though.

You have to apply 2mg of sunscreen per square cm of skin to achieve the physical barrier between your peaches and cream and that fireball for the SPF to work. (I know. More maths. It’s about a US nickel or ZA 50 cent coin sized blob for your face alone.)

posugar sunscreen

So if you’re going out all day (say a 4 hour morning on the beach) can you whack on a tot measure of factor 50 and be good to go?

(After all it’s 50 x 10 = 500 minutes, right? That’s like, a zillion hours?)

Sorry no. We move around, swim, towel off, even if we are not at the beach our clothes rub against the areas we’ve applied sunscreen, so you need to reapply every 2 hours, no matter the SPF you’ve used.

So why bother with a higher SPF ?

Well there’s this, from Sunscreen 101 :

“Testing an SPF of 30+ allows 3.23 per cent of the UV light that causes sunburn to get through the sunscreen film, whereas an SPF 50+ only allows 1.66 per cent to get through.”

But is this slight benefit of a 50+ SPF  worth it?

Not all experts think so.

The ingredients that safely create a sunscreen  are  physical barriers, like Titanium Dioxide and Zinc Oxide. (The white stuff that lingers. You know,  the stuff you see smeared on cricketers and surfers?) These reflect the UV rays.

The dubious ingredients in sunscreens are chemicals. To raise the SPF, manufacturers need to pour in more chemicals, and the one with the dodgy reputation is called Oxybenzone.  Some schools of thought suggest these chemicals contribute to hyperpigmentation and hormone problems. However, the American Academy of Dermatology states that it is safe.

CNN reporter Daniella Dellorto explains however, that the benefit of a high SPF is marginal. “Studies show that sunscreen with SPF 15 can block about 93% of all incoming UVB rays. SPF 30 blocks 97%. SPF 50 blocks 98%.”

SkinCancer.Org  dermatology expert,Dr Steven Wang, explains the down side of using very high SPF sunscreen (50+)

” above SPF 50 (which blocks an estimated 98 percent of UVB rays), the increase in UVB protection is minimal. ”

The article continues:

‘Products with very high SPFs may also encourage individuals to neglect other photoprotective behaviors, like seeking the shade and wearing sun-protective clothing… By preventing sunburn, sunscreens with very high SPFs can create a false sense of security, prompting consumers to stay out in the sun longer.  Sun damage (for example, UVA damage) can take place without skin-reddening doses of UV radiation’

Young man applying suntan lotion on a young woman's back at the beach

So lovelies, the conclusion appears to be this:

  1. Wear an SPF of at least 15 but preferably 30 or 40.  (That will give you 2 hours)
  2. Reapply every 2 hours. (Better, go inside. Over 2 hours is too long.)
  3. Lay it on thick. Big dollops.

Enjoy your happy, safe, un-sunburned summer days, Northern Hemisphere.

And cheer up Southern Hemispherites. Summer will be back before you know it. In the meantime, keep up with the sunscreen. We are, you will remember, living under the Ozone Hole. And, you don’t have to burn to get sun damage.


This is the Fitzpatrick scale. It will help you to see where you are in relation to average:

Fitzpatrick skin classification with text

Related: My favorite sunscreens. Coming soon…



One thought on “How high an SPF should you wear?

  1. Pingback: Why you need to wear sunscreen in winter | sunrisebeautyblog

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