Application Amount

Sunscreens are tested at two milligrams for every square centimetre of skin (2mg/ cm2). Although most sources convert this to at least half a teaspoon of sunscreen for face and neck, in 2013 an article was published that challenged accepted practice.

It proposed “one teaspoon to face, head, and neck [taking] into account that face and head is the most exposed site for most individuals and that for most head is totally or partially covered by hair.” The recommendation has not been widely adopted, but is sobering considering average application rates are less than half of current guidelines (as low as 0.5mg/ cm2).

So what is the effect of applying less than the recommended amount of sunscreen? Here too sources disagree! This chart from a 2003 review article is often referenced as demonstrating a non linear relationship between SPF and application rate, but is based on research published in 1997 that used theoretical calculations and not in vivo testing.

SPF 2mg/cm2 1.5mg/cm2 1mg/cm2 0.5mg/cm2 2 2 1.7 1.4 1.2 4 4 2.8 2 1.4 8 8 4.8 2.8 1.7 15 15 7.6 3.9 2 30 30 12.8 5.5 2.3 50 50 18.8 7.1 2.7

Data set: Pinnell, 2003 adapted from Wulf et al, 1997

A decade later a team from the same group used human subjects and again concluded that “the relation between SPF and sunscreen quantity follows exponential growth.” However the sample group was small, they used a non standard testing protocol and only tested a single SPF 4 product.

Conversely a 2012 study found that “there was a linear relationship between application density and the actual SPF; sunscreens with labeled SPF values of 70 and above provided significant protection.” Although the research was carried out by a commercial entity (Neutrogena) it seems to have been well executed, and was published a highly reputable journal. In the first phase 251 human subjects from three Fitzpatrick skin types were included, and an SPF 4 control was employed.

SPF 2mg/cm2 1.5mg/cm2 1mg/cm2 0.5mg/cm2 30 (lotion) 31 21.8 16 8.8 50 (lotion) 52.8 41.3 26 13.9 70 (lotion) 70.6 54.4 37.1 19.3 100 (lotion) 104.6 79.3 55.9 27.1 50 (spray) 50.7 38.6 25.7 12.6 100 (spray) 105.3 75 50.1 22.4

Data set: Neutrogena, 2011

An earlier study on two products with lower protection (SPF 15 and SPF 30) found that “despite the exponential relation, our results are different from the theoretical calculations … In fact, our results are closer to the linearity than to the exponential relation expected.” Although the sample group was a relatively small 40, the researchers utilised a standard testing protocol. In lieu of a control, the two products had the same base formula.

Overall it seems the effect of applying insufficient sunscreen is unpredictable and, if we factor in how UVB-only sunscreen performs at temperate latitudes like the UK

(see part 1/2), may leave us even more exposed in day-to-day life than the data suggests.

Reapplication Frequency

The British Association of Dermatologists (BAD) advises “apply[ing] sunscreen 15 to 30 minutes before going out in the sun to allow it to dry, and then again shortly after heading outdoors to cover any missed patches and to make sure you’re wearing a sufficient layer. Reapply it at least every 2 hours …“

Even if we initially apply the recommended amount, many of us fail to reapply especially those who wear make up. Is this really that bad? Does the oft repeated ‘every two hours’ refer to cumulative UV exposure, no matter how long that takes?

BAD’s recommendation to reapply early stems from research published in 2001 which found that “typically reapplication of sunscreen at 20 minutes results in 60% to 85% of the ultraviolet exposure that would be received if sunscreen were reapplied at 2 hours.” Although they used mathematically modelling rather than human subjects, the findings seem logical given what we know about the amount of sunscreen applied by the average person.

The type of sunscreen we choose is also of relevance. Chemical filters penetrate the ‘horny layer’ (uppermost layer of the epidermis) and, for the most part, they remain there. However research suggests some common chemical filters – octocrylene, octylmethoxycinnamate, oxybenzone – penetrate deeper into the epidermis over time.

Image credit: University of California, Riverside

This means protection is progressively reduced unless the product is reapplied. This may be of particular concern for those expecting a chemical sunscreen applied in the morning to retain a high level of protection for their evening commute.

It may also be of concern to those liberally applying formulations that include a high level of drying alcohol, because one study suggests this can enhance penetration of oxybenzone, perhaps other chemical filters given this property of alcohol is well known.

Further confirmation that reapplication is not only relevant to those getting a lot of UV exposure, sweating or swimming comes from research published in 2008 and by the same team in 2010 which reports that “the SPF decrease [for both mineral and chemical sunscreens] after 8 h is about 55% when the participants perform activities and 25% without activities.“

As we learned from the World Health Organisation’s infographic (see part 1/1) “shade can reduces UV exposure by 50% or more.” This is equivalent to a broad spectrum sunscreen of at least SPF 2, applied and reapplied correctly. Clearly an SPF that low is no substitute for reapplying sunscreen.

The last word goes to the author of the 2006 paper who says “The Skin Cancer Foundation … recommends that people go no more than 2 hours between reapplications of sunscreen. Our findings tend to support that … it might actually be necessary to reapply even more often.”

TL-DR?

So putting that together: as a UK resident I do not require sunscreen for incidental UV exposure from October to March, unless sunlight is a one of my rosacea triggers or the UV index is >2.

NICE recommend an SPF of 30+. SPF alone is a poor indicator of protection from either UVA or UVB in temperate climates because of the low angle of the sun. SPF can be as much as 50% to 60% out, due to disparity between the UV radiation used in sunscreen testing and real life exposure in different seasons or times of day.

BAD and Cancer Research UK recommend wearing a broad spectrum sunscreen from April to September. Research suggests this mitigates against the effect of latitude on SPF as well as protecting against the less visible damage from UVA (= anti-ageing and rosacea).

It is critical to apply and reapply sufficient sunscreen (1/2 to 1 teaspoon for face and neck). Many people apply a 1/4 to 1/2 of the minimum amount, which equates to an SPF <10 for a product labelled as SPF 30. Cloud cover and shade confer rubbish protection.

Some chemical filters may become less effective within an hour of application, and both chemical and mineral sunscreens may be 25% less effective after eight hours of inactivity, not simply with UV exposure.

More Information

NEXT TIME in the sunscreen series: Key ingredients