What are the common misunderstandings about sunscreen?

This answer is a draft. It stays unpublished until legal review and source verification are complete.

Sunscreen is used all over the world, but few people know the conditions under which the numbers on the label are measured. In Japan you will also see a second rating, "PA", next to the SPF. This article explains what both numbers assume, and why the amount you apply matters more than the number.

The assumptions behind the numbers

• SPF and PA values are measured under internationally standardised test methods, with 2 mg (or 2 μL) per cm² of skin applied.
• Actual use, however, has been reported at about a quarter of the proper amount in one study, and at an average of 0.56 g, roughly half the theoretical amount in another.
• The guide for the whole face is 0.7–0.9 g (two pearl-sized drops of cream, or two 1-yen-coin-sized pools of liquid — a 1-yen coin is about 2 cm across).

The performance on the label applies only when you apply the amount on the label. Applying the right amount does more than choosing a higher number.

[Verify] 2 mg/cm² is widely presented as the international standard test condition. The usage-survey figures come from several dermatology and cosmetics information sites, so check them against Japan Cosmetic Industry Association (JCIA) materials or similar before publication.

Misunderstanding 1｜With a high SPF you do not need to reapply

Sweat and friction remove it. Follow the product label and reapply.

Misunderstanding 2｜You can ignore PA

SPF is a guide to protection against UVB; PA is a guide to protection against UVA. They measure different things. PA is a rating used in Japan and has four grades, from PA+ to PA++++. UVA is known to pass easily through window glass, so check the PA rating for everyday situations too.

Misunderstanding 3｜A small amount spread thinly is enough

As above, at half the amount, the labelled performance is not delivered.

Misunderstanding 4｜You do not need it when it is cloudy or indoors

According to Ministry of the Environment materials, a considerable share of the ultraviolet light of a clear day still reaches the ground on a cloudy day. Decide by whether you spend long periods by a window.

Misunderstanding 5｜Any way of removing it is fine

How to remove it differs from product to product. Check the label, and remove it without rubbing.

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Sources

• Ministry of the Environment, "UV Environmental Health Manual" [紫外線環境保健マニュアル]

https://www.env.go.jp/chemi/uv/uv_manual.html

• Japan Cosmetic Industry Association (JCIA) (information on test standards and labelling for UV-protection cosmetics)

https://www.jcia.org/

• Japan Meteorological Agency, UV information

https://www.data.jma.go.jp/gmd/env/uvhp/

⚠️ Editorial note: the source URLs above were identified by search. Our working environment cannot reach go.jp domains, so the contents have not been checked against the text. Verify every source before publication and record who checked it and when.

About this article

What we share here are ideas and everyday practices passed down among people living in Japan, together with general information drawn from public sources. They are not official recommendations, and they are not professional instruction. Practices vary between households and regions.

This is general information. Results vary from person to person and depend on your own conditions and surroundings.

Please decide for yourself, using your own judgement, whether any of this suits your situation. We cannot accept responsibility for any damage, loss or injury arising from following this article.

Cosmetics are not medicines and are not meant to diagnose, heal or prevent any condition. If your skin reacts, stop using the product and see a dermatologist.

Sources

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