日焼け止めとは？SPF・PA・UVA・UVBの違い、選び方・塗り方・塗り直しまで解説

What Is Sunscreen? SPF, PA, UVA, UVB, How to Choose, Apply & Reapply

OBARASOTARO

“Does SPF50 last for 50 hours?” “What does PA++++ mean?” “What is the difference between UVA and UVB?” “How much sunscreen should I apply?” “Do I need sunscreen on cloudy days or indoors?”

Sunscreen is a familiar part of daily UV care, but product labels can include many terms such as SPF, PA, UV water resistance, UV absorbers and UV scattering agents.

In Japan, sunscreen products may be sold as cosmetics or quasi-drugs.

Rather than choosing only by the highest number or the greatest number of “+” symbols, it helps to understand which type of UV each rating refers to, how the rating is measured and how you plan to use the product.

To make the most of the labelled SPF and PA, it is also important to apply a sufficient amount evenly and reapply according to exposure to sweat, water and friction.

This guide explains what ultraviolet radiation is, the difference between UVA and UVB, the meaning of SPF and PA, SPF50+, UV water resistance, waterproof sunscreen, UV absorbers and scattering agents, “non-chemical” sunscreen, how much to apply, application order, reapplication, cloudy days, winter UV, UV through windows, sunscreen for children and sensitive skin, product shelf life, and how sunscreen can be combined with hats, parasols and clothing.

Contents

What is sunscreen?

Sunscreen is a UV-care product used to help protect the skin from ultraviolet radiation in sunlight.

In Japan, sunscreen products may be sold as cosmetics or quasi-drugs.

These products contain UV filters that absorb, scatter or otherwise reduce the amount of ultraviolet radiation reaching the skin.

In Japan, two of the main ratings used to compare UV protection are:

  • SPF: mainly indicates protection against UVB
  • PA: indicates protection against UVA

For products intended for use around water, Japan also uses UV water resistance ★ and ★★ labelling.

Because each rating describes something different, it is useful to look at them together rather than relying on one number alone.

What is ultraviolet radiation?

Ultraviolet radiation, commonly shortened to UV, is electromagnetic radiation with wavelengths shorter than visible light.

UV radiation is broadly divided into:

  • UVA
  • UVB
  • UVC
Type Approximate wavelength Main characteristics
UVA 315–400 nm Makes up most of the UV that reaches the Earth's surface and can penetrate more deeply into the skin
UVB 280–315 nm Most is absorbed by the atmosphere, but some reaches the surface and acts mainly on the epidermis
UVC 100–280 nm Solar UVC is almost entirely absorbed by the atmosphere and normally does not reach ground level

For everyday sun protection, UVA and UVB are the two main types to understand.

UVA vs UVB

UVA and UVB differ in how they interact with the skin and in the types of skin responses commonly associated with them.

UVA UVB
Penetration into skin Can reach deeper layers of the skin more readily Acts mainly on the epidermis
Typical skin response Immediate or persistent darkening reactions Redness and sunburn reactions appearing after exposure
Main label to check in Japan PA SPF
Window glass Can pass through ordinary glass to a greater extent Much of it is blocked by ordinary window glass

UVA has also been studied in relation to skin changes associated with photoageing, while UVB is well known for its relationship with sunburn and post-exposure pigmentation.

For this reason, checking both SPF and PA is a useful starting point when choosing sunscreen in Japan.

What is SPF?

SPF stands for Sun Protection Factor.

It mainly indicates the degree of protection against UVB-induced erythema, or the redness associated with sunburn.

SPF is determined by comparing the amount of UV exposure required to produce slight redness on sunscreen-protected skin with the amount required on unprotected skin.

In simple terms, it indicates:

how much more UVB exposure is required to produce a sunburn response on protected skin compared with unprotected skin under test conditions.

A higher SPF therefore indicates greater UVB protection under the conditions of the test.

Did you know? How to read SPF50 correctly

SPF is a familiar number, but it is a measure of relative UVB protection rather than a direct measure of time.

Actual UV exposure varies according to:

  • time of day
  • season
  • latitude
  • altitude
  • weather
  • reflection from surrounding surfaces

Real-world protection is also affected by:

  • sweat
  • water
  • towel drying
  • clothing
  • touching the face
  • the amount applied
  • uneven application

For this reason, SPF is best understood as an indicator of UVB protection strength rather than “how many hours” a sunscreen will last.

What does SPF50+ mean?

Many sunscreens sold in Japan carry the label:

SPF50+

Under guidance used by the Japan Cosmetic Industry Association, “SPF50+” may be displayed when the measured SPF is determined to be significantly higher than 50.

The “+” therefore does not simply mean “SPF51”; it represents a measured protection level above the SPF50 range.

The UV exposure involved in commuting or shopping is very different from that involved in extended summer outdoor activity, swimming, hiking or sport.

It is therefore practical to choose SPF according to the situation in which the product will be used.

What is PA and what do the “+” symbols mean?

PA stands for Protection Grade of UVA and is widely used in Japan to indicate UVA protection.

It is based on measurements of skin darkening responses caused by UVA exposure.

In Japan:

Label UVA protection level
PA+ Some UVA protection
PA++ Moderate UVA protection
PA+++ High UVA protection
PA++++ Very high UVA protection

A simple way to remember the two systems is:

SPF = mainly UVB
PA = UVA

How much sunscreen is used when SPF and PA are tested?

This is one of the most important points for understanding sunscreen ratings.

Standard SPF testing applies sunscreen at 2 mg per cm² of skin.

The SPF shown on a product is therefore measured under defined test conditions using a defined amount of sunscreen.

Environmental health guidance in Japan notes that people often apply much less sunscreen in everyday use than the amount used during testing, and that using less product can reduce the level of protection achieved in practice.

Did you know? Sunscreen performance depends on both the product and how you use it

Real-world UV protection is influenced by the combination of:

the product's SPF and PA × the amount applied × application evenness × reapplication

This is why texture and ease of use matter too: a product that you can comfortably apply in a sufficient amount on a regular basis may be more practical for everyday use.

UV absorbers vs UV scattering agents

UV filters used in sunscreen are often broadly described as:

  • UV absorbers
  • UV scattering agents
UV absorbers UV scattering agents
Basic mechanism Absorb UV radiation and convert the energy into other forms, reducing the amount reaching the skin Primarily scatter and reflect UV through particulate materials, while also absorbing part of the UV spectrum
Examples Ethylhexyl methoxycinnamate and other approved organic UV filters Zinc oxide and titanium dioxide

Some sunscreens rely mainly on one type, while others combine multiple UV filters.

Rather than treating one category as universally superior, it is more useful to compare the overall formulation, SPF, PA, feel on the skin, white cast, water resistance and suitability for the intended use.

Did you know? “Scattering agents” do more than simply reflect UV

Ingredients such as zinc oxide and titanium dioxide are often called UV scattering agents, which can create the impression that they simply reflect UV like tiny mirrors.

In practice, their protective action involves scattering and reflection as well as absorption of UV radiation.

So while the absorber/scattering-agent distinction is useful for cosmetic labelling and general explanation, the actual optical behaviour of these materials is more complex than a simple “absorb versus reflect” split.

What does “non-chemical” sunscreen mean?

In Japan, the term “non-chemical” is often used on sunscreen products.

In this context, it generally means:

a formula made without UV absorbers.

These products commonly use UV scattering agents such as zinc oxide or titanium dioxide.

Did you know? “Non-chemical” generally means “without UV absorbers”

In sunscreen terminology, “non-chemical” generally refers to the absence of UV absorbers.

The product may still contain many other cosmetic ingredients, including emollients, moisturising agents, emulsifiers and UV scattering agents.

When choosing sunscreen, it is therefore useful to look beyond the term “non-chemical” and consider the complete ingredient list, texture and how the formulation suits your skin.

How to choose sunscreen

Sunscreen becomes easier to choose when you consider how you will use it and how comfortable it is to use consistently.

1. Everyday use

For commuting, shopping and short walks, consider the amount of UV exposure involved and how long you will be outdoors.

A texture that you can comfortably apply in a sufficient amount every day is also important.

2. Extended outdoor activity

For outdoor sport, sightseeing, camping or other prolonged outdoor activities, products with higher SPF and PA ratings may be appropriate choices.

3. Swimming, beaches and water activities

For situations involving water, check UV water resistance in addition to SPF and PA.

When comparing sunscreens commonly described as “waterproof”, Japan's UV water resistance ★ and ★★ labels can help you compare how products perform under defined water-exposure test conditions.

4. Daily facial sunscreen

For a sunscreen used on the face every day, it can also help to consider:

  • spreadability
  • stickiness
  • white cast
  • dryness or comfort
  • compatibility with makeup
  • how it should be removed

These practical factors can make it easier to use enough product consistently.

The UV Index can also help guide sunscreen use

The UV Index is an internationally used measure of the strength of ultraviolet radiation.

The higher the number, the greater the potential impact of UV exposure.

UV Index Level
0–2 Low
3–5 Moderate
6–7 High
8–10 Very high
11+ Extreme

The World Health Organization recommends using sun-protection measures when the UV Index reaches 3 or higher.

Checking the daily UV Index can give a more useful picture of actual UV conditions than relying on season alone.

How much sunscreen should you apply?

A basic principle is to apply a sufficient amount evenly.

Because sunscreen textures, containers and application methods vary, follow the amount and directions recommended by the manufacturer for the individual product.

For the face, remember areas such as:

  • forehead
  • nose
  • both cheeks
  • chin
  • jawline
  • ears
  • neck

For formulas that are difficult to spread in one large amount, applying smaller amounts to several areas and then layering as needed can make even application easier.

Areas that are easy to miss

  • ears
  • back of the neck
  • hairline
  • jawline
  • backs of the hands
  • tops of the feet
  • shoulders

A quick check of exposed skin can help reduce missed areas and uneven application.

Skincare, sunscreen and makeup: what order?

A common morning routine is:

Cleanse → toner/serum/emulsion or moisturiser → sunscreen → makeup

However, some products combine several roles, such as:

  • daytime emulsions
  • UV serums
  • sunscreen primers
  • foundation products with UV protection

For this reason, follow the order recommended on the individual product.

Allowing sunscreen to spread and settle evenly before applying makeup can also help reduce unevenness caused by rubbing immediately afterwards.

How often should sunscreen be reapplied?

The Japan Cosmetic Industry Association advises reapplying sunscreen at least every two to three hours as a general guide, while adjusting according to the situation.

Reapplication is particularly important after:

  • heavy sweating
  • swimming
  • towel drying
  • friction from clothing or touching the skin

For the face when wearing makeup, products such as UV emulsions, sticks, cushions or powders may be useful depending on the directions for the individual product.

The key idea is to restore an even sunscreen film according to what has happened during the day.

What do UV water resistance ★ and ★★ mean?

In Japan, the Japan Cosmetic Industry Association introduced UV water resistance labelling under its voluntary industry standard from December 2022.

Testing based on ISO 18861 compares SPF before and after defined water-immersion cycles, and products meeting the criteria may display a water-resistance rating.

Label Water-immersion condition used in testing
UV Water Resistance ★ 40 minutes total (20 minutes × 2)
UV Water Resistance ★★ 80 minutes total (20 minutes × 4)

The symbol may sometimes appear as “★” or “☆”. The filled or unfilled appearance does not indicate a difference in performance; the important point is whether one or two stars are shown.

Did you know? The 40- and 80-minute figures are test conditions

The 40- and 80-minute figures refer to defined testing conditions used to evaluate water resistance.

Real-life conditions vary according to swimming, towel use, clothing and friction.

For this reason, sunscreen should still be reapplied according to exposure to water, sweat and rubbing even when it carries a UV water resistance rating.

UV water resistance indicates resistance to water exposure

Japan's current UV water resistance labelling evaluates how well UV protection is maintained after external water exposure to the skin.

For resistance to sweat, check the specific product claims and manufacturer guidance.

Does UV still reach the ground on cloudy days?

Cloud cover changes UV intensity, but ultraviolet radiation still reaches the ground on cloudy days.

The World Health Organization also notes that UV can remain high under some cloudy conditions.

Thin cloud, in particular, may not block UV effectively.

Instead of relying only on whether the sun is visible, it is useful to consider the UV Index and how long you expect to be outdoors.

What about UV indoors and through windows?

UVA and UVB behave differently when they encounter window glass.

Ordinary window glass blocks much of UVB, while UVA can pass through to a greater extent.

This may be worth considering when:

  • spending long periods beside a large sunny window
  • working in a strongly sunlit room
  • driving for long periods

However, UV transmission varies depending on the type of glass, UV coatings and protective window films.

A practical approach is to adjust UV protection according to your distance from the window, amount of direct sunlight, duration of exposure and type of glass.

Do you need sunscreen in winter?

UV intensity changes with the seasons, but UVA and UVB are present throughout the year.

Useful factors to consider include:

  • season
  • time of day
  • location
  • altitude
  • time spent outdoors
  • reflection from surrounding surfaces

Snow can strongly reflect UV, so exposure can still be substantial during skiing and other high-altitude winter activities.

It is practical to adjust UV protection according to the day's UV Index and your planned activities.

How should sunscreen be chosen for children?

Children's skin is thinner and can be more sensitive to irritation than adult skin.

The Japan Cosmetic Industry Association recommends selecting sunscreen products designed for children where appropriate.

For children, sunscreen can also be combined with:

  • shade
  • hats
  • covering clothing
  • rash guards during extended outdoor or water activities

Recommendations can vary by age, especially for infants, so always check the product's stated age range and directions for use.

Choosing sunscreen for sensitive skin

For sensitive skin, it is helpful to look beyond SPF and PA and consider the overall formulation and how well it suits your skin.

Points you may wish to check include:

  • whether the product is designed for sensitive skin
  • whether it contains UV absorbers
  • fragrance, alcohol or other ingredients you personally prefer to avoid
  • how the product should be removed
  • texture and comfort
  • patch-test or allergy-test statements

Rather than deciding from the “non-chemical” label alone, consider the complete formulation and your own skin response.

If your skin is particularly reactive, you may prefer testing the product on a small area first. If you are receiving treatment for a skin condition, discuss suitable products with your healthcare professional.

How should sunscreen be removed?

Removal instructions vary by product.

Some sunscreens are designed to be removed with:

  • facial cleanser
  • soap
  • body wash
  • dedicated makeup or sunscreen remover

Products with higher water resistance or makeup effects may recommend a dedicated cleansing product.

The basic approach is to follow the removal instructions provided for the individual sunscreen.

Using the appropriate cleanser and allowing it to work on the product can help minimise unnecessary rubbing of the skin.

Can you use last year's sunscreen? Shelf life and storage

Many cosmetics sold in Japan do not carry a printed expiry date.

In Japan, cosmetics that may undergo changes in properties or quality within three years after manufacture or import under appropriate storage conditions are required to display an expiry date.

Cosmetics without a stated expiry date are generally designed to maintain their quality for at least three years while unopened and appropriately stored.

Once opened, however, cosmetics are exposed to air, hands, temperature changes and the surrounding environment.

The Japan Cosmetic Industry Association recommends using opened cosmetics as soon as reasonably possible and storing them by:

  • avoiding high heat and humidity
  • avoiding places with large temperature fluctuations
  • keeping them away from direct sunlight
  • closing the cap securely

Before using an older product, check for changes in smell, colour, separation or texture.

Combine sunscreen with hats, parasols and clothing

Sun protection can be made more practical by combining sunscreen with other methods.

Useful options include:

  • shade
  • hats
  • parasols
  • long sleeves and other covering clothing
  • sunglasses

The World Health Organization also recommends using shade and protective clothing and applying sunscreen to exposed areas that cannot easily be covered.

The goal is therefore not only to consider how high the SPF is, but also to reduce the total amount of UV reaching the skin.

What sunscreen effects can cosmetics claim in Japan?

Among the efficacy expressions permitted for cosmetics in Japan are:

  • helps prevent sunburn
  • helps prevent spots and freckles caused by sun exposure

This distinction is important when interpreting the way UV-care cosmetics are described.

UV radiation is also studied in dermatology and medical research in relation to photoageing and other health effects.

When describing a commercial product, however, scientific information about UV exposure should be kept separate from the permitted claims for that specific product. Product descriptions should stay within the information supported for that item, such as SPF, PA, UV water resistance and manufacturer directions.

Quasi-drugs are regulated differently from cosmetics in Japan and may display only the efficacy and effects approved for the individual product.

Summary: look beyond SPF and PA to amount, reapplication and use situation

When choosing sunscreen, looking beyond the SPF and PA numbers to consider how and where the product will be used makes UV protection more practical.

The key points are:

  • UVA and UVB differ in how they interact with the skin and the skin responses associated with them.
  • SPF mainly indicates UVB protection, while PA indicates UVA protection.
  • SPF is a measure of relative UVB protection rather than a direct measure of how long a sunscreen lasts.
  • SPF and PA are measured under defined test conditions and application amounts, so the amount actually applied matters.
  • UV filters include absorbers and scattering agents, and “non-chemical” generally means formulated without UV absorbers.
  • Apply sunscreen evenly in a sufficient amount and use two to three hours as a general reapplication guide, adjusting for water, sweat and friction.
  • UV water resistance ★ and ★★ indicate water resistance measured under defined test conditions.
  • Combining sunscreen with hats, clothing, parasols and shade provides a broader approach to UV protection.

A practical sunscreen choice takes into account SPF, PA, UV water resistance, texture, skin compatibility and the situation in which the product will be used.

To make the most of the protection shown on the label, remember that applying enough product evenly and reapplying after sweat, water and friction are just as important as choosing the product itself.

Related Articles

Main References

  • Japan Cosmetic Industry Association. Basic Information on UV Protection.
  • Japan Cosmetic Industry Association. UV Terminology Guide.
  • Japan Cosmetic Industry Association. UV Water Resistance Labelling Q&A.
  • Japan Cosmetic Industry Association. UV Filters.
  • Japan Cosmetic Industry Association. Voluntary Standards for SPF, PA and UV Protection Testing.
  • Ministry of the Environment, Japan. Ultraviolet Radiation Environmental Health Manual 2020.
  • Ministry of Health, Labour and Welfare, Japan. Scope of Permitted Cosmetic Efficacy Claims.
  • World Health Organization. Ultraviolet Radiation.
  • World Health Organization. Radiation: Protecting Against Skin Cancer.
  • ISO 24444. Cosmetics — Sun protection test methods — In vivo determination of the sun protection factor.
  • ISO 24442. Cosmetics — Sun protection test methods — In vivo determination of sunscreen UVA protection.
  • ISO 18861. Cosmetics — Sun protection test methods — Percentage of water resistance.

Important Information

This article is intended to provide general educational information about ultraviolet radiation, sunscreen, SPF, PA and skincare.

It is not intended to diagnose, treat, cure or prevent disease, and it does not claim medical effects for any particular cosmetic or quasi-drug product.

SPF, PA, UV water resistance, directions for use, recommended age range and removal instructions vary between products. Always follow the labelling and manufacturer instructions for the sunscreen you use.

If redness, swelling, itching, strong irritation or another unusual skin reaction occurs, stop using the product. If symptoms continue, consult a dermatologist or other qualified healthcare professional.

If you are receiving treatment for a skin condition, have been given specific medical advice regarding UV exposure, or have a photosensitivity disorder, discuss suitable sun-protection measures with your doctor.

Category
Beauty & Skincare

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