Sunscreen Labels Decoded: What SPF and PA Really Mean

Every sunscreen label includes a mix of numbers, symbols, and claims. The meanings behind SPF, PA, and broad-spectrum ratings are specific and rooted in scientific testing. Learning how to read these can help you select sun protection that’s right for your needs and your skin.

Here’s a clear explanation of what sunscreen labels actually mean, without any marketing fluff.

The Two Types of UV Radiation You Need to Know

Before we decode those labels, you need to understand what sunscreen is protecting you from.

The sun emits different types of ultraviolet radiation, but two types reach your skin: UVB and UVA rays.

UVB rays (290-320 nm) are often called the ‘burning’ rays. They are shorter wavelengths that primarily affect the skin’s outer layer, the epidermis. These rays are most intense between 10 am and 2 pm during the summer and do not pass through glass. UVB accounts for only about 5-6% of all UV radiation reaching Earth’s surface, but it is powerful. UVB rays cause direct DNA damage, which can lead to mutations and skin cancer, especially squamous cell carcinoma, a type of skin cancer.

UVA rays (320-400 nm) are often called the ‘ageing’ rays. These longer wavelengths penetrate much deeper into your skin, reaching the dermis – a layer beneath the epidermis where your collagen and elastin are found. UVA makes up about 95% of the UV radiation that reaches Earth and can pass through glass windows. Whilst UVA is less energetic than UVB, it’s present in much higher quantities and penetrates deeper. Research shows that UVA rays, particularly at longer wavelengths around 365 nm, can penetrate deep into the dermis.

UVB rays can trigger enzymes that break down collagen, accelerating skin ageing. UVA rays can damage DNA through reactive oxygen species (ROS). Both types of UV rays raise the risk of skin cancer. UVB is more likely to cause cancer, but UVA should also be considered.

What SPF Really Means

SPF means Sun Protection Factor. It is a regulated way to show how well a sunscreen protects your skin from UVB rays, which mainly cause sunburn and are linked to most skin cancers.

How is SPF calculated?

SPF is calculated using the Minimal Erythema Dose (MED), which is simply the amount of sun needed to cause redness on your skin 24 hours after exposure.

During testing, volunteers have sunscreen applied to small areas of their lower back at exactly 2 mg per square centimetre, then exposed to controlled UV light. The SPF value is the ratio of the MED on protected skin to the MED on unprotected skin.​

The formula is straightforward:

SPF = MED of protected skin / MED of unprotected skin

If your bare skin would burn after 10 minutes in the sun, SPF 30 could, in theory, let you stay out for 300 minutes (10 minutes × 30 = 300 minutes). But this only happens in perfect lab conditions, with the right amount of sunscreen, careful application, and no swimming, sweating, or wiping off.

The key thing to remember is that SPF does not increase protection in a straight line. The difference in protection between higher SPFs is smaller than you might think.

• SPF 15 blocks approximately 93% of UVB rays
• SPF 30 blocks approximately 96% of UVB rays
• SPF 50 blocks approximately 98% of UVB rays
• Higher SPF numbers give a bit more protection, but no sunscreen blocks all UVB rays.

Increasing SPF from 30 to 50 increases protection by only about 2%. The jump from SPF 15 to 30 gives just a 3% increase. That might seem small, but dermatologists usually recommend SPF 30 or higher. Most people don’t use enough sunscreen to reach the listed SPF.​

People usually apply less than half of the 2 mg/cm² used in SPF testing. When applied too thinly, SPF protection is much lower than the labelled value, so higher SPF products are safer in real life. For practical application, aim to use a quarter teaspoon of sunscreen for your face and a full shot glass for your entire body. This measurement ensures you’re using enough product to match the SPF values indicated on the label.

PA Ratings: The Japanese System for UVA Protection

Whilst SPF measures how well a sunscreen protects against UVB rays, the PA rating system shows how well it protects against UVA rays. PA stands for ‘Protection Grade of UVA’ and was developed in Japan, and is commonly used in Asia.

The PA system uses the Persistent Pigment Darkening (PPD) method to measure UVA protection by looking at how much the skin tans. The PPD method checks how much the skin darkens 2 to 4 hours after UVA exposure, much like SPF measures redness after UVB exposure.

The UVA Protection Factor (UVA-PF) is calculated as:​

UVA-PF = Minimal Pigmentation Dose of protected skin / Minimal Pigmentation Dose of unprotected skin

The PA rating translates these UVA-PF numbers into an easy-to-understand system:​

• PA+: UVA-PF of 2-4 (some UVA protection)
• PA++: UVA-PF of 4-8 (moderate UVA protection)
• PA+++: UVA-PF of 8-16 (high UVA protection)
• PA++++: UVA-PF of 16 or greater (very high UVA protection)

More plus signs mean better UVA protection. A sunscreen with PA++++ gives at least 16 times more protection against UVA-related skin darkening than bare skin. Still, even the highest PA rating does not block all UVA rays. It sets a minimum level, and comparing products can be tricky because each symbol covers a wide range.

What Does ‘Broad Spectrum’ Mean?

In the United States, PA ratings are not common. The FDA uses ‘broad spectrum’ to show UVA protection instead. Other regions have different standards. Knowing this can help you make better choices.

The FDA gives a sunscreen ‘broad spectrum’ status if 90% of its UV absorption happens at 370 nm or higher. This sounds good, but there is a catch. UVA rays range from 320 to 400 nm, and 370 nm is only in the middle of that range.

Europe has a stricter rule: the International Organisation for Standardisation (ISO) requires that the UVA Protection Factor be at least one-third of the SPF. In a study of 20 sunscreens, 19 met the FDA’s standard, but only 11 met the European ISO rule. So, some US products labelled ‘broad spectrum’ may not give enough UVA protection by international standards.

The Real-World Gap

Even if you pick a sunscreen with the right SPF and broad-spectrum protection, most people still do not get enough coverage.

Studies show there are some common problems with how sunscreen is used in real life:

• How much you use matters: The recommended amount is 2 mg per square centimetre, which is about a quarter teaspoon (1.25 ml) for your face and a full shot glass (30 ml) for your whole body. Most people use only half or less, which lowers protection a lot.
• Reapplying is important: Put on sunscreen every 2 hours, and right after swimming, sweating a lot, or drying off with a towel, even if it says ‘water-resistant’. Water-resistant sunscreens keep their SPF for 40 or 80 minutes in water, depending on the label, but protection drops after that. If you are at the beach all day, you need to reapply several times, not just once in the morning.
• Don’t forget certain areas: People often miss spots like the ears, back of the neck, hands, feet, and hairline. UV photography studies show that even when people think they’ve covered everything, they often miss significant areas of their face, particularly the eyelids and periorbital regions.

Limitations and Considerations

• No sunscreen is perfect, and none can block the sun completely. Applying it properly and reapplying often is key.
• Most people get less protection than the label promises because they use too little sunscreen.
• High SPF and PA numbers can make people feel too safe. This might make them less careful and cause them to stay in the sun longer.

SPF shows how well a sunscreen protects against UVB, but people often overestimate its effect because of how they use it. PA (or ‘broad-spectrum’) shows UVA protection, but this can change depending on where you live and the label.

For the best results, pick a broad-spectrum sunscreen, use plenty, and reapply often. Look for SPF 30 or higher, broad-spectrum, and PA+++ or PA++++ if you can. This makes choosing easier.

References and Sources

1. Guan LL, Lim HW, Mohammad TF. (2021). Sunscreens and Photoaging: A review of Current literature. https://doi.org/10.1007/s40257-021-00632-5
2. Geoffrey K, Mwangi AN, Maru SM. (2019). Sunscreen products: Rationale for use, formulation development and regulatory considerations. https://doi.org/10.1016/j.jsps.2019.08.003
3. Guan LL, Lim HW, Mohammad TF. (2021). Sunscreens and Photoaging: A review of Current literature. https://doi.org/10.1007/s40257-021-00632-5
4. Sabzevari N, Qiblawi S, Norton SA, Fivenson D. (2021). Sunscreens: UV filters to protect us: Part 1: Changing regulations and choices for optimal sun protection. https://doi.org/10.1016/j.ijwd.2020.05.017
5. Li Z, Kim MA, Kim E, Jung YC, Kim JJ, Shin H. (2022). Dynamic visualization of ultraviolet dose on skin with sunscreen applied using minimum erythema dose. https://doi.org/10.1111/srt.13176
6. Brar G, Dhaliwal A, Brar AS, Sreedevi M, Ahmadi Y, Irfan M, Golbari R, Zumárraga D, Yateem D, Lysak Y, Abarca-Pineda YA. (2025). A comprehensive review of the role of UV radiation in photoaging processes between different types of skin. https://doi.org/10.7759/cureus.81109
7. Berry EG, Bezecny J, Acton M, Sulmonetti TP, Anderson DM, Beckham HW, Durr RA, Chiba T, Beem J, Brash DE, Kulkarni R, Cassidy PB, Leachman SA. (2022). Slip versus Slop: A Head-to-Head Comparison of UV-Protective Clothing to Sunscreen. https://doi.org/10.3390/cancers14030542
8. Kuritzky LA, Beecker J. (2015). Sunscreens. https://doi.org/10.1503/cmaj.150258