In-Depth Guide to Sunscreen Ingredients: UVA Protection Principles and PA Ratings Explained

I previously wrote a basic guide on sunscreen selection. This article delves deeper into the mechanisms of UVA protection. Many people only look at the SPF value when buying sunscreen, not realizing that SPF only protects against UVB. It's UVA, the real culprit behind tanning and premature aging, that deserves your primary attention.

☀️ UV Radiation Classification at a Glance

Type	Wavelength	Penetration Ability	Primary Harm
UVC (200-280nm)	Absorbed by the ozone layer, doesn't reach the ground	—	—
UVB (280-315nm)	Moderate	Sunburn, redness, skin cancer
UVA-II (315-340nm)	Stronger	Tanning, DNA damage
UVA-I (340-400nm)	Strongest, penetrates glass	Photoaging, collagen damage

Key Information:

UVA accounts for 95% of the UV radiation that reaches the ground
UVA can penetrate glass (sitting in a car or near a window can still cause photoaging)
UVA-I (340-400nm) is the most difficult wavelength range for sunscreens to block; many products offer weak protection in this range

📊 Decoding Sunscreen Metrics

SPF (Sun Protection Factor)

Only protects against UVB
SPF = Time to develop sunburn with sunscreen / Time to develop sunburn without sunscreen
SPF30 = Filters approximately 97% of UVB
SPF50 = Filters approximately 98% of UVB
SPF50+ ≠ Perfect protection; the difference beyond SPF50 is minimal

Daily Commute: SPF30 PA+++ is sufficient Outdoor Exposure: SPF50+ PA++++ is necessary

PA (Protection Grade of UVA) - Japanese System

Based on the PPD (Persistent Pigment Darkening) test
Ranges from PA+ to PA++++; more "+" signs indicate stronger UVA protection

PA Rating	PPD Value	Protection Level
PA+	2-4	Low
PA++	4-8	Medium
PA+++	8-16	High
PA++++	≥ 16	Very High

PPD (Persistent Pigment Darkening)

A common metric in Europe, directly reflecting UVA protection value
PPD ≥ 8 is the EU's recommended minimum standard
PPD ≥ 16 = PA++++

UVA-PF (Critical Wavelength)

Critical wavelength ≥ 370nm = Broad Spectrum protection
Ensures protection against long-wave UVA-I as well

🧪 Analysis of Major Sunscreen Ingredients

Chemical Sunscreens (Absorbing Type)

Avobenzone (Butyl Methoxydibenzoylmethane)

Currently the most effective broad-spectrum UVA-I sunscreen ingredient
Problem: Photounstable (degrades in sunlight)
Requires stabilizers: Octocrylene or Bemotrizinol

Tinosorb M / S (Bis-Ethylhexyloxyphenol Methoxyphenyl Triazine)

New generation of stable, broad-spectrum sunscreen ingredients
Covers both UVB and UVA
Excellent photostability, commonly used in Europe

Mexoryl SX / XL

Patented ingredients by L'Oréal
SX: Covers UVA-II and UVB
XL: Covers UVA-I
Works best when used in combination

Helioplex / Tinosorb (Multi-Ingredient Formulations)

Patented sunscreen systems from different brands
Achieve more complete spectral coverage by combining multiple complementary ingredients

Physical Sunscreens (Scattering Type)

Titanium Dioxide (TiO₂)

Primarily protects against UVB and UVA-II
UVA-I protection is weak (protection drops significantly above 400nm)
Nano-sized particles offer a more natural skin tone, but there are safety concerns (current consensus is that nano TiO₂ on the skin does not penetrate)

Zinc Oxide (ZnO)

Broad-spectrum physical sunscreen, better UVA-I protection than TiO₂
However, when used alone, the film is thick and can leave a white cast
Nano-sized zinc oxide = improved transparency while maintaining sun protection efficacy

Characteristics of Physical Sunscreens:

Stable, does not degrade
Suitable for sensitive skin (less irritating)
Thicker texture, prone to white cast (non-nano versions)

🔄 Advantages of Hybrid Sunscreens

Modern high-end sunscreens are often a hybrid of physical + chemical formulas:

Physical sunscreen provides a stable, broad-spectrum base protection
Chemical sunscreen supplements specific wavelength ranges (especially UVA-I)
The two complement each other for more complete coverage

💧 The Correct Amount of Sunscreen to Use

Many people use far too little, which is the number one reason sunscreen fails:

Standard Amount: 2mg/cm²

Full face: Approximately 1/4 teaspoon (about 1ml)
Full face + neck: Approximately 1/3 teaspoon

The Real-World Problem:

Most people use only 20-50% of the standard amount
Using half the amount → SPF effectiveness becomes the square root of the labeled value
- SPF50 with half the amount → actual protection is about SPF7

Recommended Practice: Apply two layers (each at the normal amount) to ensure even coverage.

🌊 Sunscreen Reapplication Strategy

When to Reapply:

Continuous outdoor activity: Reapply every 2 hours
After sweating or contact with water: Reapply immediately (water-resistant sunscreen does not mean reapplication is unnecessary)

Water Resistance Ratings:

Water Resistant 40 min: Retains 50% of sun protection efficacy after 40 minutes of swimming
Water Resistant 80 min: Retains 50% of sun protection efficacy after 80 minutes of swimming

⚠️ Common Misconceptions

Only looking at SPF, ignoring PA → SPF50 + PA++ offers less UVA protection than SPF30 + PA++++ → You must check both metrics.
Thinking a thicker layer of sunscreen means better protection → Sunscreen efficacy depends on even coverage, not thickness → The key is completely and evenly covering every area of your face.
Not wearing sunscreen on cloudy days → UVA can penetrate clouds (80% of UVA still reaches the ground on overcast days).
Sunscreen efficacy decreases when mixed with other products → Sunscreen should be applied last (after skincare); do not layer large amounts of other products on top of it, which can dilute it.
Physical sunscreen is always safer than chemical sunscreen → This is generally true for sensitive skin, but not all chemical sunscreens are problematic. → Consult a dermatologist for specific skin conditions.

This article is written based on knowledge of photochemistry, dermatology, and cosmetic formulation science. It does not contain brand recommendations.