UV Radiation: UVA vs. UVB

Ultraviolet radiation from the sun that reaches the Earth's surface consists of two types:

UVA (320–400nm)

Skin penetration: Reaches the dermis (deeper layer)
Effects: Destroys collagen and elastin → wrinkles, sagging, photoaging
Properties: Penetrates clouds and glass; relatively constant year-round
Tanning: Triggers melanin production causing delayed tanning

UVB (280–320nm)

Skin penetration: Reaches the epidermis (surface layer)
Effects: Causes sunburn; primary driver of DNA damage and skin cancer risk
Properties: Strongest in summer midday; largely blocked by glass
Immediate effects: Erythema (redness) appears within hours

UVC (100–280nm)

Nearly entirely absorbed by the ozone layer; does not reach the Earth's surface under normal conditions.

SPF and PA: What the Numbers Mean

SPF (Sun Protection Factor)

SPF measures protection against UVB — specifically, how much longer it takes to cause erythema (redness) compared to unprotected skin.

Example calculation:

Unprotected skin reddens in 20 minutes
SPF 50 sunscreen: 20 min × 50 = 1,000 minutes theoretical protection
Real-world conditions (sweating, friction, swimming) dramatically shorten this

Practical guide:

SPF 15–20: Daily use, cloudy days
SPF 30–50: Outdoor activities, sports
SPF 50+: Extended outdoor exposure, beach, mountains

PA (Protection Grade of UVA)

PA measures UVA protection (a Japanese standard based on Persistent Pigment Darkening):

PA+: PFA 2–4 (some UVA protection)
PA++: PFA 4–8 (moderate UVA protection)
PA+++: PFA 8–16 (high UVA protection)
PA++++: PFA 16+ (extremely high UVA protection)

Western products use "Broad Spectrum" to indicate meaningful UVA protection.

Heatstroke Risk CheckerEstimate Wet Bulb Globe Temperature (WBGT) and heatstroke risk from temperature and humidity.

Choosing the Right Sunscreen

By Activity Level

Scenario	Recommended
Daily commute	SPF 20–30 / PA++
Outdoor errands	SPF 30–50 / PA+++
Sports / outdoor work	SPF 50+ / PA++++
Beach / pool / hiking	SPF 50+ / PA++++ water-resistant

Chemical vs. Physical Filters

Chemical (UV absorbers): Absorb UV and convert to heat. Lightweight, high SPF achievable. May irritate sensitive skin.

Physical (UV scatterers): Zinc oxide, titanium dioxide — reflect UV physically. Gentler on skin; may leave white cast.

Most modern sunscreens combine both. For sensitive skin or children, mineral (physical) formulas are preferable.

How to Apply Sunscreen Correctly

Amount Matters Enormously

SPF ratings are measured at 2mg/cm² — most people apply 25–50% of this amount, which can reduce effective SPF by more than half.

Face: 2 fingertip units (~1ml)
Full body: approximately 30ml (shot glass)

Reapplication Schedule

Reapply every 2–3 hours and immediately after:

Swimming (even water-resistant formulas need reapplication)
Heavy sweating
Towel drying

Application Timing

Apply 30 minutes before sun exposure for chemical filters (allows time for binding to skin); physical filters work immediately.

After-Sun Skincare

Acute Phase (Day of / Day After Sunburn)

Cool the skin: Damp towel, cool shower — reduce inflammation
Moisturize: Ceramide/hyaluronic acid products to restore hydration barrier
Avoid irritants: No scrubs, peels, or alcohol-containing products
Hydrate internally: Drink plenty of water

Recovery Phase (Days to Weeks After)

Brightening care: Products with Vitamin C, niacinamide, or arbutin to address hyperpigmentation
Support natural skin turnover with consistent moisturization
Melanin deposits from sunburn typically fade over weeks to months

Severe sunburn with blistering, intense pain, or fever requires dermatology consultation (may be equivalent to second-degree burns).

Common Sunscreen Myths

Myth: "I don't need sunscreen on cloudy days" → 60–80% of UV reaches the ground on overcast days. UVA easily penetrates clouds.

Myth: "One application lasts all day" → Sunscreen degrades with sweat, friction, and time. Reapply every 2–3 hours.

Myth: "SPF 100 is twice as good as SPF 50" → SPF 50 blocks ~98% of UVB; SPF 100 blocks ~99%. The difference is marginal, and reapplication matters far more.

Myth: "Sunscreen is only for summer" → UVA exposure is consistent year-round. Daily SPF is the single most evidence-based anti-aging intervention.

Summary

Item	Key Points
UVA	Reaches dermis; photoaging; check PA rating
UVB	Causes sunburn, DNA damage; check SPF
Amount	Apply generously; most people under-apply
Reapplication	Every 2–3 hours; after swimming
After-sun	Cool → moisturize → brightening care