Practical Scenarios in UV Exposure

for nerds who want the numbers

Two Types of UV Radiation

UVA Radiation - Aging

315–400 nanometers

UVA penetrates deep into the skin's dermis layer, driving premature aging (photoaging), wrinkles, and contributing to melanoma risk. It passes through most window glass and remains relatively constant throughout the day and year.

UVB Radiation - Sunburn

280–315 nanometers

UVB primarily affects the skin's outer layer (epidermis), causing sunburn and playing a major role in developing skin cancer. It varies significantly by time of day, season, and latitude, with peak intensity during midday hours.

Interactive UV Exposure Calculator

UVA Irradiance · San Diego

UVA (315-400 nm) • 6 AM - 6 PM • 32.72°N, 117.16°W
Fitzpatrick Skin Type
Glass Type / Environment
Sunscreen Protection (PA Rating)
UVA Irradiance (W/m²)

How to Use This Tool

1

Choose Your Mode

Toggle between UVA Mode (blue) and UVB Mode (orange) to explore different types of UV radiation. UVA is associated with skin aging and deeper damage, while UVB causes sunburn and surface-level harm.

2

Select Your Location

Choose from 19 worldwide locations to see how latitude affects UV exposure. Locations closer to the equator (like Singapore) experience more consistent year-round UV, while higher latitudes (like London) show dramatic seasonal variation.

3

Set Your Skin Type

Select your Fitzpatrick Skin Phototype (I-VI) to see personalized exposure thresholds. Type I (very fair) burns quickly with a Minimal Erythema Dose (MED) of 200 J/m² for UVB, while Type VI (very dark) has an MED of 1000 J/m².

Type Characteristics UVB Response (MED) UVA Response (MMD)
I Very pale, white skin. Red or blonde hair. Blue/green eyes. Freckles common. Always burns, never tans
MED: Very low (~15-30 mJ/cm²)		 Minimal pigmentation response
MMD: Very low threshold
II Fair/pale skin. Blonde or light brown hair. Blue, hazel, or green eyes. Usually burns, tans minimally
MED: Low (~25-40 mJ/cm²)		 Slight pigmentation response
MMD: Low threshold
III Fair to beige skin with golden undertone. Brown hair. Brown or hazel eyes. Sometimes burns, gradually tans
MED: Moderate (~30-50 mJ/cm²)		 Moderate pigmentation response
MMD: Moderate threshold
IV Olive or moderate brown skin. Dark brown hair. Dark eyes. Rarely burns, tans easily
MED: High (~40-60 mJ/cm²)		 Good pigmentation response
MMD: Higher threshold
V Brown skin. Dark brown to black hair. Dark brown eyes. Very rarely burns, tans darkly
MED: Very high (~60-100 mJ/cm²)		 Strong pigmentation response
MMD: High threshold
VI Dark brown to black skin. Black hair. Dark brown to black eyes. Never burns, deeply pigmented
MED: Extremely high (>100 mJ/cm²)		 Maximal pigmentation response
MMD: Highest threshold
MED (Minimal Erythema Dose): The minimum UVB radiation dose that produces perceptible redness (erythema) 24 hours after exposure.
MMD (Minimal Melanogenic Dose): The minimum UVA radiation dose that produces perceptible darkening or tanning (melanogenesis) of the skin.
4

Choose Your Environment

Indicate whether you're outdoors, behind glass, or using UV-blocking materials. Remember: standard window glass blocks most UVB but allows significant UVA through— important for drivers and office workers.

5

Add Sunscreen Protection

Select your sunscreen level. For UVA, use PA ratings (PA+ through PA++++). For UVB, use SPF values (SPF 15-100). Higher values provide exponentially better protection.

6

Explore the Heatmap

The 12×13 grid shows UV irradiance (W/m²) for every month and hour (6 AM-6 PM). Hover over any cell to see the exact irradiance value and time to reach skin damage threshold. Darker colors indicate higher intensity.

Key Facts About UV Exposure

⏰
Time Matters UVB peaks between 10 AM–4 PM, while UVA remains more constant throughout daylight hours
💡
Glass Blocks UVB, Not UVA Standard window glass blocks ~60% of UVB but allows ~75% of UVA to pass through
🌍
Latitude Effects UV intensity increases closer to the equator and at higher elevations
👥
Individual Variation Skin type (Fitzpatrick I-VI) determines your susceptibility to UV damage
🧴
Protection Standards SPF protects against UVB; PA rating (or UVA-PF) protects against UVA
📊
Cumulative Damage UV exposure accumulates over time—even through windows and on cloudy days

⚗️ Scientific Methodology

This calculator uses rigorous solar geometry and atmospheric physics to compute UV irradiance:

  • Solar declination calculated from day of year using celestial mechanics
  • Solar elevation angle computed from latitude, declination, and hour angle
  • Air mass determined using the Kasten-Young formula for atmospheric path length
  • Atmospheric transmission modeled with Beer-Lambert law (τ = 0.15 for UVA, 0.35 for UVB)
  • Earth-Sun distance correction applied throughout the year

Damage thresholds are based on established photobiology research:

  • MED (Minimal Erythema Dose) for UVB: the exposure causing slight skin redness after 24 hours
  • MMD (Minimal Melanogenic Dose) for UVA: the exposure causing immediate pigment darkening
  • Values calibrated to Fitzpatrick skin phototypes using published dermatological data