Vitamin D and Altitude
Higher altitude means less atmosphere for UVB to travel through, so more of it reaches the skin. The rule of thumb: roughly 4% more UVB per 300 m (1,000 ft) of elevation gain. Combined with high snow-reflectance in mountain environments, altitude produces surprisingly high effective UV doses — a boon for vitamin D synthesis, and a hazard for sunburn and skin cancer.
The atmospheric thinning effect
- Sea level: 100% UVB reference.
- 500 m (1,650 ft): ~107%.
- 1,000 m (3,300 ft): ~113%.
- 1,500 m (Denver, Bogota, Nairobi): ~120%.
- 2,500 m (Mexico City, Addis Ababa): ~130%.
- 3,500 m (La Paz, Lhasa): ~140%.
- 4,500 m (El Alto, Everest Base Camp): ~150%.
Mountain snowfields double the dose
Fresh snow reflects 80–90% of incident UVB (vs 4% for grass). Winter mountain exposure combines high-altitude reference intensity with high reflectance — effective UV dose can exceed 180% of sea-level equivalent. Ski goggles, high-SPF lip balm, and face sunscreen are not paranoia; snow blindness (photokeratitis) and severe sunburn are common in unprepared skiers.
Highland populations
Andean, Tibetan, Ethiopian, and Bolivian highland populations show better vitamin D status on average than coastal populations at the same latitude. Adaptation: more compact stature, higher haemoglobin, and — relevant here — habitual accommodation to higher UV exposure. Skin cancer rates in Andean highland populations are higher than in coastal Peru at the same latitude.
High-altitude cities
Denver (1,600 m, 39.7° N) has notably higher measured UV than lower cities at the same latitude. Mexico City (2,240 m, 19° N) receives intense UV year-round. Addis Ababa (2,355 m, 9° N) is one of the highest-UV capital cities in the world. La Paz, Bolivia (3,640 m, 16° S) is even higher. Vitamin D deficiency in these populations, when it occurs, is more about indoor lifestyles or dark skin + limited exposure than atmospheric limitation.
Practical implications
- Living at meaningful altitude (> 1,000 m): expect better vitamin D synthesis than sea-level counterparts. Sun exposure needs may be 15–30% shorter.
- Mountain recreation (skiing, hiking above tree line): sunscreen is mandatory; sunburn and skin cancer risk climbs.
- Occasional visitors to high altitude: use SPF 30+, sunglasses, wide-brim hat; expect faster sunburn than at home.
- Chronic mountain residents still need supplementation in winter — altitude helps but doesn't solve the sun-angle problem at high latitudes.
Aviation and vitamin D
Airplane cockpit windows transmit substantial UVA (not UVB — see our windows page). Airline pilots and frequent flyers have documented elevated melanoma risk from chronic UVA exposure at altitude. This is not a vitamin D story — pilots do not make useful vitamin D during flight — but the sun-safety angle matters.