Vitamin D and Air Pollution
Air pollution reduces UVB reaching the ground — particulate matter scatters and absorbs UV, and increased ozone at ground level absorbs UVB specifically. Adults living in heavily polluted cities have lower serum 25(OH)D than adults in cleaner cities at the same latitude. Wildfire smoke events can produce dramatic short-term reductions.
How pollution blocks UVB
- Particulate matter (PM2.5, PM10) — scatters and absorbs UV. Heavy pollution can reduce UVB by 20–50%.
- Tropospheric ozone (ground-level ozone) — direct UVB absorption. Not to be confused with the protective stratospheric ozone layer.
- Sulphur dioxide, nitrogen oxides — moderate UV absorption; more relevant in industrial pollution scenarios.
- Water vapour and aerosols — general UV scattering.
City studies
- Delhi, India — despite being at latitude 28° N with abundant potential UVB, adult deficiency rates approach 90% in urban Delhi. Air pollution, dark skin, and modest outdoor time compound.
- Tehran, Iran — extremely high deficiency rates in a sunny climate; multi-year smog contributes.
- Beijing, China — measurable UVB reduction during severe smog episodes. Deficiency rates elevated compared to cleaner Chinese cities at similar latitude.
- Los Angeles, historical — 1960s–70s air pollution documented UVB reduction; improvements since have raised measurable UV.
Wildfire smoke
An increasing concern in the American West, Australia, and parts of the Mediterranean. Heavy wildfire smoke can reduce ground-level UVB by 30–80% during severe events lasting days to weeks. Combined with air quality guidance to stay indoors, wildfire seasons produce a double vitamin D hit: less UVB in the air and less time spent outside. Supplementation becomes essential during multi-week smoke events.
Volcanic and Saharan dust
Saharan dust events reach the Canary Islands, southern Europe, and even the Caribbean and southern US. Volcanic ash from major eruptions (Pinatubo 1991, Eyjafjallajökull 2010) can reduce global UVB for weeks to years — Pinatubo measurably lowered global surface UV for ~2 years. Rare events; supplement is the answer when they occur.
Historical: London smog
Coal-burning London in the 19th and early 20th centuries produced pea-soup fogs thick enough to reduce ground-level UV to near zero for days. Endemic childhood rickets in industrial northern cities — Glasgow, Manchester, Liverpool — is directly attributable to this combination of high latitude + severe pollution + poor diet. Clean Air Acts and cod liver oil largely solved the problem.
Practical guide
- Living in a chronically polluted city: assume 20–30% UVB reduction; supplement year-round.
- During wildfire smoke or extreme pollution events: increase supplementation, follow air quality guidance for time outdoors.
- Use AQI apps — when air is unhealthy, don't try to "get sun" as a compensation strategy.
- Air quality improvements matter for many reasons; vitamin D synthesis is a small one among big benefits.