Influence of high levels of cloud cover on vitamin D effective and erythemal solar UV irradiances.
Level 5 - mechanism / opinion, no new human data
Physical atmospheric measurement study without human participants (level by analogy)
PubMed 23108371 · doi:10.1039/c2pp25160d
What was done
Solar irradiances for vitamin D3 synthesis (UV(D3)) were measured concurrently with cloud cover amounts, focusing on high cloud conditions (6.5 octa or greater) across five solar zenith angle (SZA) ranges up to 80°. Researchers calculated the ratio of cloudy-day to clear-day irradiances (controlling for ozone and SZA) and modeled the exposure times needed to receive 1/3 minimal erythemal dose (MED) on a horizontal plane.
What was found
UV(D3) was reduced by high cloud cover across all SZA ranges. The average ratio of cloudy-to-clear-day UV(D3) irradiance was 0.71 for 6.5–7.5 octa cloud cover and 0.45 for >7.5 octa cloud cover. The calculated exposure times necessary to receive 1/3 MED increased as both cloud cover and SZA increased.
Why it matters
Standard public health recommendations for solar exposure times based on clear-sky models underestimate the exposure duration required to synthesize vitamin D under conditions of substantial cloud cover.
Limits
The study is based purely on physical atmospheric irradiance measurements on a horizontal plane; it did not measure in vivo human vitamin D synthesis or serum 25(OH)D levels. The abstract does not specify the sample size (number of measurement days), geographic location, or cloud type composition.
Cited by
- supports Up to 80% of UV radiation penetrates light cloud cover, whereas thick, dense storm clouds can block 70% to 90% of UVB rays.