Air pollution and vitamin D status

In response to the following comment: "Pollution levels are high in cities and clear, blue skies have become rare. I think the haze blocks a lot of the sun."

There is a large literature on this issue; the following is one example: 

This study compared vitamin D status in 200, free-living, woman, aged between 20 to 55 years, from Tehran, a high-polluted area, and Ghazvin, a low-polluted area. Level of UVB (ultra-violet B - the part of the light spectrum the skin uses to make vitamin D) was measured; less UVB means more haze or more pollution. The average blood vitamin D levels were significantly higher in women from the low pollution area and was related to the degree of haze or pollution over these cities. 

Hosseinpanah et al. The effects of air pollution on vitamin D status in healthy women: a cross sectional study. BMC Public Health. 2010 Aug 29;10:519.

"Therefore living in a polluted area plays a significant and independent role in contributing to vitamin D deficiency."

"I spent 4 days in Tehran several years ago; believe me when I say that it is one of the most congested  and polluted cities I have ever visited. More importantly vitamin D deficiency and multiple sclerosis area major emerging problem in Iran."