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.
"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."