Here are some studies about the effects of air pollution on population vitamin D levels:
Independent Association Between Air Pollutants and Vitamin D Deficiency in Young Children in Isfahan, Iran. (2014)
Conclusion: The blood levels of 97 children living in the highly-polluted city of Isfahan, Iran were analyzed. Those living in the more polluted areas of the city had lower vitamin D levels compared to children living in areas of the city with better air quality.
Gestational Exposure to Urban Air Pollution Related to a Decrease in Cord Blood Vitamin D Levels (2012)
Conclusion: A study of 375 mother-newborn pairs in Poitiers, France and Nancy, France found that pregnant women living in Nancy, with it’s higher air pollution and higher latitude, are more likely to be vitamin D-deficient and give birth to vitamin D-deficient newborns – due to the air pollution blocking the UVB light necessary for cutaneous vitamin D synthesis, the higher latitude of Nancy providing less UVB, and higher air pollution making people more reluctant to go outside. 86% of all newborns in the study had insufficient vitamin D levels and 28% were severely deficient. Infants born in summer had the highest vitamin D levels, followed by infants born in autumn and spring, with infants born during winter having the lowest vitamin D levels.
Conclusion: This study of 200 women living in highly-polluted Tehran, Iran and the less-polluted city of Ghazvin, Iran found that the women living in Ghazvin had significantly higher vitamin D levels than the women of Tehran. 54% of women in Tehran were severely deficient.
Urban Tropospheric Ozone Increases the Prevalence of Vitamin D Deficiency Among Belgian Postmenopausal Women With Outdoor Activities During Summer. (2008)
Conclusion: This study of 85 postmenopausal women living in Belgium at the same latitude found that those living in less-polluted rural areas had an average vitamin D level twice as high as those living in more-polluted Brussels. 84% of urban-dwelling women had inadequate vitamin D levels versus 38% for rural-living women.
The Impact of Atmospheric Pollution on Vitamin D Status of Infants and Toddlers in Delhi, India. (2002)
Conclusion: The vitamin D levels of approximately 50 infants and toddlers living in two different parts of Delhi, India – the more highly-polluted area Mori Gate and less-polluted area Gurgaon – were analyzed. The babies living in the more-polluted Mori Gate had an extremely low average vitamin D blood level of only 12.4 ng/ml, less than half the average level of those living in the Gurgaon area. 12 of the Mori Gate babies had levels below 12 ng/ml, with three of them having nearly undetectable levels, and one infant showing signs of rickets.