Introduction:
Vitamin D deficiency is a major health problem, which is unexpectedly prevalent in Iran. The ultraviolet (UV) rays of the sun are considered to be the foremost source of vitamin D in humans. In contrast, several environmental factors could decrease UV transmission to the earth, thereby reducing vitamin D absorption. Considering that the key role of environmental factors in vitamin deficiency has been neglected, the present study aimed to investigate the associations between environmental factors (e.g., geographical and air pollution parameters) with the prevalence of vitamin D deficiency in Iran. Methods: The duration of the prevalence of vitamin D deficiency were extracted from the previous studies conducted in different cities in Iran, where vitamin D deficiency was reported. Afterwards, the environmental factors that were reported to affect sunlight transmission through the atmosphere were collected based on the place and time of vitamin D deficiency as mentioned in the reviewed studies via different geographic databases. The associations between the environmental factors and prevalence of vitamin D deficiency were determined. In total, 35 studies were reviewed completely. Results: The results indicated significant correlations between the prevalence of vitamin D deficiency and some environmental factors (e.g., cloudy and clear days). However, no significant association was observed between vitamin D deficiency and other environmental factors, such as geographical parameters (e.g., sunshine, longitude, latitude, elevation, humidity, and temperature) and air pollution (e.g., number of days with dust and visibility of ≤2 km). Conclusion: According to the results, there were significant associations between the prevalence of vitamin D deficiency andenvironmental factors, such as cloudy and clear days. Therefore, adequate exposure to sunlight for the absorption of vitamin D is strongly recommended.