Shoukat, S., S. Irshad, N. Fida, A. Mustafa, R. Shah, and M. Khan. “Impact of Vitamin D on Respiratory Function and Immune Health on Patients With Chronic Obstructive Pulmonary Disease: A Systematic Review”. Pakistan Journal of Chest Medicine, vol. 28, no. 4, Dec. 2022, pp. 522-9, https://www.pjcm.net/index.php/pjcm/article/view/876.