RT Journal Article SR Electronic T1 Vitamin D deficiency and the asthma epidemic JF Thorax JO Thorax FD BMJ Publishing Group Ltd and British Thoracic Society SP 293 OP 293 DO 10.1136/thx.2007.091728 VO 63 IS 3 A1 S O Shaheen YR 2008 UL http://thorax.bmj.com/content/63/3/293.1.abstract AB