Associations between serum 25(OH)D concentrations and prevalent asthma among children living in communities with differing levels of urbanization: a cross-sectional study oleh Suzanne L. Pollard, John J. Lima, Karina Romero, Carla Tarazona-Meza, Edward Mougey, Katherine Tomaino, Gary Malpartida-Guzmán, Nadia N. Hansel, William Checkley, GASP Study Investigators