Patient factors	Environmental factors	Social factors
Prematurity	Exposure to tobacco smoke	Older siblings
Low birthweight	Exposure to other air pollutants	Daycare attendance
Chronic lung disease, particularly bronchopulmonary dysplasia; abnormal airway anatomy	Living at an altitude higher than 2500 m (~8200 ft)a	Twins and other multiple births
Hemodynamically significant congenital heart disease, especially if associated with left-to-right shunting	Crowded household
Age less than 3 months
Immunodeficiency
Down syndrome
Neurologic disease
Native American

^aExamples: Asia, Lhasa, Tibet; North America, Leadville, CO, USA; South America, Quito, Ecuador; Africa, Adi Keyh, Eritrea. The highest inhabited towns on the other three continents are below 8200 feet. Europe, Ushguli, Georgia, at 6900 feet; Australia, Perisher Village, at 5600 feet; and Antarctica, both small settlements sitting close to sea level