Definition | Total (45) | Cohort (6) | Cross-sectional (30) | Case–control (9) | |
Home environment | Water and sanitation, garbage disposal, house cleaning, housing construction, endotoxins | 9 | 2 | 6 | 1 |
Animal contact | Contact with pets, farm animals and cockroaches inside/outside the house | 34 | 5 | 23 | 6 |
Contact with other children | Overcrowding in home, day-care attendance and having older siblings | 22 | 6 | 11 | 5 |
Early-life infection | Acute respiratory and gastrointestinal infections during infancy, worm infections | 11 | 4 | 6 | 1 |
Demonstrated infection | Intestinal parasites or chronic viral or bacterial infections diagnosed by serology or stool sample | 21 | 4 | 11 | 6 |
Numbers represent the number of studies that measured at least one exposure related to each category, even if the results were not published.