Characteristics of the 2844 participants
| Characteristics | n (%) |
| Gender | |
| Female | 1398 (49.2) |
| Male | 1446 (50.8) |
| Enrolment status | |
| Farmer | 1491 (52.4) |
| Spouse | 1353 (47.6) |
| State | |
| Iowa | 2055 (72.3) |
| North Carolina | 789 (27.7) |
| Current asthma status | |
| Case | 1041 (36.6) |
| Non-case | 1803 (63.4) |
| Smoking status | |
| Never | 1884 (66.2) |
| Former | 839 (29.5) |
| Current | 121 (4.3) |
| Season of endotoxin measurement (n=2208)* | |
| Summer | 628 (28.4) |
| Spring | 586 (26.5) |
| Fall | 492 (22.3) |
| Winter | 502 (22.7) |
| FEV1 | |
| Median (25th–75th percentiles) (L) | 2.5 (2.0–3.1) |
| FVC | |
| Median (25th–75th percentiles) (L) | 3.4 (2.8–4.2) |
| FEV1/FVC | |
| Median (25th–75th percentiles), proportion | 0.75 (0.69–0.79) |
| Age | |
| Median (25th–75th percentiles) (years) | 62.8 (54.8–71.3) |
| Pack-years in ever smokers | |
| Median (25th–75th percentiles) | 9 (1.5–26.9) |
| Number of cigarettes per day in current smokers | |
| Median (25th–75th percentiles) | 10 (5–20) |
| Endotoxin in house dust (n=2208) | |
| Median (25th–75th percentiles) (EU/mg) | 43.5 (20.1–73.5) |
*House dust endotoxin data were available for 2208 participants after removing the 177 for whom a visit was also made to a spouse.