Table 1 Characteristics of the study population by cohort and overall

Child sex

 Female

355 (50.9%)

204 (53.3%)

559 (51.7%)

 Male

343 (49.1%)

179 (46.7%)

522 (48.3%)

Child age at clinic visit (years)

 8

394 (56.4%)

211 (55.1%)

604 (55.9%)

 9

230 (33.0%)

146 (38.1%)

376 (34.8%)

 10

74 (10.6%)

26 (6.8%)

100 (9.3%)

First born

 Yes

276 (39.5%)

200 (52.2%)

476 (44.0%)

 No

422 (60.5%)

174 (45.4%)

596 (55.1%)

 Missing

0 (0%)

9 (2.3%)

9 (0.8%)

Postnatal second-hand smoke exposure

 Yes

163 (23.4%)

7 (1.8%)

170 (15.7%)

 No

518 (74.2%)

347 (90.6%)

865 (80.0%)

 Missing

17 (2.4%)

29 (7.6%)

46 (4.3%)

Maternal education at enrollment

 Less than high school

58 (8.3%)

18 (4.7%)

76 (7.0%)

 High school completion

311 (44.6%)

44 (11.5%)

355 (32.8%)

 Graduated college or technical school

233 (33.4%)

132 (34.5%)

365 (33.8%)

 Some or more graduate school

95 (13.6%)

186 (48.6%)

281 (26.0%)

 Missing

1 (0.1%)

3 (0.8%)

4 (0.4%)

Maternal race

 Black or African American

433 (62.0%)

38 (9.9%)

471 (43.6%)

 Multiple races

37 (5.3%)

10 (2.6%)

47 (4.3%)

 White

219 (31.4%)

285 (74.4%)

504 (46.6%)

 Other

9 (1.3%)

46 (12.0%)

55 (5.1%)

 Missing

0 (0%)

4 (1.0%)

4 (0.4%)

Maternal ethnicity

 Hispanic or Latino

11 (1.6%)

24 (6.3%)

35 (3.2%)

 Not Hispanic or Latino

687 (98.4%)

355 (92.7%)

1042 (96.4%)

 Missing

0 (0%)

4 (1.0%)

4 (0.4%)

Maternal history of asthma

 Yes

118 (16.9%)

63 (16.4%)

181 (16.7%)

 No

558 (79.9%)

310 (80.9%)

868 (80.3%)

 Missing

22 (3.2%)

10 (2.6%)

32 (3.0%)

Maternal urinary cotinine (ng/mL)

 Median [Min, Max]

0.221 [0.00141, 197]

0.00919 [0.00919, 159]

0.0937 [0.00141, 197]

 Missing

4 (0.6%)

0 (0%)

4 (0.4%)

Adjusted household income (USD)

 Mean (SD)

41,000 (28,400)

112,000 (61,200)

66,000 (54,700)

 Median [Min, Max]

31,000 [2,490, 83,400]

111,000 [6,160, 213,000]

51,600 [2,490, 213,000]

 Missing

40 (5.7%)

22 (5.7%)

62 (5.7%)