Table 6. Parental (paternal+maternal) occupational social contact level and risk of childhood ALL by rural/urban status, the Northern California Childhood Leukaemia Study, 1995–2002, ages 0–4 and ages 5–14.
| Rural/urban | Parental social contacta | Cases, n (%) | Controls, n (%) | OR (95% CI)b | P-value for interactions |
|---|---|---|---|---|---|
| Ages 0–4 | |||||
| Urban | Low | 48 (31.8) | 55 (28.7) | Reference | |
| Urban | Medium | 22 (14.6) | 28 (14.6) | 1.54 (0.72–3.30) | |
| Urban | High | 62 (41.1) | 83 (43.2) | 1.18 (0.68–2.07) | |
| Rural | Low | 7 (4.6) | 11 (5.7) | 0.89 (0.28–2.81) | |
| Rural | Medium | 3 (2.0) | 5 (2.6) | 1.15 (0.22–6.01) | |
| Rural | High | 9 (6.0) | 10 (5.2) | 1.15 (0.36–3.68) | 0.97§ |
| Urban | 0–47.4c | 62 (41.1) | 50 (26.0) | Reference | |
| Urban | 47.5–73.0 | 39 (25.8) | 64 (33.3) | 0.59 (0.33–1.08) | |
| Urban | 73.1 or more | 31 (20.5) | 52 (27.1) | 0.72 (0.35–1.48) | |
| Rural | 0–47.4 | 9 (6.0) | 13 (6.8) | 0.49 (0.16–1.55) | |
| Rural | 47.5–73.0 | 3 (2.0) | 11 (5.7) | 0.26 (0.05–1.33) | |
| Rural | 73.1 or more | 7 (4.6) | 2 (1.0) | 2.81 (0.50–15.76) | 0.04 § |
| Rural × occupational social contact months | 0.19 | ||||
| Ages 5–14 | |||||
| Urban | Low | 35 (35.7) | 36 (31.0) | Reference | |
| Urban | Medium | 10 (10.2) | 17 (14.7) | 0.70 (0.22–2.23) | |
| Urban | High | 34 (34.7) | 45 (38.8) | 0.83 (0.38–1.84) | |
| Rural | Low | 7 (7.1) | 10 (8.6) | 0.49 (0.13–1.82) | |
| Rural | Medium | 2 (2.0) | 0 (0.0) | — | |
| Rural | High | 10 (10.2) | 8 (6.9) | 1.85 (0.44–7.81) | 0.03 § |
| Urban | 0–47.4c | 37 (37.8) | 32 (27.6) | Reference | |
| Urban | 47.5–73.0 | 16 (16.3) | 28 (24.1) | 0.54 (0.21–1.34) | |
| Urban | 73.1 or more | 26 (26.5) | 38 (32.8) | 1.04 (0.43–2.53) | |
| Rural | 0–47.4 | 7 (7.1) | 10 (8.6) | 0.46 (0.12–1.73) | |
| Rural | 47.5–73.0 | 5 (5.1) | 3 (2.6) | 2.19 (0.36–13.15) | |
| Rural | 73.1 or more | 7 (7.1) | 5 (4.3) | 2.70 (0.53–13.72) | 0.06§ |
| Rural × occupational social contact months | 0.10 | ||||
ALL=acute lymphoblastic leukaemia; CI=confidence interval; OR=odds ratio.
The ‘low’ exposure group in the combined parental analysis consists of children whose both parents had a low occupational contact level, the ‘high’ exposure group consists of children with either one of the parents having an occupation with high social contact level, and the remaining children are categorised as the ‘medium’ group.
The ORs were adjusted for annual household income, birth order, total child-hours spent in day care, and the number of other children in household before the index child attended first grade in school using conditional logistic regression.
Occupational social contact months=(1.0 × months of employment with low social contact)+(1.5 × months of employment with medium social contact)+(2.0 × months of employment with high social contact).
P-value was derived from log-likelihood ratio test comparing full model with interaction terms with the submodel without interaction terms. Bold values signifies P-values⩽0.05.