TABLE 1.
Characteristics of study subjects by neighborhood socioeconomic status and season, Los Angeles County, California, 1994–1996*
| Low SES†,‡ |
Middle SES§ |
High SES¶ |
||||
|---|---|---|---|---|---|---|
| Summer | Winter | Summer | Winter | Summer | Winter | |
| Traffic-related pollution exposure | ||||||
| Distance-weighted traffic density (%) | ||||||
| <20th percentile | 16 | 16 | 21 | 20 | 23 | 23 |
| 20–<80th percentile | 64 | 64 | 60 | 60 | 56 | 57 |
| ≥80th percentile | 20 | 20 | 19 | 20 | 21 | 20 |
| Risk factors | ||||||
| Maternal race/ethnicity (%) | ||||||
| White | 3 | 2 | 4 | 4 | 27 | 28 |
| Hispanic | 83 | 82 | 81 | 80 | 53 | 52 |
| African American | 13 | 14 | 8 | 9 | 7 | 11 |
| Other | 2 | 2 | 7 | 7 | 12 | 12 |
| Maternal age (%) | ||||||
| <20 years | 21 | 20 | 17 | 17 | 11 | 12 |
| 20–29 years | 55 | 55 | 55 | 56 | 49 | 50 |
| 30–34 years | 15 | 16 | 18 | 17 | 25 | 24 |
| ≥35 years | 10 | 9 | 11 | 10 | 15 | 14 |
| Protective factors | ||||||
| Maternal education (%) | ||||||
| <9 years | 35 | 36 | 31 | 31 | 15 | 14 |
| 9–11 years | 33 | 32 | 29 | 29 | 18 | 19 |
| 12 years | 23 | 22 | 25 | 24 | 27 | 27 |
| 13–15 years | 7 | 8 | 10 | 10 | 18 | 19 |
| ≥16 years | 2 | 2 | 5 | 5 | 21 | 20 |
| Payment for delivery (%) | ||||||
| Private insurance | 15 | 16 | 23 | 24 | 49 | 51 |
| Government insurance | 83 | 82 | 74 | 74 | 47 | 45 |
| Self/uninsured | 1 | 1 | 3 | 2 | 4 | 4 |
| Prenatal care (%) | ||||||
| None/after first trimester | 28 | 28 | 26 | 27 | 18 | 20 |
| During first trimester | 71 | 71 | 74 | 72 | 82 | 80 |
| Control variables | ||||||
| Infant’s sex (%) | ||||||
| Male | 51 | 51 | 51 | 51 | 51 | 51 |
| Female | 49 | 49 | 49 | 49 | 49 | 49 |
| Parity (%) | ||||||
| 1st birth | 35 | 35 | 37 | 37 | 44 | 45 |
| 2nd or subsequent birth | 65 | 65 | 63 | 63 | 56 | 55 |
| Time since previous livebirth (%) | ||||||
| ≤12 months | 3 | 3 | 3 | 3 | 2 | 2 |
| >12 months | 96 | 96 | 97 | 97 | 97 | 98 |
| Previous LBW† or preterm infant (%) | ||||||
| 0 | 99 | 99 | 99 | 99 | 99 | 99 |
| ≥1 | 1 | 1 | 1 | 1 | 1 | 1 |
| Year of birth (%) | ||||||
| 1994 | 34 | 35 | 33 | 34 | 32 | 33 |
| 1995 | 35 | 34 | 35 | 35 | 35 | 35 |
| 1996 | 30 | 30 | 32 | 31 | 34 | 32 |
| Live near freeway (%) | 14 | 15 | 13 | 12 | 14 | 13 |
| Background annual average pollutants (mean (SD†)) |
||||||
| Carbon monoxide, ppm | 2.0 (0.4) | 1.8 (0.5) | 1.7 (0.5) | |||
| PM10,† μg/m3 | 4.0 (0.4) | 4.0 (0.4) | 3.9 (0.4) | |||
| Nitrogen dioxide, pphm† | 4.4 (0.4) | 4.1 (0.7) | 3.6 (0.7) | |||
| Ozone, pphm | 1.7 (0.3) | 1.9 (0.4) | 2.3 (0.4) | |||
*
Some percentages may not sum to 100% because of missing values or rounding.
†
SES, socioeconomic status; LBW, low birth weight; SD, standard deviation; PM10, particulate matter with a diameter of <10 μm; pphm, parts per hundred million of air by volume.
‡
No. of observations for “low SES”: summer (n = 5,482); winter (n = 5,545).
§
No. of observations for “middle SES”: summer (n = 5,779); winter (n = 5,859).
¶
No. of observations for “high SES”: summer (n = 7,534); winter (n = 7,148).