Ambient air pollution and term birth weight in Texas from 1998 to 2004

Figures & data

Table 1. Descriptive statistics of the study population and exposures (n = 1,548,904)

Variables relating to birth and mother	Value
Birth weight (g)	3 382.8 ± 467.2
Low birth weight [< 2 500 g (%)]	2.8
Neonate's gender (%)
Male	50.9
Female	49.1
Type of birth (%)
Primary cesarean section	14.4
Repeat cesarean section	11.3
Vaginal birth	72.8
Vaginal birth after cesarean	1.4
Unknown	0.1
First child (%)
Yes	59.8
No	38.7
Unknown	1.5
Month prenatal care began (%)
No care	1.6
First 3 months of pregnancy	79.4
4th–6th month of pregnancy	14.5
7th month of pregnancy or later	3.1
Unknown	1.4
Gestational length (weeks)	39.6 ± 1.52
37–38 (%)	24.9
39–40 (%)	50.7
41–42 (%)	20.2
43–44 (%)	4.2
Alcohol use by mother (%)
Yes	0.9
No	98.4
Unknown	0.7
Tobacco use by mother (%)
Yes	4.9
No	94.5
Unknown	0.6
Mother's education (%)
<12 years	32.6
12 years	29.2
13–15 years	16.6
>15 years	20.2
Unknown	1.4
Mother's race/ethnicity (%)
Non-Hispanic white	34.7
Non-Hispanic black	10.6
Hispanic	50.7
Other^a	4.0
Mother's marital status (%)
Married	68.6
Unmarried	31.3
Other	0.1
Mother's age (years)	26.5 ± 6.1
<20 (%)	13.7
20–24 (%)	27.4
25–29 (%)	27.4
30–34 (%)	20.8
35–39 (%)	8.9
>39 (%)	1.7
	0.0
County level weather data
Temperature (˚F)	69.5 ± 5.4
Dew point temperature (˚F)	56.6 ± 8.0
Wind speed (miles per hour)	8.7 ± 1.6
Year of birth (%)
1998	13.0
1999	13.4
2000	14.0
2001	14.2
2002	15.1
2003	15.3
2004	15.1
Gestational pollution exposures	(mean ± SD)
NO2 (IQR 2.4 ppb)	15.0 ± 1.9 ppb
CO (IQR 98 ppb)	451 ± 91 ppb
SO2 (IQR 1.6 ppb)	2.3 ± 0.8 ppb
O3 (IQR 5.9 ppb)	25.4 ± 4.9 ppb
PM10 (IQR 2.7 µg/m^3)	27.4 ± 4.1 µg/m^3
PM2.5 (IQR 1.0 µg/m^3)	12.6 ± 1.0 µg/m^3
Public Health Service Regions (%)
1	2.0
3	31.8
4	2.0
5	1.5
6	26.0
7	6.2
8	8.8
9	0.6
10	4.4
11	12.6
Notes: Values are percentages or mean ± SD.
^aIncludes North American Indian, South or Central American Indian, Chinese, Asian, Indian, Japanese, Korean, Hawaiian, Samoan, Filipino, Vietnamese, Other Asian, and Guamanian.

Table 2. Difference in birth weight associated with selected nonpollution variables (95% confidence interval) from a model without air pollution variables

Variable	Difference in birth weight (g)
Child's sex
Female (reference)
Male	122.5 (123.9 to 121.1)
Mother's education (years)
12 (reference)
<12	−4.7 (–6.6 to –2.7)
13–15	19.6 (17.3 to 21.8)
>15	20.3 (18.0 to 22.6)
Unknown	−0.2 (–6.5 to 6.1)
Tobacco use by mother
No (reference)
Yes	−170.5 (–173.9 to –167)
Alcohol use by mother
No (reference)
Yes	−14 (–21.5 to –6.4)
Mother's marital status
Married (reference)
Unmarried	−32.2 (–33.9 to –30.4)
Mother's race/ethnicity
Non-Hispanic white (reference)
Hispanic	−64.1 (–66.0 to –62.3)
Non-Hispanic black	−168.1 (–170.7 to –165.5)
Other	−202.9 (–206.7 to –199.1)
Month prenatal care began
Month 1–3 (reference)
Month 4–6	−10.1 (–12.1 to –8.0)
Month ≥ 7	−27.4 (–31.5 to –23.4)
No care	−67.5 (–73.1 to –62.0)
Mother's age (years)
30–34 (reference)
<20	−92.1 (–95.0 to –89.1)
20–24	−59.7 (–62.0 to –57.5)
25–29	−21.5 (–23.6 to –19.5)
35–39	7.9 (5.1 to 10.7)
>39	−8.8 (–14.4 to –3.3)
Birth order
Firstborn (reference)
Not firstborn	75.4 (72.8 to 76.1)
Gestational length (weeks)
39–40 (reference)
37–38	–208.1 (–209.8 to –206.3)
41–42	102.3 (100.4 to 104.2)
43–44	43.5 (39.9 to 47.1)
Public Health Service Regions
1	−81.7 (−86.7 to 76.7)
3 (reference)
4	3.7 (−0.93 to 8.3)
5	−10.8 (−16.6 to −4.9)
6	−9.7 (−11.5 to −7.9)
7	0.6 (−2.5 to 3.6)
8	−40.3 (−43 to −37.6)
9	−76.6 (−85.8 to −67.4)
10	−88.4 (−92.0 to −84.8)
11	−34.8 (−37.3 to −32.4)
Note: Significant at p < 0.001 for all associations except for mother's education category “Unknown” and Public Health Service Regions 4 and 7.

Table 3. Single pollutant model for change in birth weight per IQR increase in pollution for the gestational period (95% confidence interval)

Pollutant	n	Difference in birth weight (g)/IQR
NO2	930 296	1.4 (−2.43, 5.24)
CO	953 735	1.85 (−0.15, 3.84)
SO2	630 162	−4.99 (−8.11, −1.87)^a
O3	1 081 804	−2.72 (−4.33, −1.11)^a
PM2.5	759 465	2.49 (0.58, 4.41)^a
PM10	1 014 645	1.30 (0.14, 2.46)^a
Note: Model was adjusted for convariates listed in Table 2, as well as for year of birth and weather data.^aSignificant at p < 0.001.

Figure 1. Change in birth weight per IQR increase in gestational exposure to pollutant, for single- (indicated as none or no adjustment by another pollutant) and two-pollutant linear models (adjusted for Texas Health Service Regions 1–11). The point reflects the central estimate; the vertical line represents the 95% confidence interval.

Table 4. Single-pollutant model for change in birth weight per IQR increase in pollutant for trimester exposure

Pollutant	Trimester	Range among the trimester models for change in birth weight (g) per IQR increase in pollutant
O3	1st	–1.45 to –1.31
	2nd	–1.23 to –1.00
PM2.5	1st	1.96 to 2.19
Note: For comparability, the IQR for gestational exposure was used.
Model was adjusted for covariates listed in Table 2, as well as year of birth and weather data.

Table 5. Change in birth weight per IQR increase in pollutant for gestational period, by race/ethnicity (95% confidence interval)

	Difference in birth weight (g) per IQR of pollutant
Pollutant	Non-Hispanic black mother	Hispanic mother	Non-Hispanic white mother
O3	–3.89 (–6.6 to –1.17)	–1.87 (–3.95 to 0.21)	–2.97 (–5.15 to –0.78)
SO2 ^a	–1.81 (–7.54 to 3.93)	–6.39 (–9.62 to –3.16)	0.34 (–4.22 to 4.9)
PM2.5 ^a	1.93 (–3.95 to 7.82)	3.35 (1.37 to 5.33)	–3.44 (–7.01 to 0.12)
PM10 ^a	9.62 (6.28 to 12.96)	1.59 (0.35 to 2.83)	–2.3 (–4.19 to –0.41)
Notes: Model was adjusted for covariates listed in Table 2, as well as year of birth and weather data. ^aSignificant at p < 0.05 for comparison of results from infants with non-Hispanic black mothers to infants of non-Hispanic white or Hispanic mothers.