Table 4.

Associations between exposure to ${\mathrm{PM}}_{2.5}$ across quartiles of glutathione-related oxidative potential (${\text{OP}}^{\text{GSH}}$) over different periods of exposure during pregnancy and birth outcomes in Ontario, Canada (2006–2012).

Birth outcome and exposure time period	Percentile of ${\text{OP}}^{\text{GSH}}$				p-Value for effect modification
	$<25\text{th}$	25–50th	50–75th	$>75\text{th}$
	Term low birth weight OR (95% CI)
First trimester	0.99 (0.87, 1.14)	1.00 (0.85, 1.18)	1.06 (0.91, 1.23)	1.28 (1.10, 1.48)	0.03
Second trimester	0.95 (0.86, 1.04)	0.98 (0.85, 1.11)	1.01 (0.87, 1.17)	0.94 (0.83, 1.06)	0.93
Third trimester	0.92 (0.83, 1.03)	0.90 (0.78, 1.04)	0.96 (0.78, 1.19)	0.97 (0.85, 1.12)	0.43
Whole pregnancy	0.85 (0.72, 1.00)	0.90 (0.69, 1.17)	1.04 (0.82, 1.33)	1.22 (0.94, 1.57)	0.01
Preterm birth
Chronic exposure OR (95% CI)	—	—	—	—	—
First trimester	0.87 (0.82, 0.92)	0.65 (0.30, 1.41)	0.94 (0.87, 1.02)	1.00 (0.88, 1.13)	0.72
Second trimester	0.95 (0.85, 1.06)	1.11 (0.81, 1.52)	0.96 (0.87, 1.06)	1.00 (0.86, 1.15)	0.89
Third trimester	0.98 (0.93, 1.02)	0.97 (0.90, 1.03)	1.03 (0.94, 1.11)	0.97 (0.87, 1.09)	0.29
Whole pregnancy	0.91 (0.69, 1.19)	1.01 (0.61, 1.90)	0.94 (0.84, 1.06)	0.98 (0.79, 1.21)	0.39
Acute exposure HR (95% CI)	—	—	—	—	—
Last 4 wk of pregnancy	0.96 (0.92, 1.00)	0.97 (0.90, 1.05)	0.85 (0.73, 1.00)	0.96 (0.86, 1.08)	0.65
Same day of delivery	0.97 (0.95, 1.00)	0.99 (0.96, 1.02)	1.01(0.99, 1.03)	1.02 (1.01, 1.04)	0.04
Term birth weight $\mathrm{\beta }$ (95% CI)
First trimester	5.25 ($-6.62$, 17.13)	$-0.39$ ($-7.98$, 7.19)	$-5.56$ ($-16.25$, 5.12)	4.48 ($-5.26$, 14.22)	0.55
Second trimester	2.71 ($-7.48$, 12.90)	3.35 ($-4.14$, 10.84)	2.99 ($-8.20$, 14.29)	$-1.03$ ($-10.49$, 8.44)	0.95
Third trimester	5.88 ($-3.60$, 15.37)	8.50 (1.26, 15.74)	9.42 ($-2.19$, 21.03)	8.23 ($-0.57$, 17.04)	0.62
Whole pregnancy	10.31 ($-5.27$, 25.90)	8.94 ($-3.01$, 20.89)	6.04 ($-10.33$, 22.41)	10.46 ($-3.47$, 24.39)	0.71

Note: Estimates of association are for IQR increases in exposure to ${\mathrm{PM}}_{2.5}$ ($2.6{\mathrm{\mu }\mathrm{g}/\mathrm{m}}^3$). Models represent pooled city-specific estimates derived using two-stage random-effects meta-analysis and logistic regression [term low birth weight (LBW) and ORs for preterm birth in association with chronic exposures], Cox proportional hazard models (HRs for preterm birth in association with IQR increases in exposure to ${\mathrm{PM}}_{2.5}$ ($7.1{\mathrm{\mu }\mathrm{g}/\mathrm{m}}^3$) during the last 4 wk of pregnancy or on the day of delivery), or linear regression (term birth weight). Models were adjusted for maternal age at delivery, marital status, maternal cigarette smoking during pregnancy, infant sex, parity, previous caesarean section delivery, maternal comorbidities (i.e. asthma, hypertension, type 1 and 2 diabetes mellitus, preeclampsia, and gestational diabetes), year of birth, month of birth, census dissemination-area (DA) median family income, census DA proportion of population who are visible minority, and census DA proportion of the adult female population aged 25–64 y old who completed postsecondary education; gestational age was also included in term LBW and term birth weight models; mean temperature and mean relative humidity were also included in preterm birth acute exposure models. Random-effects multivariate meta-regression models were used to test potential effect modification by between-city differences in ${\text{OP}}^{\text{GSH}}$. The outcome variables in the meta-regression models in this study were the pooled estimates, and the explanatory variable (i.e. potential effect modifier) was the categorical variable of ${\text{OP}}^{\text{GSH}}$ at the city level. Effect modification was considered statistically significant if the effect modifier’s p-value was less than 0.05. —, data not available; CI, confidence interval; HR, hazard ratio; OR, odds ratio; ${\mathrm{PM}}_{2.5}$_, fine particulate matter air pollution with aerodynamic diameter $\le 2.5\mathrm{\mu }\mathrm{m}$; ${\mathrm{PM}}_{2.5}\times {\text{OP}}^{\text{AA}}$, ascorbate-related oxidative burden; ${\mathrm{PM}}_{2.5}\times {\text{OP}}^{\text{GSH}}$, glutathione-related oxidative burden; $\mathrm{\beta }$, beta coefficient.