Maternal exposure levels to air pollution. (a) Box-plot of maternal exposure levels to background pollutants three months before pregnancy (BP) and during each trimester of pregnancy (t1, t2, t3) in the total sample of 370 mothers. (b) Box-plot of maternal exposure to benzene, ethylbenzene, toluene and xylenes for seven consecutive days of the second trimester in the sub-group of 56 non-smoking mothers with BETX (Benzene, Ethylbenzene, Toluene, Xylenes) assessment. The bottom and top edges of the boxes are located at the first quartile and third quartile of the sample. Within the boxes, the median is displayed as a line (second quartile). The circles displayed beyond upper and lower fences are outliers and stand for values three times above or below the interquartile range (the difference between the first and third quartile). Bz = benzene; EB = ethylbenzene; Tol = toluene; Xyl = xylenes.