Gatzke-Kopp L, et al. - In the present study, researchers investigated if the magnitude of exposure to environmental smoke across the first four years of life demonstrates a linear association with later externalizing symptoms among children. Salivary cotinine measures were determined to quantify the exposure when children were 6, 15, 24, and 48 months of age, rendering more precise quantification of realized exposure than can be determined from the parental report of cigarettes smoked. The Family Life Project, a study of child development in areas of rural poverty, provided data for n = 1,096 (50% male; 44% African American) children. As per the analyses, cotinine had a linear correlation with children’s symptoms of hyperactivity and conduct problems. After controlling for family poverty level, parental education, parental history of ADHD, hostility, depression, caregiver IQ, and obstetric complications, this correlation remained significant. This highlights the significance of decreasing children’s exposure to environmental smoke, including sources that extend beyond their parents.