Maternal tobacco smoking during pregnancy is associated with increased activity, decreased attention, and diminished intellectual abilities.[44] However, a recent study finds that maternal tobacco smoking has no direct causal effect on the child's IQ. Adjusting for maternal cognitive ability as measured by IQ and education eliminated the association between lower IQ and tobacco smoking.[45] But another study instead looking at the relationship between environmental tobacco smoke exposure, measured with a blood biomarker, and cognitive abilities among U.S. children and adolescents 6–16 years of age, found an inverse association between exposure and cognitive ability among children even at extremely low levels of exposure. The study controlled for sex, race, region, poverty, parent education and marital status, ferritin, and blood lead concentration.[46]