Previous studies have demonstrated that children born to mothers who smoke during pregnancy are at increased risk for attention-deficit/hyperactivity disorder (ADHD).  In a recent review, Huang and colleagues looked at data from 15 cohort studies and 5 case-control studies in order to better quantify this risk.  The analysis included a total of 50,044 children with ADHD and 2,998,059 controls. The researchers calculated that maternal smoking during pregnancy was associated with an increased risk of ADHD in the exposed offspring (OR: 1.60).  Furthermore, the risk of ADHD was greater for children born to mothers who were heavy smokers (OR: 1.75) compared to children whose mothers were light smokers (OR: 1.54).

A new study has looked at the impact of environmental smoke exposure occurring during the early years of the child’s life.  In order to quantify exposure, cotinine levels in the children’s saliva were measured.  Cotinine is a byproduct of nicotine, and blood or saliva levels of cotinine reflect recent exposure to nicotine in tobacco smoke and are more accurate for quantifying level of exposure than estimating the number of cigarettes smoked. 

Cotinine levels were measured in 1096 children participating in the Family Life Project, a study of child development in areas of rural poverty.  Levels were assessed at 6, 15, 24, and 48 months of age

The researchers observed a linear relationship between children’s cotinine levels and their symptoms of hyperactivity and conduct problems. This association remained significant even after controlling for family socioeconomic status, parental education and IQ, parental history of ADHD, and obstetric complications. Furthermore, this association was significant after excluding mothers who smoked during pregnancy from the analysis.

This is the first study to demonstrate that environmental exposure to nicotine early on in a child’s life is associated with increased risk for attention and behavior problems, even in children whose mothers did not smoke during pregnancy.  When a mother smokes during pregnancy, the developing brain may be exposed to higher levels of nicotine; however, this study indicates that exposure to lower levels of nicotine during the first four years of the child’s life can also have deleterious effects.  

Although smoking rates have decreased over the last few decades, the number of children exposed to secondhand smoke remains high.  According to a recent study from the CDC, between 2013 and 2016, 35.4% of children were exposed to secondhand smoke from tobacco products .The rates are even higher in African-American children and children of lower socioeconomic status.   Because this exposure to secondhand smoke may have deleterious effects, we must query mothers not only about their smoking habits but also about the child’s exposure to family members, friends, and care providers who may smoke.  

Ruta Nonacs, MD PhD

Gatzke-Kopp L, Willoughby MT, Warkentien S, Petrie D, Mills-Koonce R, Blair C.  Association between environmental tobacco smoke exposure across the first four years of life and manifestation of externalizing behavior problems in school-aged children. J Child Psychol Psychiatry. 2019 Dec 3.