Exposure to cigarette smoke in utero and in infancy has been linked to numerous adverse health outcomes, including prematurity, growth restriction, asthma and sudden infant death syndrome. Exposure through breast feeding has recently been scrutinized because studies have shown that breast-fed infants of mothers who smoke have urine cotinine levels from two to 10 times higher than bottle-fed infants who were exposed to environmental tobacco smoke. Mascola and colleagues performed a study to compare the urine cotinine levels in breast-fed and bottle-fed infants of mothers who smoked. They also examined the effects of breast feeding on urine cotinine levels in infants whose mothers did not smoke but who had environmental exposure to other household members who smoked. Finally, the authors examined whether restricting the location of smoking in the house affected the degree of infant exposure.

A cohort of 330 mother-infant pairs were selected from a longitudinal study of the effects of prenatal and postnatal smoking on respiratory illnesses and pulmonary function in children. Detailed information was obtained from the mothers, including age, race, medical and obstetric history, past and current smoking history and tobacco smoke exposure from other household members. Mothers who smoked included those who admitted to having at least one cigarette per day or whose corrected urine cotinine levels exceeded 200 ng per mg creatinine (1,135 nmol per L), a level unlikely to represent environmental exposure). Infants were considered to be breast-fed if the mothers reported any current breast-feeding, regardless of supplementation. The infants were considered to be environmentally exposed to tobacco smoke if they lived with a person who smoked or spent more than two hours twice weekly with a person who smoked. Urine specimens were collected from the infants at one or more visits during the first year of life. The specimens were analyzed for cotinine concentration by radioimmunoassay.

There were no significant differences among the infant-mother pairs in maternal age, education level, percentage of households with smokers, percentage of mothers who breast-fed, infant birth weights or length, or infant age at entry into the study. Urine cotinine levels were significantly higher in the infants whose mothers smoked compared with infants whose mothers did not smoke. Furthermore, breast-fed infants of mothers who smoked had median levels of cotinine that were 10-fold higher than those in bottle-fed infants of mothers who smoked. This finding was independent of the number of cigarettes smoked daily. Infants whose mothers reportedly smoked in the same room had increases in urine cotinine levels, regardless of feeding type, compared with infants whose mothers always smoked in rooms away from the infant.

The authors conclude that breast-fed infants of mothers who smoke have urine cotinine levels 10-fold higher than bottle-fed infants whose mothers smoke. Their findings suggest that the degree of exposure to nicotine through breast milk is much more substantial than was previously thought. The study also indicates that efforts to restrict smoking to certain areas of the home may have only a negligible effect on the child's exposure to environmental tobacco smoke. Physicians should encourage mothers to stop smoking both prenatally and after birth, and should inform them that harmful chemicals from cigarettes can be transmitted to their infants from the environment and through breast milk.