Passive smoke and low birth weight. Evidence of a threshold effect

To examine the relationship between different levels of maternal passive tobacco smoke exposure and low-birth-weight infants.

Data from the National Health Interview Survey were analyzed.

Women who did not smoke during pregnancy and gave birth to children within 6 years of the date of the interview in 1988 were the source of the data (N = 3253). Passive smoke exposure was categorized as very low, low, moderate, and high.

Low-birth-weight (< 2500 g) infants and mean birth weight in grams.

There was no significant difference in the rates of low birth weights when passive smoke exposure was dichotomized as no exposure vs some exposure (5.0% vs 5.6%, respectively; P = .55). However, when the level of exposure to passive smoke was considered as a continuum, high exposure was associated with both lower mean birth weights in grams (P = .007) and a greater likelihood of a low birth weight (P = .01), indicating a threshold effect of exposure. Logistic regression analysis indicated that, after controlling for potentially confounding variables, individuals with high exposure are 1.57 times (95% confidence interval [CI], 0.98 to 2.51) as likely as those with low exposure to have a low-birth-weight infant. Among nonwhites, individuals with high exposure to passive smoke were 2.31 times (95% CI, 1.06 to 4.99) as likely to have a low-birth-weight infant as those with low exposure to passive smoke.

A threshold effect of exposure to passive smoke and low birth weight was discovered.