Every breath you take: the effect of postpartum maternal smoking on childhood asthma.

• Author: Sabia, Joseph J.

1. Introduction

   In June 2006, the U.S. Surgeon General's Office released a report titled, "The Health Consequences of Involuntary Exposure to Tobacco Smoke," which summarizes evidence from the public health and medical literatures on the adverse effects of exposure to secondhand tobacco smoke. Upon presenting the report to the media, Surgeon General Richard Carmona offered the following comments:

   The scientific evidence is now indisputable: secondhand tobacco smoke is not a mere annoyance. It is a serious health hazard that can lead to disease and premature death in children and non-smoking adults ... Children exposed to secondhand smoke are at an increased risk for Sudden Infant Death Syndrome (SIDS), acute respiratory infections, ear problems, and more severe asthma (Carmona 2006). Carmona further emphasized that "there is no safe amount of secondhand smoke" to which a child can be exposed. Many states have adopted a variety of antismoking policies based on this evidence. (1) The Surgeon General's report concludes that while there is some medical evidence of a causal link between environmental tobacco smoke and asthma among school-age children, "the evidence is suggestive, but not sufficient to infer a causal relationship between secondhand smoke exposure from parental smoking and the onset of childhood asthma" (Report of the Surgeon General 2006, p. 9). Examining the appropriateness of inferring a causal relationship between parental smoking and childhood asthma requires more careful empirical attention than has been seen in the public health literature. This is the key contribution of the current study.

   Today, nearly 5 million children have asthma, with the most rapid rate of increase occurring for those under the age of five (American Academy of Pediatrics 2000). Asthma remains the leading cause of pediatric emergency room use and hospital admission (National Institute of Environmental Health Sciences 2007). Over the last decade, federal agencies have turned their attention toward better surveillance of and more research on asthma. The Department of Health and Human Services (DHHS) issued a 1997 report titled "Action against Asthma," which set goals for reductions in asthma prevalence by 2010. In May 2000, the President's Task Force on Environmental Health Risks and Safety Risks to Children--a joint venture of the DHHS and the Environmental Protection Agency--issued a report titled "Asthma and the Environment: A Strategy to Protect Children," which set forth similar goals. The National Heart, Lung and Blood Institute's Asthma Education and Prevention Program has also produced several reports on this topic. Each of these agencies, along with the recent report from the Surgeon General, has cited exposure to secondhand smoke as a strong correlate of childhood asthma.

   This study carefully examines the relationship between postpartum maternal smoking and childhood asthma among populations at highest risk of asthma--racial minorities and children of less-educated parents. In contrast to previous studies in the public health literature, this study more carefully addresses unmeasured heterogeneity in attempting to isolate the causal effect of secondhand smoke on childhood asthma. The findings presented here suggest that postpartum maternal smoking may have important adverse effects on children's health. Confirming several previous studies in the literature, cross-section estimates reflect consistent evidence of a positive association between maternal smoking and childhood asthma. Postpartum maternal smoking is associated with a 2 to 3 percentage point higher probability of asthma by age one and a 4 to 5 percentage point higher probability of asthma by age three. Importantly, these results are robust to the inclusion of individual fixed effects. Difference-in-difference models that compare the change in asthma rates for children whose mothers began smoking between their child's first and third birthday with mothers who remained nonsmokers find that maternal smoking is associated with a 4 to 7 percentage-point increase in asthma rates. This result is robust to several falsification tests, suggesting that other time-varying unmeasured maternal health input choices cannot explain this positive relationship.

2. Asthma-Smoking Literature

   Asthma is a chronic inflammatory lung disease characterized by repeated instances of breathlessness, wheezing, and coughing (Department of Health and Human Services 1997). This disease has been discussed quite extensively in the medical and public health literatures but has largely been ignored in the economics literature. But as policymakers consider policy alternatives to reduce asthma rates--such as raising cigarette taxes--economists will be increasingly called upon for cost-benefit analyses. As a primer for such future research, a 1998 DHHS study estimated the annual cost of asthma, including medical costs and productivity losses, to be over $11 billion per year. Death rates due to asthma are on the rise, with an over 300% increase from 1977 to 1995. Race differences also persist. In 1995, the death rate among African Americans (11.5 per million) was over four times higher than that among whites (Mannino, Homa, and Pertowski 1998).

   The medical community has reached a consensus that asthma is a disease of airway inflammation "resulting from a complex interplay between environmental exposures and genetic and other factors" (Asthma Priority Area Workgroup 2000, p. 9). Secondhand tobacco smoke from cigarettes, cigars, and pipes is composed of over 3800 unique chemical compounds (National Research Council 1986). Concentrations of suspended particulate matter are two to three times higher in homes with smokers than homes without smoking, and this particulate matter is believed to be negatively associated with respiratory health (Lefcoe and Inculet 1971; Dockery et al. 1982; American Academy of Pediatrics 2000).

   While there is minimal understanding of the environmental causes of asthma, the environmental triggers of child asthma have become more well known. Among the principal allergens that trigger asthma attacks among young children are house dust mites, cockroaches, mold, and animal hair (for examples, see Bierman 1996; Warner et al. 1996; National Asthma Education and Prevention Program 1997; Platts-Mills and Carter 1997; and Institute of Medicine 2000). Air pollutants, including ozone and upper respiratory viruses, are also positively correlated with asthma attacks (Koren 1995; Busse, Gern, and Dick 1997).

   Several studies have examined the relationship between exposure to secondhand tobacco smoke and the development of asthma in young children. However, almost all are cross-sectional studies that do not account for unobservable characteristics that may be correlated with both maternal smoking and the child's development of asthma, thus raising doubts as to the appropriateness of a causal interpretation of estimates.

   Using the Third National Health and Nutritional Examination Survey, Gergen et al. (1998) examine the impact of environmental tobacco smoke on the respiratory health of a nationally representative sample of children aged two months to five years. They found that among children two months to two years of age, maternal smoking of greater than 20 cigarettes per day is associated with a significantly higher rate of diagnosed asthma. Weizman et al. (1990) find similar results using the National Health Interview Survey. (For a review of much of the medical literature, see Strachan and Cook 1998.)

   A series of smaller, localized studies have also found a positive association between parental smoking and child asthma (for example, Burchfield et al. 1986; Evans, Levison, and Feldman 1987; O'Connor et al. 1987; Murray and Morrison 1989; Krzyzanowski, Quackenboss, and Lebowitz 1990). It is difficult to interpret these studies causally--and to generalize the results of local studies--given the failure to carefully address potential biases caused by unmeasured heterogeneity.

   No large-scale studies in the economics literature have examined the impact of secondhand tobacco smoke on child asthma rates. Most studies have examined the impact of prepartum maternal smoking on infant health outcomes, usually focusing on birth weight (see, for example, Evans and Ringel 1999), and have explored how increases in cigarette taxes could lead to improved health outcomes for children (Ringel and Evans 2001; Colman, Grossman, and Joyce 2003). Other work has examined the impact of air pollutants on child asthma (Neidel12004). Less attention has been paid to the negative health effects associated with exposure to household-level secondhand smoke.

   This paper contributes to the existing empirical literature in three key ways. First, while much of the health economics literature has focused on the effect of smoking during pregnancy on child health outcomes, this study examines whether there are important adverse child health effects of postpartum maternal smoking. Second, in examining the relationship between postpartum maternal smoking and childhood asthma, this study improves upon the public health literature by more carefully addressing potential biases that may result from unobserved heterogeneity. Finally, this study examines the effect of parental smoking on childhood asthma among populations at high risk of developing asthma--racial minorities and children of less-educated mothers.

3. Methodology

   Cross-Section Estimates

   The existing public health literature on the relationship between parental smoking and childhood asthma has generally presented cross-section estimates of the child health production function. To replicate the existing literature, probit and linear probability models of the following form are estimated (2):

   [A.sup.*.sub.ijt] = [[beta].sub.0] + [[beta].sub.1] [S.sub.jt] + [X'.sub.it][[beta].sub.3] + [[epsilon].sub.ijt], (1)

   where [A.sup.*] is a latent propensity asthma...