Developing asthma in childhood from exposure to secondhand tobacco smoke: insights from a meta-regression.

OBJECTIVE: Studies have identified associations between household secondhand tobacco smoke (SHS) exposure and induction of childhood asthma. However, the true nature and strength of this association remains confounded in many studies, producing inconsistent evidence. To look for sources of potential bias and try to uncover consistent patterns of relative risk estimates (RRs), we conducted a meta-analysis of studies published between 1970 and 2005. DATA SOURCES: Through an extensive literature search, we identified 38 epidemiologic studies of SHS exposure and the development of childhood asthma (that also controlled for atopy history) from 300 potentially relevant articles. DATA SYNTHESIS: We observed substantial heterogeneity within initial summary RRs of 1.48 [95% confidence interval (CI), 1.32-1.65], 1.25 (1.21-1.30), and 1.21 (1.08-1.36), for ever, current, and incident asthma, respectively. Lack of control for type of atopy history (familial or child) and child's own smoking status within studies and age category altered summary RRs in separate meta-regressions. After adjusting for these confounding characteristics, consistent patterns of association emerged between SHS exposure and childhood asthma induction. Our summary RR of 1.33 (95% CI, 1.14-1.56) from studies of incident asthma among older children (6-18 years of age) is 1.27 times the estimate from studies of younger children and higher than estimates reported in earlier meta-analyses. CONCLUSIONS: This new finding indicates that exposure duration may be a more important factor in the induction of asthma than previously understood, and suggests that SHS could be a more fundamental and widespread cause of childhood asthma than some previous meta-analyses have indicated.

Statistical distributions of daily breathing rates for narrow age groups of infants and children.

Children may be more susceptible to toxicity from some environmental chemicals than adults. This susceptibility may occur during narrow age periods (windows), which can last from days to years depending on the toxicant. Breathing rates specific to narrow age periods are useful to assess inhalation dose during suspected windows of susceptibility. Because existing breathing rates used in risk assessment are typically for broad age ranges or are based on data not representative of the population, we derived daily breathing rates for narrow age ranges of children designed to be more representative of the current U.S. children's population. These rates were derived using the metabolic conversion method of Layton (1993) and energy intake data adjusted to represent the U.S. population from a relatively recent dietary survey (CSFII 1994-1996, 1998). We calculated conversion factors more specific to children than those previously used. Both nonnormalized (L/day) and normalized (L/kg-day) breathing rates were derived and found comparable to rates derived using energy estimates that are accurate for the individuals sampled but not representative of the population. Estimates of breathing rate variability within a population can be used with stochastic techniques to characterize the range of risk in the population from inhalation exposures. For each age and age-gender group, we present the mean, standard error of the mean, percentiles (50th, 90th, and 95th), geometric mean, standard deviation, 95th percentile, and best-fit parametric models of the breathing rate distributions. The standard errors characterize uncertainty in the parameter estimate, while the percentiles describe the combined interindividual and intra-individual variability of the sampled population. These breathing rates can be used for risk assessment of subchronic and chronic inhalation exposures of narrow age groups of children.