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.

Breast milk and lipid intake distributions for assessing cumulative exposure and risk.

Breast milk consumption is the primary route of infant exposure to certain lipophilic toxicants that have accumulated over decades in maternal adipose tissue, as well as to less persistent toxicants from maternal exposure during lactation. Such infant exposures occur at a time of rapid growth and development when susceptibility to certain toxicants can be greatest. Breast milk and lipid intake rates are presented for the 0-6 and 0-12 month age periods for infants fed according to the American Academy of Pediatrics' current recommendations (exclusive breast-feeding for 0-6 months and continued breast-feeding to 12 months). Intake rates are normalized to infant bodyweight to account for the covariance of consumption and bodyweight. Frequency distributions describe the population variability in intake. For age 0-12 months, daily average milk intake is 100.7 +/- 22.7 g/kg day (mean +/- SD), with a 95th percentile of 153.5 g/kg day. Breast milk intake distributions are also developed for infants exclusively breast-fed (no significant calories from non-breast milk sources) over their first year, and for the entire (nursing and non-nursing) infant population. For short-term exposures, intake can be derived from the regression equation presented here. Lipid intake estimated assuming a 4% lipid content (current risk assessment practice) is compared and found comparable to that derived from measured lipid content. The national trend of increased breast-feeding found in surveys further supports including the breast milk pathway in risk assessment.