PROBABILITY DENSITY-FUNCTIONS DESCRIBING 24-HOUR INHALATION RATES FORUSE IN HUMAN HEALTH RISK ASSESSMENTS

A Monte Carlo simulation was undertaken to estimate the amount of air inhaled over a typical 24-hour period by six age groups of Canadians. The objective of the simulation was to derive probability density func tions that could be used to describe inhalation rates in probabilistic health risk assessments involving airborne contaminants. The six age groups considered were those typically employed in human health risk a ssessments in Canada: infants caged 0 to 6 months), toddlers caged 7 m onths to 4 years), children (aged 5 to 11 years), teenagers caged 12 t o 19 years), adults (aged 20 to 59 years), and seniors (aged 60 years and older). The resulting distributions are considered equally applica ble to Americans as Canadians, however, since the study relied heavily on time-activity information gathered in the USA. Existing time-activ ity and breathing rate studies were reviewed in order to define random variables describing probable durations that North Americans spend at various levels of activity and their probable inhalation rates while at each level of activity. These random variables were combined in a M onte Carlo simulation to empirically generate probability density func tions describing 24-hour inhalation rates for each age group. The simu lation suggested that most age groups' 24-hour inhalation rates can be represented with log-normal probability density functions. Arithmetic mean values and standard deviations for these distributions are as fo llows: approximately 9.3 +/- 2.4 m(3)/day for toddlers; 14.6 +/- 3.0 m (3)/day for children; 15.8 +/- 3.7 m(3)/day for teenagers; 16.2 +/- 3. 8 m(3)/day for adults; and 14.2 +/- 3.3 m(3)/day for seniors. The dist ribution of infants' 24-hour inhalation rates was found to be better r epresented by a normal distribution than a log-normal distribution. Th e mean and standard deviation for the infants' normal distribution are 2.1 +/- 0.58 m(3)/day. Inhalation rates were also estimated separatel y for male and female toddlers, children, teenagers, adults and senior s. These estimates suggested that males inhale on average 8% to 27% mo re air than females of the same age. Because infants' activity pattern s and minute volumes did not appear to be strongly correlated with gen der, a single probability density function was deemed satisfactory to describe male and female infants' 24-hour inhalation rates.