136
Pesticides, Organic Contaminants, and Pathogens in Air
reasons, CALMET was unable to resolve, especially in the daytime, the fne
structure of the meteorology prevailing in the modeling domain explaining
the lower correlation observed with CALMET/CALPUFF.
The lack of representative meteorology, variability in fux estimates,
source–receptor spatial relationship, and inherent uncertainty in measurements are among the reasons for moderate correlation between measured
and modeled concentrations, especially for daytimes. With refned source
estimates and better meteorological data, improved model predictions can be
expected. We also note that this result came from a single feld experiment.
Ideally it should be replicated to generate average values.
8.3.2.4 Flux from Multiple Fields: Applications to Exposure Calculations
Nearly 80% of the MeBr applications in the Salinas Valley occurred in the
vicinity of the city of Salinas potentially exposing the population of the city
of Salinas to MeBr. MeBr has been shown to cause developmental toxicity in
animal studies and neurotoxicity in all species studied including humans
(Lim, 1992). The U.S. EPA has established a chronic exposure reference concentration of 5 µg/m 3 . The CDPR has established a 24-hour exposure limit
of 816 µg/m 3 (210 ppb) (Nelson, 1992) and ensures compliance by enforcing
buffer zones between the application area and places where people live or
conduct activities for a signifcant period of time (e.g., schools).
8.3.2.5 Flux from Multiple Fields: Exposure Assessment
The margin of exposure (MOE) is the ratio of the no observed adverse effect
level (NOAEL) (mg/m 3 ) from experimental animal studies to the human
exposure (mg/m 3 ) concentration. The MOEs for acute exposure can be estimated from the highest airborne MeBr concentration measured at each location during the monitoring period while those for chronic exposure were
calculated using the annual time-weighted average (ATWA) of estimated
weekly ambient concentrations of MeBr at each receptor (Table 8.4). The
CDPR uses an MOE of at least 100 to protect the health of the general public
from acute exposure to MeBr (Lim, 1992). The U.S. EPA regards an MOE of
100 adequate to protect the health of the general population from chronic
exposure to MeBr (IRIS, 1992).
The MOEs for acute exposure were about 10,000 and for chronic exposure
were >100 for all monitoring stations. However, with the higher emission rate
(Yates et al., 1996), the ISCST3-predicted concentrations are generally twice
those measured and the CALPUFF predicted concentrations are 1.6 times the
measured. Using these higher predicted concentrations, the MOE at Salinas
lies close to 100. Thus, conditions of heavier than average usage in a given
locale and exposures to infants and children and other sensitive population
subgroups suggest that close attention should be given to ambient exposures
in regions, such as the Salinas Valley, where the fumigant is widely used.
Pesticides, Organic Contaminants, and Pathogens in Air
reasons, CALMET was unable to resolve, especially in the daytime, the fne
structure of the meteorology prevailing in the modeling domain explaining
the lower correlation observed with CALMET/CALPUFF.
The lack of representative meteorology, variability in fux estimates,
source–receptor spatial relationship, and inherent uncertainty in measurements are among the reasons for moderate correlation between measured
and modeled concentrations, especially for daytimes. With refned source
estimates and better meteorological data, improved model predictions can be
expected. We also note that this result came from a single feld experiment.
Ideally it should be replicated to generate average values.
8.3.2.4 Flux from Multiple Fields: Applications to Exposure Calculations
Nearly 80% of the MeBr applications in the Salinas Valley occurred in the
vicinity of the city of Salinas potentially exposing the population of the city
of Salinas to MeBr. MeBr has been shown to cause developmental toxicity in
animal studies and neurotoxicity in all species studied including humans
(Lim, 1992). The U.S. EPA has established a chronic exposure reference concentration of 5 µg/m 3 . The CDPR has established a 24-hour exposure limit
of 816 µg/m 3 (210 ppb) (Nelson, 1992) and ensures compliance by enforcing
buffer zones between the application area and places where people live or
conduct activities for a signifcant period of time (e.g., schools).
8.3.2.5 Flux from Multiple Fields: Exposure Assessment
The margin of exposure (MOE) is the ratio of the no observed adverse effect
level (NOAEL) (mg/m 3 ) from experimental animal studies to the human
exposure (mg/m 3 ) concentration. The MOEs for acute exposure can be estimated from the highest airborne MeBr concentration measured at each location during the monitoring period while those for chronic exposure were
calculated using the annual time-weighted average (ATWA) of estimated
weekly ambient concentrations of MeBr at each receptor (Table 8.4). The
CDPR uses an MOE of at least 100 to protect the health of the general public
from acute exposure to MeBr (Lim, 1992). The U.S. EPA regards an MOE of
100 adequate to protect the health of the general population from chronic
exposure to MeBr (IRIS, 1992).
The MOEs for acute exposure were about 10,000 and for chronic exposure
were >100 for all monitoring stations. However, with the higher emission rate
(Yates et al., 1996), the ISCST3-predicted concentrations are generally twice
those measured and the CALPUFF predicted concentrations are 1.6 times the
measured. Using these higher predicted concentrations, the MOE at Salinas
lies close to 100. Thus, conditions of heavier than average usage in a given
locale and exposures to infants and children and other sensitive population
subgroups suggest that close attention should be given to ambient exposures
in regions, such as the Salinas Valley, where the fumigant is widely used.