4 Spray Drift Contribution of Air Concentrations
The spraying of crops is often done over large areas and uses significant amounts of
pyrethroids which can result in spray (aerosol) drift or vapor phase (volatilization)
transport of the pyrethroids reaching residences several hundred meters or more
away [18]. The degree that the spray drift may impact air concentrations at surrounding residences is dependent upon the distance from the application to the
receptor residence, the meteorological conditions (e.g., wind speed, wind direction,
temperature, precipitation), application method, nozzle type, and the height the spray
is released from [19]. Field measurements of spray drift can be difficult and
expensive. Therefore, mathematical modeling of the drift has been used to predict
the extent of the impacted areas and the concentration gradient for different scenarios, which can help guide the US EPA minimize the impact of spray applications on
the surrounding environment and residences [20]. Recently, remote sensing instruments have been deployed to estimate the relative amount of deposition and spatial/
temporal air concentrations and are used for model evaluation [21, 22]. Drift has
been found to occur during every application and can account for approximately
2–25% of the pesticide loss during application with the drift spreading from a few
yards to several hundred miles [23]. Various mathematical models of the drift have
been developed. One computational fluid dynamic (CFD) model of spray droplets
suggests that the air pesticide droplet concentration would decrease by two orders
magnitude from 100 to 1 μg/L (1,000 μg/m
3 ) over a 200 m distance from its release
[24]. A study based on samples collected between 1995 and 2015 looking at the
variations in pesticide levels in house dust with distance from agricultural fields in
North America showed, that the amount of pesticide drift decreased sharply and
nonlinearly with distance from the source [25]. They reported that the geometric
mean pesticide dust levels were 64% lower in homes 250 m from fields compared to
homes only 23 m away and that homes near farms in which the pesticides were
applied more recently or frequently were 2.3 higher than other homes near fields
without recent pesticide applications.
5 Outdoor Air Levels
Few recent studies that have measured outdoor or personal worker pyrethroid air
concentrations. This is due to sampling and analytical challenges and the need to
evaluate the exchange between the vapor and particle phase for the semi-volatile
pyrethroids. Current studies more commonly use biomarker measurements to assess
exposure rather than air monitoring. As discussed below, biomarkers do not differentiate inhalation exposure from other exposure routes, and several biomarkers are
non-specific, reflecting exposures to multiple pyrethroids and pyrethrins. A method
developed within the last few years measures semi-volatile organic compounds
(SVOCs) by having participants wear a silicone wristband for several days which
Indoor and Outdoor Pyrethroid Air Concentrations
231
The spraying of crops is often done over large areas and uses significant amounts of
pyrethroids which can result in spray (aerosol) drift or vapor phase (volatilization)
transport of the pyrethroids reaching residences several hundred meters or more
away [18]. The degree that the spray drift may impact air concentrations at surrounding residences is dependent upon the distance from the application to the
receptor residence, the meteorological conditions (e.g., wind speed, wind direction,
temperature, precipitation), application method, nozzle type, and the height the spray
is released from [19]. Field measurements of spray drift can be difficult and
expensive. Therefore, mathematical modeling of the drift has been used to predict
the extent of the impacted areas and the concentration gradient for different scenarios, which can help guide the US EPA minimize the impact of spray applications on
the surrounding environment and residences [20]. Recently, remote sensing instruments have been deployed to estimate the relative amount of deposition and spatial/
temporal air concentrations and are used for model evaluation [21, 22]. Drift has
been found to occur during every application and can account for approximately
2–25% of the pesticide loss during application with the drift spreading from a few
yards to several hundred miles [23]. Various mathematical models of the drift have
been developed. One computational fluid dynamic (CFD) model of spray droplets
suggests that the air pesticide droplet concentration would decrease by two orders
magnitude from 100 to 1 μg/L (1,000 μg/m
3 ) over a 200 m distance from its release
[24]. A study based on samples collected between 1995 and 2015 looking at the
variations in pesticide levels in house dust with distance from agricultural fields in
North America showed, that the amount of pesticide drift decreased sharply and
nonlinearly with distance from the source [25]. They reported that the geometric
mean pesticide dust levels were 64% lower in homes 250 m from fields compared to
homes only 23 m away and that homes near farms in which the pesticides were
applied more recently or frequently were 2.3 higher than other homes near fields
without recent pesticide applications.
5 Outdoor Air Levels
Few recent studies that have measured outdoor or personal worker pyrethroid air
concentrations. This is due to sampling and analytical challenges and the need to
evaluate the exchange between the vapor and particle phase for the semi-volatile
pyrethroids. Current studies more commonly use biomarker measurements to assess
exposure rather than air monitoring. As discussed below, biomarkers do not differentiate inhalation exposure from other exposure routes, and several biomarkers are
non-specific, reflecting exposures to multiple pyrethroids and pyrethrins. A method
developed within the last few years measures semi-volatile organic compounds
(SVOCs) by having participants wear a silicone wristband for several days which
Indoor and Outdoor Pyrethroid Air Concentrations
231