Ecological Risk Assessment

Pyrethroids are extremely toxic to many aquatic organisms, and thus could pose a substantial ecological risk (see section 3.10). In this section, we have employed a species sensitivity distribution from the USEPA's Ecotox Database125 for permethrin using 41 aquatic species based on 96 hour LC50s (see Figure 3.4). Species sensitivity distributions are used to calculate the concentrations at which a specified proportion of species will be affected, referred to as the hazardous concentration (HC) for p% of the species (HCp). The resulting HC5 is 0.047 mgl-1, which amounts to approximately 33% of the maximum concentrations seen in the environment (see section 3.9). The minimum concentration observed in the environment is 0.0054 mgl-1, which would result in 0.35% of the species reaching their respective LC50 value. At the maximum concentrations seen in the environment (3 mgl-1), which are rarely observed, 65% of the species would be affected (see Figure 3.4). These results are supported by aquatic risk assessments performed for pyrethroids. The toxicity of pseudopyrethroids, like etofenprox, is lower with respect to aquatic organisms than other pyrethroids currently used; thus they represent a lower risk to the aquatic environment.126

A probabilistic aquatic risk assessment conducted by Maund et al.124 for cotton-growing areas focused on pyrethroid exposure in static water bodies as a worst-case scenario. They found that exposures were several orders of

Figure 3.4 Acute species sensitivity distribution constructed from the lethal concentrations that kill 50% of a population (LC50) for permethrin, demonstrating the proportion of species affected for aquatic organisms.

magnitude lower than those that would cause effects based on laboratory and field studies. Davis et al.127 conducted a deterministic ecological risk assessment for truck-mounted ULV applications of pyrethroids, and found that the risks to mammals, birds, aquatic vertebrates, and aquatic and terrestrial invertebrates were negligible. These results were subsequently supported using actual environmental concentrations after aerial and truck-mounted ULV applica-

tions. - Studies by Schleier III and Peterson using caged house crickets as a surrogate for medium- to large-sized terrestrial invertebrates showed that ULV applications of permethrin did not result in increased mortality. These results can most likely be applied to smaller insects as well, because house crickets have been found to be more sensitive to pyrethroids than adult convergent lady beetles and larval fall armyworms.71

3.8 Biomonitoring and Epidemiology

Current human biomonitoring and epidemiological studies show that pyre-throid exposures to the general population are low and adverse effects are highly unlikely. The main route of exposure for the general public to pyrethrins and pyrethroids is through dietary intake.7 Urinary metabolite data from both the USA and Germany show that exposure to pyrethroids in the general population is similar, with the highest exposure coming from the most com-

monly used pyrethroids, permethrin and cypermethrin, , , - with infrequent exposure to pyrethroids like cyfluthrin and deltamethrin.90 The average daily intake of permethrin in the USA due to diet has been estimated at about 3.2 mgday-1, which is approximately 0.1% of the acceptable daily intake.135 Children have been found to have higher levels, which may be attributed to their higher ingestion rates of household dust.132 Even when residential areas have been treated directly with truck-mounted ULV applications of pyrethroids, urinary metabolites have shown no statistical difference when compared with results from untreated areas.136 The use of biomarkers to monitor pyrethroid exposure may be problematic because the estimation of daily absorbed doses of pyrethroids from volume-weighted or creatinine-adjusted concentrations can lead to substantial under- or over-estimation when compared with doses reconstructed directly from amounts excreted in urine during a period of time.137

Occupational application of pyrethroids resulting in the highest concentrations of metabolites in urine samples are from indoor pest-control operators. However, occupational exposures to pyrethroids do not seem to lead to adverse effects.138 Weichenthal et al.74 reviewed the epidemiological evidence relating to occupational exposures and cancer incidence in agricultural workers applying permethrin, and found an increased odds ratio, but the associations were small and imprecise because of small sample sizes and clear exposure-response relationships were not observed. This is most likely because pyrethroids are slowly absorbed across the skin which prevents high levels of exposure. Karpati et al.139 found no increase in asthma cases after truck-mounted ULV applications in residential neighborhoods in New York, New York, USA. Epide-miologically, the USEPA found that the weight of evidence shows no clear or consistent pattern of effects to indicate an association between pyrethrins or pyrethroid exposure and asthma and allergies.140

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