Advancing Options for Management and Mitigation of
Spray Drift
Risk Mitigation Measures in Europe to Prevent Plant Protection Product losses to the Environment
Human Risk Assessment
In addition to protecting the environment, it is a regulatory requirement in the EU that members of the public are not subjected to unacceptable risk as a result of pesticide emissions. The EFSA Guidance on non-dietary exposure(1) describes two groups, bystanders and residents, who may be exposed to pesticides as a result of off-target drift. Both bystanders and residents are persons whose location may lead to exposure, but whose presence is incidental and unrelated to the pesticide application process. Whereas bystander exposure is considered to be a short term (acute) event, residents may live or work in the vicinity of pesticide application operations, potentially leading to a longer term exposure. For the purpose of risk assessment, this means that whereas 75th percentile data are typically used for residents, corresponding 95th percentiles from the same datasets are typical for bystander assessment. Bystander and resident assessments are carried out for both adults and children.
Bystanders and residents may be exposed via four pathways, three of which are a result of pesticide emissions, i.e. exposure to spray drift during application, exposure to volatilised pesticide and exposure to spray drift fallout on contaminated surfaces, such as contaminated lawn which is used for leisure activities. The relevant routes of exposure are as follows:
(1) Guidance on the assessment of exposure of operators, workers, residents and bystanders in risk assessment for plant protection products; EFSA Journal 2014;12(10):3874
calculator version: 24 April 2015
Spray Drift
Exposure to spray drift during application is highly dependent on the method used to apply the pesticide. The EFSA Guidance distinguishes between downward (boom sprayer) application and upward (orchard/vine sprayer) application. Whilst it is conceivable that exposure could occur during application by hand held devices, there are no readily available, robust exposure data; in most cases exposure during application by large scale, vehicle mounted equipment would represent the worst case exposure scenario for bystanders and residents.
In the EFSA Guidance, the relevant exposure data for arable crops regarding spray drift and drift fallout are derived from BREAM (Bystander and Resident Exposure Assessment Model) (2).
BREAM has been developed within an UK initiative and predicts deposition on bystanders more realistically than measurements taken with horizontal drift collectors (3D versus 2D). Its underlying principles have recently come under increased scrutiny because it uses a mechanistic model to predict airborne spray ground deposits, and an empirical component that relates airborne spray to deposits on the human body, from which exposure can be calculated.
Investigations are ongoing whether or not the higher centiles predicted by BREAM, which are relevant for the risk assessment, are influenced disproportionally by the high variability of this relationship.
The empirical data is very variable, and when this is included in a probabilistic model such as BREAM, this makes a very large contribution to the resulting variability of the model output, and therefore high values of the upper centiles. Currently a project (BREAM 2) is ongoing to reduce the variability and uncertainty in the relationship between airborne spray and bystander contamination. First results will be expected by end of 2017.
(2) Kennedy, MC, Butler Ellis, MC, Miller, PCH (2012). BREAM: A probabilistic Bystander and Resident Exposure Assessment Model of spray drift from an agricultural boom sprayer. Computers and Electronics in Agriculture. 88, 63-71
The BREAM project did not investigate exposure due to spray drift from high crop (airblast) applications. Consequently, the data used in the EFSA model are taken from a much earlier study (Lloyd et al., 1987[3]) during which bystander exposure to tracers applied at 470 L/ha via conventional nozzles was measured. The surrogate bystanders were positioned 8 m downwind of the middle of the last row during treatment of an entire orchard. To account for additional, more distant, passes of a sprayer, and for the possibility of closer proximity than 8 m, the EFSA guidance proposes that the dermal values be increased by a factor of 10.5. Additionally, as exposure was only measured for adult bystanders, the EFSA guidance makes a correction for the relative body surface areas to predict dermal exposure for the child (x 0.3). Inhalation exposures are corrected by taking into account appropriate breathing rates for children.
A project called "BREAM 2" is currently dressing some of the uncertainties and causes of variability. Results are expected by the end of 2017.
(3) Lloyd GA, Bell GJ, Samuels SW, Cross JV and Berry AM, 1987. Orchard sprayers: comparative operator exposure and spray drift study. Agricultural Science Service, Agricultural Development and Advisory Service, Ministry of Agriculture Fisheries and Food, UK.
The relevant data are tabulated below:
As the only available exposure values for orchard applications were for 8 m (5 m from orchard edge), the same values were proposed for 5 m and 10 m. This is likely to over-estimate exposure resulting from drift at 10 m. Due to this sparsity of data and the EFSA Working Group recommendation that more modern data are required to produce a robust model, the European Crop Protection Association (ECPA) is in the process of generating data for orchard applications.
Surface deposits
The assumption is made that a proportion of the spray applied to crops may deposit off target on adjacent property. If this drift gives rise to surface deposits on a lawn, then members of the public may be exposed during leisure activities. For adult bystanders and residents, exposure is considered to be limited to the dermal route. However, for children, oral exposure may also occur due to hand to mouth and object to mouth events. The degree of exposure is directly proportional to the amount of pesticide which deposits on the lawn and this in turn depends on the percentage of the application rate which drifts off target. The EFSA guidance adopts drift data from both the BREAM project and Ganzelmeier/Rautmann as follows:
(a) – BREAM
(b) – Ganzelmeier/Rautmann (75th percentile not published)
Based on very limited data, for the application of granules, drift is assumed to be a precautionary 3% of the application rate for broadcast and manual applications.
Vapour
The risk assessment for exposure to vapour is independent of the assessment of inhalation exposure to aerosol during spray application. The EFSA guidance includes only 2 default values for air concentration based on extremely limited datasets. The magnitude of the predicted exposure is determined solely by the vapour pressure of the active ingredient (this determines the default air concentration) and is completely independent of application rate, application method, formulation type and distance downwind. For moderately volatile compounds (vapour pressure ≧0,005 Pa and <0,01 Pa), the exposure should be calculated assuming a default concentration of 15 𝛍g/m³, while for compounds with low volatility (vapour pressure < 0,005 Pa) a default concentration in the air of 1 𝛍g/m³ should be used. The 15 𝛍g/m³ derived from a study conducted by the Californian EPA monitoring chlorpyrifos residues in air adjacent to orange orchard that has been treated with 1100 g chlorpyrifos /ha (vapour pressure: 0,0014 Pa)(4) using air assisted sprayers (2). The meteorological conditions recorded during the chlorpyrifos study included temperatures up to 42°C.
The 1 𝛍g/m³ originally derived from a dataset published by Siebers et al (2003). For applications in cereals (250 g/ha) using field crop boom sprayers vapour exposure to lindane (vapour pressure = 0,056 Pa at 25°C) has been predicted. Measured air residues revealed a time weighed average of 0,29 𝛍g/m³ and 0,58 𝛍g/m³. The meteorological conditions during the trial included temperatures up to 28°C.
(4) California Environmental Protection Agency (1998). Report for the application and ambient air monitoring and chlorpyrifos (and the oxon analogue) in Tulare County during spring/summer 1996.
(5) Siebers JN, Binner R and Wittich KP (2003). Investigation on downwind short-range transport of pesticides after application in agricultural crops. Chemosphere 51, 397-407.
Risk Mitigation Measures
As is evident from the data presented for the spray drift and surface deposits pathways, increased distance from the source leads to lower exposure. In the EFSA calculator which accompanies the non-dietary exposure guidance, increased buffer zones are offered as a risk mitigation measure. Consequently, for boom sprayers, the first tier assessment which assumes a buffer zone of 2-3 m may be refined by selecting 5 m or 10 m, giving lower exposure to spray drift during application and lower surface deposits on adjacent property (BREAM data). In the US, direct exposure to spray drift during application is illegal and addressed through enforcement.
Although the lack of drift data for bystanders and residents (vertical deposition) for upward spraying leads to the use of the same exposure data for 5 m and 10 m downwind, the trends observed with horizontal collectors (Ganzelmeier/Rautmann) indicate that the same kind of attenuation is likely. The ongoing ECPA work will provide valuable supporting data. In the meantime, exposure to spray drift and surface deposits can only be predicted at a downwind distance of 5 m for upward spraying.
The EFSA guidance also acknowledges the value of drift reduction technology as a risk mitigation measure. The data reviewed by the Working Group (including Guidelines for the testing of plant protection products Part VII, April 2000. Federal Biological Research Centre for Agricultural and Forestry Federal Republic of Germany) demonstrated 50-90% drift reduction for a range of nozzles. From this review, although it is pointed out that the measurements were only taken to a height of 50 cm, it was agreed that a reliable factor to include in the model would be 50%. This risk mitigation measure leads to a reduction in exposure to spray drift during application and in exposure to surface deposits, but also sees a decrease in exposure for the operator. It is stated that further data would be required to support drift reduction higher than 50%, but this could be an opportunity for the applicant to generate specific data.
In terms of bystander and resident exposure to vapour drift, there are no available risk mitigation measures or refinement available in the model.
As the safety of bystanders and residents has become a prime consideration in the registrability of pesticides, the risk assessments have become increasingly precautionary. In order to ensure that the health of members of the public remains a priority, development of drift reduction technology, be it nozzle design, the use of appropriate adjuvants and formulations, or other drift reducing equipment such as shrouded sprayers is to be encouraged. At the same time, it is important to generate data which demonstrate the reduction in drift which these technologies can provide to support quantitative refinement of the risk assessment which may be needed to successfully register important pesticides. At the same time, it is important to engage regulatory authorities, some of whom have expressed a reluctance to consider drift reduction technology or increased buffer zones as a valid risk mitigation measure, so that they may see that these technologies will be adopted by farmers and that significant reduction in drift is realistically achievable.