Protective No-Spray Zones
Each different pesticide has unique chemical properties that determine its inherent hazard and the way it interacts with living things. Each pesticide is not equally hazardous to all living things. For example, an insecticide can be highly toxic to insects and at the same time much less toxic to mammals or aquatic life. Similarly, a fungicide or herbicide will have a strong effect on its target pest but might have little effect on people. Some pesticides do not cause significant concern because they have very low toxicity to non-target organisms, may break down rapidly in the environment and/or do not accumulate in animals’ bodies.
By contrast, there are also pesticides that are highly toxic to people and other organisms as well as to their target pests. It is important to remember that all pesticides are not the same.
The APVMA with its cooperating agencies assesses each one individually to determine its inherent hazard. Once the short term and long term hazards of a pesticide are understood from numerous scientific studies, the next step is to examine risk.
Risk from a pesticide can be understood by considering both the pesticide’s inherent hazard properties combined with potential exposure to the pesticide. Both ‘hazard’ and ‘exposure’ are important in measuring risk. Even the most toxic chemical known presents no risk at all if there is no chance of exposure. In the same sense, a pesticide only modestly toxic to people could present unacceptably high levels of risk if exposure to it were high. In setting limits of exposure, the APVMA analyses what might happen in the range of extremely slight up to high levels of exposure. This analysis is done by examining scientific studies on a variety of species of laboratory animals and environmental indicator species. For nearly all chemicals, living organisms have the ability to tolerate trace exposures without harm. The science of toxicology studies this phenomenon in great detail to understand where the level of ‘no harm’ is found for each chemical.
Specific features of a particular chemical, the environment and the physiology of an organism that might be harmed (people as well as other living things in the environment) all interact in complex ways to result in outcomes potentially different for each combination.
When all of these features are considered together, a threshold of risk for each pesticide can be determined for each of the three major areas of the APVMA’s responsibility – risk to human health (this threshold is set by the Dept of Health and Ageing), risk to the environment (this threshold is set by the Department of Sustainability, Environment, Water, Population and Communities) and risk to Australia’s international trade (this threshold is set by the APVMA).
Exposures above each threshold are deemed not acceptable and those below it can be considered negligible because large safety margins are built in to each threshold. Each pesticide has its own unique thresholds for the various risk areas, and the threshold values for a given pesticide will almost always be different for each of the three risk areas.
The next necessary step that must be done is to determine how much exposure might occur in real world situations. For spray drift, the level of potential exposure is assessed from downwind spray drift deposition data. Deposition data details how much of a pesticide is deposited from spray drift at various points downwind from the application area. Many studies have been done to find out how spray droplets behave in the atmosphere when moving downwind. Examples of deposition data for different kinds of application methods can be found in the APVMA’s standard spray drift scenarios located on its website. One example is illustrated in the following graph where downwind deposition curves (a deposition curve is a graphic plot of deposition data) for each of three standard droplet size spectra are shown for three different wind speeds. This example is for a typical aerial fixed wing application.
For this illustration of the concept, two risk thresholds are shown, one substantially lower than the other. This kind of situation often happens with a pesticide where for one type of risk, say an environmental risk, its threshold could be at one level while for the very same pesticide, a human health or a trade risk threshold could be at another level.
When all the information is assembled in this way, one can readily see how protective no-spray zones can be determined. The point where the downwind deposition curve intersects a risk threshold is the distance downwind from the application area that must be protected. In the example below, the 0.025 threshold might be the maximum amount of a pesticide that could be tolerated without measurable harm to the environment while the 0.009 threshold might apply to a trade risk for that same chemical. For each risk, spray drift deposition above those respective levels would be unacceptable. Depending upon the size of the droplet spectrum allowed on the product’s label, the label would need to specify a no-spray zone for the environment that matched the distance where the deposition curves for that droplet spectrum crossed the environmental threshold. A no-spray zone for the trade risk would be determined in the same way.
In this graph, zero on the horizontal axis represents the downwind edge of the application area. The wind is blowing from left to right. The numbers along the horizontal axis represent the distance in metres downwind from the application area. The vertical axis of the graph is scaled as a fraction of applied field rate – for this example the value of 0.01 near the bottom of the graph means 1% of the pesticide rate applied to the field.
This example demonstrates why the necessary no-spray zones for low thresholds need to be larger than for higher thresholds. It also illustrates why the use of smaller droplet spectra requires larger no-spray zones than larger droplet spectra because smaller droplets drift farther. In the graph illustrated here, the Fine droplet spectrum would be ruled out entirely for the 0.009 risk threshold. For the Medium droplet size spectrum for a 20 km/hr wind speed (the green curve), that deposition curve crosses the 0.025 threshold at 152 metres (determined from the actual numbers) and crosses the 0.009 risk threshold at 708 metres. If the product label limited aerial applications to a Medium droplet size spectrum, then for these risks, two no-spray zones of approximately those distances would be required.
Numbers such as 152 or 708 metres are too precise to be appropriate for real world observance of no-spray zones. Therefore, each no-spray zone determined by the risk assessment process needs to be approximated into a number that is easy for users to visualize and remember. When the predicted number falls somewhere between ‘even’ numbers, the APVMA moves it upward to some higher even number unless the predicted value was only a very small amount over the closest lower even number.
It is also the case that, because of the complexities of the various factors, the APVMA has greater confidence in the values determined for smaller no-spray zones than those determined for larger ones. The APVMA addresses this difference in confidence by using smaller increments of standard ‘even’ numbers for smaller buffer zones and larger standard increments for larger buffer zones. The APVMA uses the values in the table below.
|Ground Application (Note: 300 metres is the limit of the ground data sets)||Aerial Application(Note: 800 metres is the validated limit of the aerial computer model AGDISP used by the APVMA)|
|100||50m increments from 200 to 800|
When a predicted no-spray zone falls between the steps of the standard increments, the APVMA moves it to the next higher step. However, when the predicted value only slightly exceeds one of the steps, a judgment needs to be made. Each situation can be different.
In most cases, the APVMA will allow a 10% overshoot factor, but it should be understood that it would consider each situation on a case-by-case basis. This issue will relate mainly to no-spray zones less than 100 metres (because for greater distances, 10% will push close to or into the next increment). Some pesticides, their use patterns and the quality of their data packages might provoke greater concern and therefore might require a more conservative decision.