Atmospheric dispersion of pesticides

The present authorisation procedure for agricultural pesticides includes an evaluation of the risks pesticide use poses to non-target organisms living on and immediately adjacent to the treated fields. In recent years, it has become increasingly apparent that airborne dispersion is also resulting in these pesticides finding their way into surface waters and nature reserves a long way from the areas of use. Consequently, the Minister of Health, Welfare and Sport, also on behalf of other members of the government, has requested the Health Council of the Netherlands to review the current level of knowledge about the atmospheric dispersion of agricultural pesticides and to make recommendations about both the necessity of including such dispersion in the risk evaluation and the possibilities that exist for doing so. The Health Council’s Committee that was established for this purpose presents its findings in this advisory report. It does so on the basis of the results of an international workshop the Committee organised on the subject.

Extent and significance of atmospheric dispersion

The extensive use of chemical pesticides results in these substances being ubiquitous in air and rainwater and in their dispersion through the atmosphere across large areas. The resulting atmospheric deposition also affects areas outside the immediate surroundings of the areas of use. It is several orders of magnitude lower than the deposition on the treated fields and at least one or two orders of magnitude lower than that on adjacent verges, ditches and so forth. There is still a lot of uncertainty about the ecological significance of this atmospheric input. However, rainwater quality does not meet the standards based on toxicity data for the quality of surface water. The Committee does not, therefore, exclude the possibility of ecological damage outside the immediate surroundings of the treated fields. Also considering the large scale of the problem, the Committee believes a reduction of the risks of atmospheric dispersion is required to guarantee that wildlife is conserved. The most effective way is to reduce the use of chemical pesticides, but the authorisation policy also needs to be tightened. The policy is currently only concerned with limiting environmental damage on and around the treated fields that is caused by direct application and by drops of sprayed liquid landing in adjacent ditches. However, the Committee does not believe this is sufficient to prevent damage occurring at greater distances as a result of atmospheric dispersion. The fact that the resulting deposition is relatively small does not alter the situation. The exposure pattern that results from atmospheric transport differs considerably from that caused by direct application or spray drift. It is characterised by a chronic exposure to possibly a whole range of pesticides. Moreover, in nature reserves, situations may prevail that affect the behaviour (transformation, mobility) of pesticides (low pH, low availability of nutrients, low temperature and, consequently, little microbial activity) and occurring populations and communities of organisms may also be relatively more vulnerable.

Emission into the atmosphere

The Committee subscribes to the workshop’s conclusion that it is preferable to make the additional risk assessment according to a tiered procedure. The first tier of the procedure involves a simple classification of pesticides into, on the one hand, substances for which atmospheric dispersion most probably presents no problem and, on the other hand, substances for which this needs to be further evaluated. The first sifting should be based on the degree to which the pesticide may enter the atmosphere during or after application. The percentage of the applied amount that is permitted to enter the air should be subject to a limit, preferably based on the substance’s toxicity, persistence, bio-accumulation and (anticipated) application volume.

Risks of medium-range atmospheric transport

Pesticides that are identified as problem substances in the first tier must be evaluated in a second tier to determine the risks that arise from medium-range atmospheric dispersion (up to a few dozen kilometres from the area of use). This must be based on a comparison between the anticipated exposure of organisms and their anticipated sensitivity. Models are available for estimating the atmospheric transport of pesticides to nature reserves. Soil- and water-quality models can then be used for estimating exposure in various environmental compartments. However, there is a lack of data on pesticide behaviour (e.g. transformation rate, attachment to organic matter) under the conditions that prevail in nature reserves. There is also a lack of toxicity data, at least for representative species. The Committee therefore calls for the evaluation to be based on information - to be supplied by the manufacturer in accordance with the present procedure - that relates to conditions in agricultural areas and to standard test organisms. In order to compensate for the uncertainty about the validity of the data for nature reserves and for exposure to several substances simultaneously, more stringent provisional requirements can be set for the ratio between exposure and sensitivity than for the evaluation of the risks to organisms on and nearby the fields. The Committee calls for research into how the conditions that prevail in nature reserves affect the behaviour of pesticides and into the sensitivity and recovery capability of the organisms and populations that live there. Such research should show the degree to which more stringent requirements for the aforementioned ratio would be appropriate. If a pesticide fails to satisfy the ratio criterion, the manufacturer could be given the opportunity to demonstrate by providing additional research data that unacceptable impacts would not occur in practice.

Risks of long-range atmospheric transport

The second tier should also include making an estimate of the substance’s potential for long-range dispersal (more than one thousand kilometres). This can be estimated by calculating an atmospheric transport potential. The potential quantifies either the time it takes for half of the emitted substance to disappear from the atmosphere, or the distance the substance travels in that time, given a postulated, constant wind speed. The transformation of the pesticide in air has to be known for the calculation. The best measure of this is the reaction rate constant for the reaction of a substance with OH radicals in air, kOH. The Committee calls for manufacturers to be obliged to provide this figure for risk assessment. This should be determined in accordance with an internationally accepted protocol, which has yet to be drafted. The Committee recommends that the figure for the atmospheric transport potential above which a further risk evaluation owing to long-range transport is required should be made dependent on the substance’s toxicity, persistence, bioaccumulation and the (anticipated) volume used.

The risk evaluation for long-range transport may depend on the ratio between anticipated exposure and the sensitivity of the living organisms in the remote areas. The Committee recommends also basing this calculation on the information the manufacturer already has to supply. However, the limiting value for this ratio should be made more stringent than that for the evaluation of the risks of medium-range transport, because there is more uncertainty about the validity of the data used. Further research should show the degree to which the conditions and the vulnerability of organisms in remote areas necessitate a more stringent limiting value.

In general, the uncertainties in the calculation of the anticipated exposure at a given distance from the area of use increase as the transport distance increases. Consequently, the calculation of the ratio between exposure and sensitivity for organisms in distant nature reserves will be subject to greater uncertainty. This may be an argument for the advance prohibition of substances with a large atmospheric transport potential. Given the cross-border character of the long-range atmospheric transport of pesticides, this would require international agreement.

Operationalisation

The Committee believes it ought to be possible to have the outlined procedure operational within approximately five years. To be able to determine whether such a modified acceptance procedure would be adequate to limit the risks of atmospheric dispersion of pesticides, the Committee believes it will be necessary to monitor air and precipitation quality. This applies all the more so because only the risks of individual pesticides are considered in the acceptance procedure. The monitoring data may show that problems occur (e.g. excessive concentrations in rainwater), owing to the authorisation of several pesticides with the same active ingredients or with the same working mechanism. In those cases, it will be necessary to examine whether this need have consequences for the entire group of pesticides or only for specific ones, for example, those that can be most readily missed. The monitoring results can also be used for further validation and improvement of the models used. Finally, the Committee points out that modification of the authorisation procedure is only possible at the international level, in the first place at the level of the European Union.