Long-term exposure, field studies

Studies for pesticide risk assessment focus on the situation in a water body near the field edge, with the peak of exposure soon after application due to drift, runoff, or drainage. In most cases, endpoints are related to the initial concentration of the test item (which can encompass multiple applications). In contrast to this, the focus of EQS derivation is mostly on protection against effects of long-term exposure. 

A long-term large field exposure of North American deciduous trees to doubled atmospheric ozone and CO2 in Wisconsin was studied relative to terpenoid... 

The nonvisual or subtle effects of air pollutants involve reduced plant growth and alteration of physiological and biochemical processes, as well as changes in the reproductive cycle. Reduction in crop yield can occur without the presence of visible symptoms. This type of injury is often related to low-level, long-term chronic exposure to air pollution. Studies have shown that field plantings exposed to filtered and unfiltered ambient air have produced different yields when no visible symptoms were present (5). Reduction in total biomass can lead to economic loss for forage crops or hay. 

Research interest in POPs appears to be a relatively recent phenomenon with a dramatic increase in the POPs related publications since 1993. Despite a significant number of research studies most of this is limited to studies of field exposure and analytical methods with few studies on risk assessment, alternatives to POPs or management and disposal techniques. Some participants pointed out that there have been few studies in China assessing the long-term effects of POPs on people or critical ecosystems, studies that would be particularly useful for decisionmakers and for increasing public awareness of the problems posed by such chemicals. 

Heagle and associates found a reduction in yield of sweet com and soybean after exposure to ozone at 0.10 ppm for 6 h/day over much of the growing season. These exposures were carried out in field chambers set over soybean plots in the field. They suggested that a threshold for measurable effects on these crops would lie between ozone (oxidant) concentrations of 0.05 and 0.10 ppm for 6 h/day. These values are realistic in terms of growing-season averages in the eastern United States. More of these studies could help to clarify dose-response relationships for economically important crops. Table 11-5 summarizes these long-term, chronic studies. 

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