Even though dissipation may occur from photochemical, chemical and microbial degradation, the introduction of pesticides into the environment, particularly as sprays, has led to adverse effects on organisms other than the intended target. The duration and intensity of exposure to pesticides of non-target species are affected by many factors related to their habitat and behaviour. These include the population density of non-target species at the time of application, the physical nature of the area treated as well as the type of crop or vegetation (including refuges for non-target species), the time of year when pesticides are applied and the age and sex of fauna. The effects of many pesticides on nontarget species are often restricted to the locality in which they were applied. For non-target fauna and flora, knowledge of local environmental concentrations may be useful in predicting the likelihood of acute toxic effects on sensitive species. However, concentrations within areas of application may not be the sole determinants of risk to non-target species. The risk posed by a pesticide depends on the sensitivity of individual organisms and the degree of exposure to the chemical.
For example, soil-applied herbicides may be present in relatively high concentrations and have little direct effect on soil fauna, or accumulate through food webs/chains. Of course such herbicides may pose problems for local flora and even following crops. Conversely, chemicals present in low concentrations or having low persistence could be classified as safe, whereas in practice they may present risks due to their intrinsic high toxicity, bioconcentration or bioaccumulation in certain species.