Naturally occurring chemicals environmental influences

Creep is the measurement of the increase of strain with time under a constant applied stress stress relaxation is the measurement of change of stress with time under constant strain and set is a measure of the recovery after the removal of an applied stress or strain. For all of these properties there are two distinct causes, one physical and one chemical. The physical effect is due to the viscoelastic nature of polymers, which means that the response to an applied stress or strain is not instantaneous but develops with time. The chemical effect is due to the aging of the material under environmental influences. The tw o causes cannot be totally separated as both will always occur. However, at short times, normal ambient temperatures and in the absence of other environmental effects the ph -sical effects will dominate, but at longer times, higher temperatures and the introduction of other degrading agents the chemical effects will increase. 

For some pesticide compounds, such as dini-troaniline herbicides (Weber, 1990), phototransformation occurs primarily in the vapor phase, rather than in the dissolved or sorbed phases. Perhaps the most environmentally significant pesticide phototransformation in the atmosphere, however, is the photolysis of the fumigant methyl bromide, since the bromine radicals created by this reaction are 50 times more efficient than chlorine radicals in destroying stratospheric ozone (Jeffers and Wolfe, 1996). Detailed summaries of the rates and pathways of phototransformation of pesticides and other organic compounds in natural systems, and discussions of the physical and chemical factors that influence these reactions, have been presented elsewhere (e.g., Zepp et al, 1984 Mill and Mabey, 1985 Harris, 1990b). 

Although the influence of relevant biotic and abiotic variables on the fate and effects of chemicals can, to a certain extent, be explored through controlled experiments and observations of natural systems, the combinations of factors that can be tested in practice are very limited. Mesocosm and field studies are often expensive to perform, can be difficult to replicate sufficiently, and are frequently complicated to interpret. Because they typically represent one unique scenario (species composition and density, temperature, light, nutrient level, and timing of pesticide application in relation to the environmental conditions), questions are often raised about the generality and robustness of the results. Likewise, unexpected or uncontrollable events may occur (e.g., it may have been an unusually rainy, sunny, hot, or cool season), the influence of which on the estimate of risk can be difficult to assess. 

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