Prediction of potential harm, or risk assessment, is essential to planning for any vegetation management method, but the concepts and process are often not understood. This discussion is a highly simplistic description of the basic elements of toxicology and estimation of risk in excess of the high normal background. All chemical risk is directly related to the dose acquired by the subject or population. In the case of cancer, added human risk that may be associated with very low doses is expressed as a probability that is estimated by extrapolation from observations at high dose rates. Reasons for such an indirect approach and weaknesses of the present process are described.