This guidance covers the general methods to be used in determining the first-order degradation model input. Frequently, biotransformation laboratory studies do not follow a firstorder degradation pattern; yet standard pesticide fate models (e.g. PRZM, EXAMS) require firstorder degradation inputs. This interim guidance, which is based in part on FOCUS (2006), establishes a simple procedure for determining a first-order rate constant from biotransformation/degradation studies, which include standard biotic studies conducted in soil, water, or mixed media.You may need a PDF reader to view some of the files on this page. See EPA’s About PDF page to learn more.