DT50

DT50, short for degradation time 50%, is a parameter used in environmental fate assessments to denote the time required for the concentration of a substance to decline to 50% of its initial value under specified conditions. It is effectively the half-life of a chemical in a given medium and is distinct from biological half-lives in organisms, reflecting environmental processes such as biodegradation, hydrolysis, photolysis, and volatilization.

DT50 is highly context-dependent, with separate values for soil, water, sediment, and air. Factors influencing DT50 include temperature, moisture, microbial activity, pH, light exposure, and the presence of other chemicals. DT50 values are typically obtained from laboratory studies under standardized conditions or from field measurements, and are used in regulatory risk assessments to estimate environmental persistence and potential exposure.

Calculation and interpretation: For first-order degradation, DT50 equals ln(2) divided by the rate constant (k). If degradation does not follow first-order kinetics, DT50 may be derived from empirical data or modeled accordingly. DT50 values can range from hours to years, depending on the substance and the medium; many pesticides show soil DT50s ranging from days to months, while certain persistent substances exhibit much longer times to 50% degradation.

Limitations: DT50 assumes relatively constant conditions and homogeneous systems; real environments vary spatially and temporally. Therefore DT50 should be reported with the specific medium, conditions, and methodology used to derive it. It is one component of a broader assessment of environmental persistence and risk.

See also: half-life, t1/2, environmental fate, biodegradation, regulatory guidelines (e.g., OECD, EPA).