Calculate EPA-style hazard quotients and, only for compatible components, their screening hazard index.
EPA Hazard Quotient and Hazard Index Formulas
A hazard quotient applies to one component: HQ = exposure / matched reference value. A hazard index is the sum of compatible hazard quotients: HI = Σ HQ_i. Calling a single ratio an “HI” is incorrect unless the assessment contains only one component and the distinction is made explicit.
Reference doses and concentrations must match the exposure route and time period. EPA mixture guidance also cautions that dose addition is most defensible for components that affect the same target organ or toxic effect. Do not add unrelated routes, durations, or endpoints merely because their units can be converted.
How to Interpret a Hazard Index
HQ and HI are screening ratios, not probabilities of illness. A value above 1 does not prove an adverse effect will occur, and a value at or below 1 does not guarantee no risk. The underlying toxicity values, exposure assumptions, sensitive populations, uncertainty, and component grouping require qualified risk-assessment review.
Primary references: EPA Reference Dose: Description and Use and EPA Supplementary Guidance for Chemical Mixtures.
