Spridning av radioaktivt material från en strålkälla genom olyckshändelse. Olyckor vid kärnreaktorer kan omfatta stora befolkningsgrupper genom utsläpp av radioaktivitet till omgivningen och genom ned fall, eller ett fåtal individer med höga, skadliga doser.

"Chemical environmental hazardous substances are those emissions, either in the form of gas, liquid, or solid, that contain chemicals known to be harmful to the environment and human health, posing risks such as pollution of air, water, and soil, disruption of ecosystems, and adverse effects on living organisms."

A Proportional hazards model, such as the Cox regression model, is a statistical approach used to analyze the relationship between covariates and survival times in medical research. It allows for the estimation of hazard rates, which represent the instantaneous risk of an event occurring (e.g., death, disease progression), while accounting for the influence of multiple factors. The assumption in this model is that the effect of a covariate on the hazard rate remains constant over time, meaning that the hazard ratio between any two groups remains proportional throughout the study period. This model is widely used in clinical trials and epidemiological studies to evaluate treatment effects, identify prognostic factors, and estimate survival distributions.