The method involving the irradiation duration influence in radiation induced health effect is proposed. At any doses, the radiating risk is expressed by the following formula: R = 1-exp (- E), where R is the life span risk of radiation induced stochastic diseases. For empirical representation of dose-effect ratio, the following assumption was used: the threshold is the function of the ratio of ... [Show full abstract] irradiation duration, T and characteristic time of reparation and elimination rates, τ, i.e. D0 = f(T/τ). For acute irradiation, the threshold, D0 → 0, and the dose-effect ratio is linear, so the radiation injury is maximal. From some data, the D0 value has been described by the following formula: D0 = 0.8 (T/τ)0.301, Sv. The proposed formula allows introducing simply an equal-effective of reduced dose as the measure of radiation effect risk, Deff = D/R. It is shown, that the time factor of irradiation is important for radiation health effect development. Therefore, such factor should be taken into account for Standards of Radiation Safety. Introduction of reduced doses allows to manage the collective doses and radiation protection optimization techniques.