Abstract: The definition and categories of hazardous thresholds are put forward. According to the related literature at home and abroad, the harm criteria for fire and explosion are determined. Then, the emergency exposure thresholds for common toxic substances are compared. Based on this, the harm threshold criteria for different harms under different accident scenario are given, which are classified into threshold of high lethality, threshold of starting lethality, threshold of irreversible effect, and threshold of reversible effect. These hazardous thresholds can be applied to the fields of safety assessment, land use planning and emergency planning for major hazard installations or industrial zones.