To demonstrate the safety of a given chemical use or a decontaminated site, it is important to evaluate its counter term, "risk," as accurately as possible. According to ISO/IEC Guide51, “safety” is defined as “freedom from risk which is not tolerable”. Tolerable risk can change depending on social conditions, regional characteristics, and historical background and other factors. A single accident or event, or a technological development, can trigger a major change in society's attitude toward risk, and a level of risk that was previously accepted without problems may become unacceptable to date. This talk will present examples of risk assessment practices or studies (e.g., chemical substances, radiation, infectious disease, and marine plastics) in which the speaker has been involved or are currently involved, and discuss opportunities, challenges, and evolving roles of risk assessment approach for complex environmental issues.