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The concept of prevention in international environmental law was first introduced in the Stockholm Declaration of 1972 and later reiterated in the Rio Convention of 1992. It emphasizes taking measures to avoid environmental damage if no action is taken, based on predictable, definite scientific evidence. In contrast, the precautionary principle focuses on situations where scientific uncertainty exists, and involves taking measures to avoid potential environmental risks, even if the likelihood of damage is uncertain. The precautionary approach is provisional until additional scientific evidence confirms the existence of a real risk, and some commentators have expanded it to include measures aimed at avoiding uncertain risks in addition to grave and irreversible ones.