The cytotoxicity of various dilutions of sodium hypochlorite was evaluated using three independent biological models. Dilutions as low as 1:1000 caused complete hemolysis of RBC's in vitro. Undiluted and 1:10 dilutions produced moderate to severe irritation to rabbit eyes which healed after 24 to 48 h. Intradermal injections of undiluted, 1:1, 1:2, and 1:4 dilutions of NaOCl produced skin ulcerations following intradermal injections. Although the use of 5% NaOCl for biomechanical preparation of root canals is a clinically acceptable and highly effective procedure, it must be used judiciously and with great caution to prevent it from reaching the periapex where it can elicit severe inflammatory reactions.
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