The general rule with any harmful chemical is that it can act as a dangerous poison if used in sufficiently high dosage. Similarly, a poisonous chemical given in small amounts will not be harmful and could be beneficial. However, for each individual the safety level will be different, and the response to a low dosage cannot be predicted. It may be safe and appropriate to utilise very small amounts of boric acid in rice, but there is no guarantee that the cooked rice will be free of toxic effects that might appear in very susceptible people. It may be best to consult a major company that markets rice for their advice.