I'm designing a self-rescue chest belt which has a balloon that inflates in case of drowning. I need to use a factor of safety, especially for the volume of air inside the balloon. So, how can I choose the best factor of safety so that my system is economical and well-designed? If you want to answer, please provide me with a reference to prove where or based on what I choose the factor of safety. I think it should be high, larger than 5, since it is related to human life.