What is the least toxic flux?

Could you please elaborate on what is considered the least toxic flux? I'm interested in understanding the characteristics that make a flux less harmful, and how it compares to other types of fluxes in terms of toxicity. Is there a specific material or composition that typically results in lower toxicity? Also, how does the toxicity level of the least toxic flux affect its performance or usability in various applications? Thank you for your assistance in clarifying this matter.