if someone had passed out in a rusty area, why could I not hold my breath to enter and get them?

As long as I had confidence that I could get back out (maybe to get more air) before breathing, why would I be at risk.

And waiting for help, might result in their death!

Are you worried the helper cannot estimate how long they can hold their breath? Or worried that the breath they take in, might not be fresh air (with normal O2)?