I'm not UAL, but just food for thought, the regional I was at previously had a crew get CO poisoning from running a pack off of a huffer, resulted in an emergency air return and them (and the pax) being grounded for a while until their systems could clean themselves out. I expect not many carriers maintain the air carts to a standard for breathing air. It was our policy to never turn packs on until the huffer was completely disconnected after engine start.