I suspect that they don’t. But why not?
I would imagine that it costs a pretty penny to fill those balloons with fresh helium every year. I would also imagine that very little of the “used” helium gets contaminated during the pre-parade, parade and post-parade activities. I would further imagine that it would not be terribly difficult to suck the “used” helium out of the balloons and back into the tanks.
Am I full of hot air? [grinny here]
They probably don’t, because helium is so cheap.
The problem with helium s that once you let it go, it’s gone. You don’t get it back. It’s a inert gas, lighter than anything (except hydrogen) and it ges to the top of the atmosphere. Eventually it gets a kick from solar particles and flies off into space. Helium is created at very slow rate via radioactive decay, and gets trapped atop oil deposits in imermeable rock. We’re tapping into this and releasing an ages-old supply in a relatively short time. Eventually, it’ll be gone.
And that’s not good, because helium is on of the few things you can use for cooling to ultra-low temperatures. It’s a helluva lot safer than hydrogen. And you can’t build a dilution refrigerator using hydrogen.
Some time ago, scientists got concerned about this, and built helium–recovery systems to trap th helium used in experiments. Our lab in Utah had one. But t hadn’ been used in years. I think that helium fell n price, and it just wasn’t worth it.
I suspect that supercomputers crammed nto tiny chips are going to brin a high demand fo helium, one of these days. W’ll see what hapens.
The irony…the 2nd most common element in the universe, and we run out of it!