Helium doesn't cause cancer, so from that perspective it doesn't matter.
43200 times
23000 times!
You breathe helium out pretty quickly when you inhale it. In order for the effect to last the rest of your life, you'd need to breathe enough helium to be able to breathe it out for the rest of your life. Unless "the rest of your life" is measured in seconds (a minute or two at the most), this is impossible. If you asphyxiate yourself with helium, your voice will sound the same way forever (i.e. silent, since you'll be dead). I'm not certain how many balloons that would take, precisely, since it depends on the volume of the balloons.
about 4 times
You always breathe in 3-dimensions, never in 2d
80
a lot
8 to 19 times
Whilst having an asthma attack you would normally breathe about 97 times a minute.
18000
How many times do you breathe in a year.
100times