You should NEVER breathe pure helium

Never.  It is potentially lethal, as these teenagers found out:

After drinking on the drive, and downing more drinks in the condo, it came time for [14-year-old] Ashley to take her turn on a tank of helium that everyone else was inhaling to make their voices sound funny.

"That helium tank got going around... It got to my daughter. My daughter didn't want to do it. It was peer pressure. They put a mask up to her face. They said it would be OK. `It's not gonna hurt you. It'll just make you laugh and talk funny.'"

Instead, she passed out and later died at a hospital, the result of an obstruction in a blood vessel caused by inhaling helium from a pressurized tank... Dr. Mark Morocco, associate professor of emergency medicine at the Ronald Reagan Medical Center in Los Angeles, said what happens is similar to when a scuba diver surfaces too quickly. A gas bubble gets into the bloodstream, perhaps through some kind of tear in a blood vessel, and can block blood flow to the brain, causing a stroke.

The gas is also commonly seen in suicide kits — mail-order hoods sold out of Oregon and elsewhere that can be attached to a helium tank by people who want to kill themselves. In those cases, the helium crowds out the oxygen, asphyxiating a person. Death from inhaling helium is so rare that the American Association Poison Control Centers lumps it in with other gases, such as methane and propane. Only three deaths were recorded in 2010, said spokeswoman Loreeta Canton...

The family moved from Grants Pass, Ore., to Eagle Point about a year ago, and Ashley had just gotten over the difficulty of adjusting to eighth grade in a new school. Justin Earp said the kids had four wine coolers each in the car, and four mixed drinks at the condo, before they started passing around the helium. Police said it was an 8-gallon canister, the kind you can buy at many stores. The kids were taking hits directly from the tank.

The cause of death is described as a gas embolism, which could occur if the nipple from the cylinder was placed directly in the mouth and the flow was high.  The more common mode of death would be through anoxic asphyxia, in which the inhaled pure helium displaces oxygen in the lungs; that can occur even after inhaling low-pressure gas via a loose-fitting face mask.

Helium is, as noted, rare as a cause of death, and is less commonly recognized as a toxic gas because it is lighter than air and unlike carbon dioxide and methane, it doesn't tend to accumulate in spaces (except in rare industrial settings).

If someone wants to alter their voice to imitate "Donald Duck," the effect can be achieved by inhaling a helium-oxygen mixture ("Heliox"), which typically contains 20% oxygen, 80% helium - though other proportions are available for specialized medical uses.  Those canisters are more difficult to find (and I suspect considerably more expensive) than the helium cylinders used to inflate balloons.

If you're at a party, and a helium cylinder is present, with tubing coming off the regulator leading to a face mask, look at the label on the cylinder.  If it doesn't specify that it is a helium-oxygen mixture, give it a pass and warn everyone else.