While neither Helium nor Argon will damage tissue (they are both noble gases, and therefore chemically inert), they are dangerous to breathe because they displace the oxygen in the lungs. The brain becomes oxygen deprived and acts like it is drowning, similar to what happens when huffing.

Probably the only safe way to breathe helium (especially for more than one breath at a time)is to breathe the helium oxygen mixtures that deep sea divers use. This is a mixture of ~20% oxygen and ~80% helium that is used at extreme depths. The helium replaces the nitrogen normally found in air, because at high pressure nitrogen can dissolve in the blood, causing nitrogen narcosis and the bends.

When you breathe Helium instead of Air, the pitch of your voice will be higher because of the lower molecular weight of helium. This effect follows directly from the kinetic theory of gases. The mean kinetic energy of a gas is given by:

Uk = (1/2) m v2 = (3/2) k T

where Uk is the kinetic energy of the gas, m is the mass, v is the mean velocity of the gas molecules, and T is the temperature. k is a constant, given by R/NA (the universal gas constant divided by Avogadro's number.)

The velocity of the gas molecules, and thus the speed of sound can now be calculated by:

v = √(3 RT/M)

where M is the molecular weight of the gas (gram/mol.)

The last formula shows that a lower molecular weight results in a higher speed of sound. Note that also changes in temperature have an effect, although this effect is small under "normal" conditions (remember that the absolute temperature is used.)

Sound is produced in your vocal chords by resonance of sound waves in your throat and mouth (i.e. a cavity). The resonance of (sound) waves in a cavity is given by:

v = f × lambda

where f is the frequency (pitch) of the standing wave, and lambda is the wavelength. The wavelength is essentially fixed for a given geometry of a cavity. Thus an increase in the speed of sound will result in a higher frequency, or pitch of the resonated sound.

Air has an (average) molecular weight of 28.85 gram/mol. Helium has a much lower molecular weight: 4.00 gram/mol, and will result in a much higher pitched voice (A "Donald Duck" voice). On the other hand, argon has a higher molecular weight than air: 39.95 gram/mol, and will result in a lower pitched voice (A "Goofy" voice). The noble gas krypton (83.8 gram/mol) would give you an even lower pitched voice.

Both helium and argon are noble gases, which means that they do not react with any other elements (though rare exceptions exist.) If you were dealing with pure gases, one hit of the gas probably would not do any damage; the oxygen concentration in your blood doesn't drop that fast. Also, the argon will not displace all the air in your lungs, since both gases will mix.

However, I recommend that you do not attempt this, since the gas cylinders that you buy for filling helium balloons may contain toxic impurities. Gas manufacturers produce many types of commercial gases, and will not guarantee absolute gas purity unless this is desired by the customer (higher quality means higher price.) Breathing even trace amounts of toxic gases such as carbon monoxide can seriously harm you, and possibly kill you.

Not that it stopped me from trying this when I was younger...

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