Decorative helium balloons, unlike those filled with simple air, float and make interesting, festive decorations. On the other hand, helium balloons can also be expensive, and if they're only used for a short time this can lead to a low return on investment. Putting half air and half helium in a balloon allows you to split the difference.

A balloon filled with half air and half helium turns the composition of the atmosphere within it on its head — in the Earth's atmosphere, helium is a trace element. Inside the balloon, the atmosphere is composed of about 50 percent helium, 39.1 percent nitrogen, 10.5 percent oxygen and .5 percent argon. Depending on local conditions, the balloon may also contain between 0 and 2 percent water vapor, with trace quantities of carbon dioxide, methane, ozone, neon, krypton and hydrogen. Despite the oxygen content, it is not safely breathable.

Helium is lighter than air, so although a balloon half-filled with helium will look the same as a balloon filled completely with air, it will behave slightly differently. Assuming that it's spherical in size, a 12-inch balloon has a volume equal to 4/3 x pi x 216, or 904.8 cubic inches. Half of that, 452.4 cubic inches, is equivalent to .26 cubic feet. Because helium has a lifting capacity of about .84 ounces per cubic foot, a half-helium 12-inch balloon can lift about .22 ounces. Since a 12-inch balloon weighs less than this amount, a half-helium balloon will still float, though not as well as a completely filled one.

Although a half-helium balloon floats better than a completely air-filled balloon, it will also deflate faster, based on principles governed by Graham's law, which holds that the rate at which gas escapes from a container is directly related to the molecular weight of the gas. Because helium is so much lighter than nitrogen and oxygen, the primary components of the atmosphere, it will escape at a faster rate. If you intend to keep your half-helium balloon airborne for a substantial length of time, you'll have to periodically replace the lost helium.

One of the less visible changes to occur if you inflate a balloon half-full of helium involves your wallet. Although air is free, helium is comparatively expensive — between $1 and $3 per cubic foot, not including any costs associated with purchasing or renting a helium tank. Helium is a finite resource — it's so light it literally floats away from the Earth; helium has to be mined, like oil or natural gas, from dwindling stockpiles. The price of helium has been kept artificially low by the U.S. government's desire to privatize its helium reserves, and is expected to increase sharply once this process is completed.