How many helium balloons is it safe to buy a child?

Yesterday I was reading a few articles I'd put in my favorites folder over the past few years, one in particular that you might remember was the 'Balloon Boy' hoax. It got me thinking, could you buy enough balloons at say a fair ground to accidentally get a child airborne?

First we need to know that it takes roughly one litre of helium to lift one gram so to lift a child weighting 25kg we would need 25,000 litres of helium, that's quite a lot. If we assume that an average helium filled balloon is 25cm in diameter we can use the equation 4/3 x pi x r x r x r to find the volume.

By taking pi to 2 decimal places and the radius as 12.5cm we get:

4/3 x pi x 12.5 x 12.5 x 12.5 = 8,177.08 cubic centimetres, which is just over 8 litres.

If we assume that the balloon and the string weight 2 grams, each balloon could lift just 6 grams in addition to its own weight. For your child that weights 25kg you'd need 4,167 balloons and roughly £8,500 in cash to lift him or her off the ground, probably not something anyone will do by accident.