Latex balloons are made from rubber trees and decompose at the same rate as an oak leaf.
How is latex made?
Latex is harvested by tapping the rubber tree (this doesn't harm the tree). Each tree planted to harvest rubber consumes CO2, thus reducing global warming. After the tree no longer produces latex, it is harvested and it's wood turned into lumber. New trees are planted every year to ensure a continued supply of latex.
This video made on behalf of Qualatex shows how latex balloons are made:
Qualatex have produced a poster which you can print and display (or use online) to explain to your customers how latex balloons are a natural product and 100% biodegradable. The rate of degradation of a latex balloon is highly variable and influenced by many variables such as temperature, light and the presence of latex consuming microbes.
A brilliant way to reuse foil balloons (especially if the foil balloon was damaged and couldn't be inflated) is to use them to cover a cake board. This board is then used as a base for a new balloon design. This article from Sue Bowler's 'The Very Best Balloon Blog' shows how to do this and some creative ideas on using covered cake boards in your balloon designs. There is also an instruction video below from Sue showing the technique.