The letters PVA stand for Poly Vinyl Alcohol, and it was invented in Japan in the late 1920`s.
There are many uses for the PVA we know in the pharmaceutical trade, agriculture, printing, textile, and of course the fishing trade.
PVA is produced from the saponificatuon of of polyvinyl acetate, or in other words it is produced when a fat is mixed with a polyvinyl acetate, (a kind of plastic containing double Carbon bonds).
PVA itself usually comes in a white crystal powdered form. To make our bags this form is made into a film, which is then made into our bags or spun into thread which is then made into mesh.The film can be made to nearly any thickness, however if its too thin, the bags will have no strength in them and if its too thick the bags will not melt within an acceptable time period. It is finding an acceptable balance between strength and breakdown that is important.
The general chemical structure of PVA in the kind we use can be seen in the diagram below.
C = Carbon
H = Hydrogen
O = Oxygen
When it is put into water the bonds that hold the structure together break and thus the PVA melts.
As you can see PVA is very safe both to handle and for the environment when it has broken down, as it is only made from simple elements.