There are three main areas of concern:
Volatile organic compunds - emitted as the ink dries
Heavy metals - contained in certain pigments (particularly metallic colours) and can result in environmental and worker health hazards
Non-renewable resources - the oil content in non-vegetable oil inks is petroleum-based
These issues are less of a problem where the mineral oil content is replaced with vegetable oil.
The oil content - mineral or vegetable - of a sheet-fed litho ink will make up around 35 percent of the ink. The remainder will consist of pigment, resins, fillers and varnishes. The term 'vegetable based' is, therefore, rather misleading, but it is so widely used that it is probably impossible to re-educate. It should also be noted that the vegetable oil which is used in inks is not pure, as it is reprocessed and contains additives.
Prior to the 1960s, less harmful vegetable oil inks were commonly used for all printing applications. Then petroleum-based (or mineral-based) inks were introduced, and because, at that time, they were cheaper and performed better, they gradually became the norm - despite the health and environmental issues.
Vegetable oil inks replace the mineral oil content to varying degrees. For a sheet-fed litho or heatset web offset ink, for which heat is used in the drying process, the oil content will be around 35 percent. For a coldset web offset ink that dries by absorption, rather than with heat, this rises to around 75 percent.
In recent years, vegetable oil inks have improved greatly - they certainly match the performance of mineral oil inks, and some say they are superior. They are now widely used for sheet-fed litho printing. Vegetable oil inks have much lower rates of VOC emissions than mineral oil inks. Also, in contrast to mineral oils, vegetable oils are derived from renewable resources, and the inks made from them are more easily removed from waste paper during de-inking. Another plus is that the pigments in the vegetable oil inks do not usually contain heavy metals.
At present there are no vegetable oil inks suitable for digital or screen-printing or fluorescent inks for litho printing.
Read about problems with deinking some digitally printed products here