The used definition of an environmental tax was elaborated by Eurostat and OECD and makes comparable studies possible between different countries:

"...it has been chosen to single out the tax base that seem to have a particular environmental relevance, and to consider all taxes levied on these tax bases as environmentally related regardless of motives behind their introduction, their names etc."

According to the definition it is the tax base that determines whether or not the tax is an environmental tax.