The index measures the degree to which environmental policies put an explicit or implicit price on polluting or environmentally harmful behaviour. The index ranges from 0 (not stringent) to 6 (highest degree of stringency) and is based on the degree of stringency of 14 environmental policy instruments, both market-based and non-market-based, primarily related to climate and air pollution. These policy instruments include environmental taxes on SOx, NOx, diesel, and CO2; trading schemes in CO2; renewable energy and energy efficiency certificates; feed-in tariffs on solar and wind energy; deposit and refund schemes; emission limit values on NOx, SOx, PMx and sulphur content limits in diesel, as well as government expenditure on research and development within renewable energy.