As detailed in Chapter 2, the classification considers policies that avoid pollutant emissions, either by providing the energy services more efficiently or because they are provided in a way that does not involve the combustion of fossil fuels or bioenergy. It also includes policies that reduce the impact of such combustion, either by mandating control technologies that reduce the resulting emissions, or by introducing measures that otherwise reduce their impact once emitted (e.g. regulations on stack heights, improved ventilation), or by encouraging a switch to combustion of a less-polluting fuel (e.g. gas instead of coal, or a lower-sulfur oil product). In addition, there is a range of supporting measures, such as pollution charges, emissions trading schemes or the removal of fossil- fuel subsidies that create changes in relative pricing as an economic incentive to change behaviour. These can result in either a reduction or an avoidance of pollutant emissions, depending on how they affect the investment calculations or operational decisions. Some of these measures can be formulated relatively quickly, such as the reform of subsidies to polluting fuels or the introduction of stricter efficiency standards, but implementation may face significant barriers and it still takes time for measures of the latter type to work through into the stock of existing equipment or the vehicle fleet. Other measures typically require significant investment and transition periods to implement in full, such as a shift in the energy mix in favour of low-carbon sources.