HSTs are predominantly electric powered and therefore emissions from HST
operations are considered to be linearly related to energy consumption and the
sources used to generate the electricity. The higher the level of renewable sources
and nuclear power used to generate the electricity, the lower the level of emission
associated with HST operations. Usually, it is assumed that the electricity is
supplied from the national grid and emission is calculated based on the average
electricity generation mix (Commission for Integrated Transport, 2001). The use
of electric power also means virtually zero emissions from the HST along the line
and at the stations.
The most harmful pollutants related to HST operation are sulphur dioxide
(SO2) and nitrogen oxides (NOx). The former affect the environment mainly by
contributing to LAP, and the latter to both LAP and climate change. In general,
HST operations are not considered to contribute significantly to climate change,
while their contribution to LAP can be significant depending mainly on the levels
of SO2 emission associated with HST operations (Givoni, 2005). These levels
depend mainly on the share of coal used to generate the electricity (Button, 1993).
Usually, power plants are located away from densely populated areas, which
means that the actual impact from HST operation on LAP is lower then suggested
by the mix and amount emitted due to the relatively low number of people
exposed to the emission.