It does not emit harmful gases when in motion, but is also calculated to have a lower overall environmental impact than its internal combustion siblings.
For example, it recovers a significant amount of electricity during regenerative braking, while at the same time not wearing out the brake shoes. The construction of an electric motor is much simpler than that of an explosion engine, especially compared to a petrol engine.
Simpler construction, fewer parts, which means less impact on the environment.