High air quality in the tunnel is achieved in two ways: ventilation and purification. Large fans draw air in from both entrances, and polluted air is expelled through the ventilation tunnel to Tynjadalen. The Lærdal Tunnel is the first in the world to be equipped with an air treatment plant, located in a 100-metre (330 ft) wide cavern about 9.5 kilometres (5.9 mi) northwest of Aurlandsvangen. The plant removes both dust and nitrogen dioxide from the tunnel air. Two large fans draw air through the treatment plant, where dust and soot are removed by an electrostatic filter. Then the air is drawn through a large carbon filter which removes the nitrogen dioxide.[3]