SARAJEVO, Bosnia-Herzegovina (AP) — Sarajevo authorities have banned half of the city's cars from driving on city streets in an attempt to ease air pollution.

The city government ordered the introduction of alternative driving days for cars with even and odd numbered license plates and made public transportation free until the situation improves. Some outdoor construction work has also been halted. The ban went into effect on Sunday.

Sarajevo is located in a narrow valley, which hampers the free flow of air. For more than a week, dense smog has completely obscured the view of the city from surrounding hills.

Official statistics from the past seven days show that the concentration of hazardous air particles have been between six and 10 times above the acceptable limit.