In the 1980s it became evident that smog from factories and an ever-increasing fleet of automobiles, as well as a lack of adequate free space due to congestion, had evolved into the city's most important challenge. A series of anti-pollution measures taken by the city's authorities in the 1990s, combined with a substantial improvement of the city's infrastructure (including the Attiki Odos motorway, the expansion of the Athens Metro, and the new Athens International Airport), considerably alleviated pollution and transformed Athens into a much more functional city. In 2004 Athens hosted the 2004 Summer Olympics.