Since 1851, when London held the first world exposition themed on the "Industry of all Nations", more than 100 world expos have been held.

But the continued use of the expo venues after the events and the concept of sustainable exhibitions are relatively new.

Most of the structures from more than 160 years of expos were temporary and were taken down after the events closed. Only a number of landmark buildings or structures from several fairs were notable exceptions.

For example, the most famous of these is the Eiffel Tower in Paris, built for the 1889 expo and now the most recognizable symbol of the city.

A few other venues have remained, such as Seattle's Space Needle, Treasure Island in the middle of San Francisco Bay, and Mies Van Der Rohe's German Pavilion in Berlin.

With the cost from the 1851 expo of $1.65 million rising to $176.9 million in 2012, the issue of usage post-event has also gradually been emphasized.

The Crystal Palace from the first World's Fair in London was designed for other uses, but it did not have a noticeable plan for reopening or rebuilding.

Since 1982, China has organized the China Pavilion for about 15 world expos; Expo Shanghai 2010 is the first Universal International Exposition held in China.

With more than 200 nations participating and drawing a record-breaking 73 million visitors, the Shanghai Expo left some indelible marks on the city, which can still be seen today.

According to previous news reports, after the Shanghai Expo, its popular China Pavilion reopened to the public for six months. After that, it was reopened as a museum of Chinese history and culture.

Besides the China Pavilion, a number of other pavilions remained at the site and continued to draw crowds, such as the Saudi Arabian pavilion. The Shanghai expo park also went through redevelopment.

According to its plan, the Shanghai Expo venue on the Puxi side will be home to both an expo museum and a contemporary art museum. The Pudong side will see many international companies setting up new offices.

The Expo Milano 2015 is themed "Feeding the Planet, Energy for Life". According to an exclusive interview with Fabrizio Grillo, secretary-general of the Italy Pavilion, the expo site is already planned for reuse after the event proper concludes.

"The Italian pavilions will stay on the site, as well as the major tree structure. We want the tree to stay there because it has become a symbol of this expo," Grillo said. "So in the frame of the continuity of this expo the two structures will remain there for the future, because they are also wonderful attractions for tourists."

For the other pavilions, Grillo said most of them are expected to be dismantled. "We are holding discussions; maybe one or two pavilions will remain after the closing of the expo."

Half of the site's area will become green zones, because that is part of the initial plan of the local government; the other half will be used as an innovation town, he said.

Italian universities with research centers, technology parks, institutions and representative offices of companies may be invited to the site, he said.