Mayors from around the world have gathered in San Francisco to share ideas on preserving the environment in a time of urban growth. The meetings give the West Coast city a chance to highlight its environmental achievements.

Local officials proudly claim that theirs is the greenest big city in America. San Francisco is the first US city ever to host talks for United Nations World Environment Day, which will be celebrated Sunday. This year's theme of Green Cities focuses on the steps that local officials can take to improve the environment.

Outside city hall, where some 70 mayors are meeting, a temporary house has been built to make a point. It is made of discarded materials, including metal scraps from a machine shop and glass from a building site, which are re-used in the two-story structure.

San Francisco chief building inspector Laurence Kornfield says the display conveys a message that the city encourages creative building techniques. "My goal is to show that we're interested in approving alternate methods of construction, alternative materials. We want to show that we are interested in sustainability and green building. So that's my message," he said.

Officials say San Francisco is also a leader in using renewable energy sources, such as solar and wind power. A huge array of solar panels atop the Moscone Center, the city's convention hall, provides one-quarter of the center's energy needs. Michael Kim of the San Francisco Public Utilities Commission says that solar power, if used at key points in California's electrical grid, could relieve strain on the overtaxed system, typically seen afternoons when power demand is highest. He notes that is also the time when the sun is shining brightly.

"So energy generated onsite could rely the transmission congestion that may be requiring areas where they are thinking about putting in more transmission lines, if there's more onsite generation during these peak times, then that will not be necessary," he said.

Another key component of San Francisco's green strategy is recycling. Michael Sangiacomo is the president of Norcal Waste Systems, an employee-owned company that contracts with the city to collect garbage. He says the business traces its roots to early San Francisco firms that called themselves scavenger companies. "That word scavenger was there because these people actually went through everything they collected and pulled out anything that could be recycled," he said.

He says recycling paper, bottles and metal was common 100 years ago, and is once again happening.

His company sponsors artists like Andrew Junge who search through trash for material for their art works. The San Francisco sculptor creates art objects from old boxes, material like Styrofoam, and discarded appliances, working at a studio near a recycling center. He says he is giving truth to the adage that one man's trash is another's treasure.

"It's overwhelming, the amount of material that's in here, and it's a constant inspiration as far as what shows up and what can be made from what shows from what shows up. Every day is an adventure," he said.

Local official Jack Macy says recycling is a way of life for San Franciscans. He says many families and restaurant owners recycle table scraps, producing compost, a rich organic compound that farmers use for fertilizer. Offices recycle paper. Construction contractors recycle waste from building sites. Mr. Macy says this allows the re-use of materials such as concrete, and ensures that the energy that went into producing them does not go to waste.

"For every ton that we throw away on a municipal level, 70 tons of waste is generated in the resource extraction and the processing and the distribution. All of that energy or resources are displaced or saved when we reuse and recycle material," he said.

The San Francisco official says many developing countries, which can become polluters as they start to industrialize, are ahead of the industrial world at another level. He says economic necessity has forced people in poor countries to recycle what they have, offering a lesson that the developed world is starting to rediscover.