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Gurdarshan Gill of India is sworn in as a U.S. citizen during a naturalization ceremony May 22, 2007 in San Francisco

The rust-belt cities of Pittsburgh, Cleveland and St. Louis are all drawing a higher proportion of highly skilled immigrants than Silicon Valley, according to a new analysis of census data published Thursday by the the Brookings Institution.

"We don't think of these areas as high-tech hubs, but they have been really active in trying to recruit and encourage the welcoming of foreign workers into their industries to rejuvenate their economy," said the think tank's Matthew Hall.

Those cities also have a lower cost of living than San Francisco or San Jose, where companies and government expect highly educated immigrants to gravitate, according to Brookings.

"Pittsburgh is an easier place to afford to live the American dream and get your foot in the door. That might sound like a pretty good option to a lot of people," Hall said.

Brookings findings represent "a danger sign" for the Bay Area's tech sector, said Russell Hancock, head of Joint Venture Silicon Valley, a public-private think tank. 

"Some people think we should be promoting American citizens and American jobs, but over two-thirds of our scientists and entrepreneurs and Indian and Chinese," Hancock said.

Vivek Wadwa, a visiting scholar at UC Berkeley's School of Information, said he doesn't think Silicon Valley has anything to worry about — yet.

"I was surprised at how dramatic the change was," he said, but "it just means there is more competition."

Nationally, highly skilled immigrants in the United States now outnumber lower-skilled ones — a trend that holds true in the Bay Area, the report found.

The think tank found that 30 percent of the country’s working-age immigrants, regardless of legal status, have at least a bachelor’s degree, while 28 percent lack a high school diploma.