WUHAN, China - The final years of the U.S. housing boom and a disastrous series of Gulf Coast hurricanes created a golden opportunity for Chinese drywall manufacturers.

With domestic suppliers unable to keep up with demand, imports of Chinese drywall to the United States jumped seventeenfold in 2006 from 2005.

That imported drywall is now at the center of complaints of foul odors seeping from walls.

Hundreds of homeowners, most of them in Florida, have reported corrosion of air conditioners, mirrors, electrical outlets and jewelry.

State and federal authorities have traced the problems to Chinese-made drywall, but have not fully determined the causes. Some Chinese experts suspect the culprit is a radioactive phosphorus substance - phosphogypsum - that is banned for construction use in the United States but has been used by Chinese manufacturers for almost a decade.

Copies of Chinese customs reports obtained by the Los Angeles Times, along with interviews, indicate that drywall made with phosphogypsum was shipped to the United States in 2006 by at least four China-based manufacturers and trading companies.

The health risk of phosphogypsum is uncertain, but industry specialists say its wide use and the possibility it was exported are troubling, especially in light of recent incidents in which Chinese imports such as pet food, toys and candy were found to be contaminated with toxic or unsafe substances.

"Considering the fact that phosphogypsum can cause corrosion, something should be done," said Ding Dawu, a geoscientist and an authority on gypsum processing in China. "Right now, there are no complaints in China because most people don't know much about gypsum board, and there are no standards against it."

Phosphogypsum contains radium, which under prolonged exposure can lead to a higher risk of lung cancer. That is why the U.S. Environmental Protection Agency banned phosphogypsum for use in construction in 1989.

Paul Papanek, a board member of the Western Occupational and Environmental Medical Association, said the health effects of contact with phosphogypsum are not immediate. Medical studies about how often the substance causes cancer are inconclusive, he said.

Chinese building-material managers say they have seen an increasing number of drywall makers mixing phosphogypsum in production. They said the corrosion of coils and metals seen in American houses was consistent with drywall made with that ingredient.

So far, tests in the United States of Chinese-made drywall used in American homes have not turned up evidence of phosphogypsum. In Florida, four samples taken from troubled houses showed no indication of radium, said Lori Streit, a scientist at Unified Engineering Inc., which conducted the analysis.

Streit says the rotten-egg-like odor and corrosion are associated with sulfuric acids. Some industry officials say that could mean the drywall was made with gypsum from mines in eastern China's Tai Mountain area, where ores have unusually high levels of sulfur compounds.

U.S. Consumer Product Safety Commission spokesman Joe Martyak says his agency asks U.S. Customs and Border Protection to inspect items for which there are mandatory testing requirements, such as children's toys. But there are no such conditions for drywall, he said.

No one knows how much phosphogypsum gypsum board from China was shipped abroad. But in 2006, Chinese exports of drywall to the United States totaled a record 503 million pounds, according to Chinese customs statistics. That's enough for 32,000 homes.