PARKLAND, Fla. (AP) — At the height of the U.S. housing boom, when building materials were in short supply, American construction companies used millions of pounds of Chinese-made drywall because it was abundant and cheap.

Now that decision is haunting hundreds of homeowners and apartment dwellers who are concerned that the wallboard gives off fumes that can corrode copper pipes, blacken jewelry and silverware, and possibly sicken people.

Shipping records reviewed by The Associated Press indicate that imports of potentially tainted Chinese building materials exceeded 500 million pounds during a four-year period of soaring home prices. The drywall may have been used in more than 100,000 homes, according to some estimates, including houses rebuilt after Hurricane Katrina.

"This is a traumatic problem of extraordinary proportions," said U.S. Rep. Robert Wexler, a Florida Democrat who introduced a bill in the House calling for a temporary ban on the Chinese-made imports until more is known about their chemical makeup. Similar legislation has been proposed in the Senate.

The drywall apparently causes a chemical reaction that gives off a rotten-egg stench, which grows worse with heat and humidity.

Researchers do not know yet what causes the reaction, but possible culprits include fumigants sprayed on the drywall and material inside it. The Chinese drywall is also made with a coal byproduct called fly ash that is less refined than the form used by U.S. drywall makers.

Dozens of homeowners in the Southeast have sued builders, suppliers and manufacturers, claiming the very walls around them are emitting smelly sulfur compounds that are poisoning their families and rendering their homes uninhabitable.

"It's like your hopes and dreams are just gone," said Mary Ann Schultheis, who has suffered burning eyes, sinus headaches, and a general heaviness in her chest since moving into her brand-new, 4,000-square foot house in this tidy South Florida suburb a few years ago.

She has few options. Her builder is in bankruptcy, the government is not helping and her lender will not give her a break.

"I'm just going to cry," she said. "We don't know what we're going to do."

Builders have filed their own lawsuits against suppliers and manufacturers, claiming they unknowingly used the bad building materials.

The Consumer Product Safety Commission is investigating, as are health departments in Virginia, Louisiana, North Carolina, Florida and Washington state.

Companies that produced some of the wallboard said they are looking into the complaints, but downplayed the possibility of health risks.

"What we're trying to do is get to the bottom of what is precisely going on," said Ken Haldin, a spokesman for Knauf Plasterboard Tianjin, a Chinese company named in many of the lawsuits.

The Chinese ministries of commerce, construction and industry and the Administration of Quality Supervision Inspection and Quarantine did not respond to repeated requests for comment. Chinese news reports have said AQSIQ, which enforces product quality standards, was investigating the complaints but people in the agency's press office said they could not confirm that.

Meanwhile, governors in Louisiana and Florida are asking for federal assistance, and experts say the problem is only now beginning to surface.

"Based on the amount of material that came in, it's possible that just in one year, 100,000 residences could be involved," said Michael Foreman, who owns a construction consulting firm. The company has performed tests on some 200 homes in the Sarasota area and has been tracking shipments of the drywall.

Federal authorities say they are investigating just how much of the wallboard was imported. Shipping records analyzed by the AP show that more than 540 million pounds of plasterboard — which includes both drywall and ceiling tile panels — was imported from China between 2004 and 2008, although it's unclear whether all of that material was problematic or only certain batches.

Most of it came into the country in 2006, following a series of Gulf Coast hurricanes and a domestic shortage brought on by the national housing boom.

The Chinese board was also cheaper. One homeowner told AP he saved $1,000 by building his house with it instead of a domestic product.

In 2006, enough wallboard was imported from China to build some 34,000 homes of roughly 2,000 square feet each, according to AP's analysis of the shipping records and estimates supplied by the nationwide drywall supplier United States Gypsum.

Experts and advocates say many homes may have been built with a mixture of Chinese and domestic drywall, potentially raising the number of affected homes much higher.

So far, the problem appears to be concentrated in the Southeast, which blossomed with new construction during the housing boom and where the damp climate appears to cause the gypsum in the building material to degrade more quickly. In Florida alone, more than 35,000 homes may contain the product, experts said.

In Louisiana, the state health department has received complaints from at least 350 people in just a few weeks. Many of the affected homeowners rebuilt after Hurricane Katrina only to face the prospect of tearing down their houses and rebuilding again.

In another cruel twist, some of the very communities that have been hit hardest by the collapse of the housing market and skyrocketing foreclosure rates are now at the epicenter of the drywall problem.

Foreman warns of a "sleeping beast" in the thousands of bank-owned condos and houses across the country, with no one in them to complain.

Outside the South, it's harder to pinpoint the number of affected homes. And in drier climates such as California and Nevada, it may be years before homeowners begin to see — and smell — what may be lurking inside their walls.

The drywall furor is the latest in a series of scares over potentially toxic imports from China. In 2007, Chinese authorities ratcheted up inspections and tightened restrictions on exports after manufacturers were found to have exported tainted cough syrup, toxic pet food and toys decorated with lead paint.

Scientists hope to understand the problem by studying the chemicals in the board. Drywall consists of wide, flat boards used to cover walls. It is often made from gypsum, a common mineral that can be mined or manufactured from the byproducts of coal-fired power plants.

Plaintiffs in the lawsuits, as well as U.S. wallboard manufacturers, say the tainted drywall was made with fly ash, a residue of coal combustion more commonly used in concrete mixtures.

Fly ash can be gathered before it ever reaches the smokestack, where technology is used to remove sulfur dioxide from the emissions. The process of "scrubbing" the smokestack emissions creates calcium sulfate, or gypsum, which can then used to make wallboard, experts say.

Haldin, the Knaupf Tianjin spokesman, says some domestic drywall is also made from the less-refined fly ash.

But Michael Gardner, executive director of the U.S. Gypsum Association, said American manufacturers gather the gypsum from the smokestacks after the scrubbing, which produces a cleaner product.

The Consumer Product Safety Commission has dispatched teams of toxicologists, electrical engineers and other experts to Florida to study the phenomenon. The commission is also working with the Environmental Protection Agency and the Centers for Disease Control and Prevention to determine whether there is a health hazard.

A Florida Department of Health analysis found the Chinese drywall emits "volatile sulfur compounds," and contains traces of strontium sulfide, which can produce the rotten-egg odor and reacts with air to corrode metals and wires.

But the agency says on its Web site that it "has not identified data suggesting an imminent or chronic health hazard at this time."

"We're continuing to test," said Susan Smith, a spokeswoman for the department, which has logged 230 complaints from homeowners.

Dr. Patricia Williams, a University of New Orleans toxicologist hired by a Louisiana law firm that represents plaintiffs in some of the cases, said she has identified highly toxic compounds in the drywall, including hydrogen sulfide, sulfuric acid, sulfur dioxide and carbon disulfide.

Prolonged exposure to the compounds, especially high levels of carbon disulfide, can cause breathing problems, chest pains and even death; and can affect the nervous system, according to the CDC.

"It is absolutely shocking what is happening," Williams said.

Dr. Phillip Goad, a toxicologist hired by Knaupf Plasterboard Tianjin, sampled drywall from 25 homes, some that contained the company's wallboard and some that did not.

"The studies we have performed to date have identified very low levels of naturally occurring compounds," Goad said. "The levels we have detected do not present a public health concern. The chemicals are naturally occurring. They're produced in ocean water, in salt marsh air, in estuaries."

But those who are living with it are convinced that something is making them sick, including dozens of homeowners in a single subdivision in Parkland, about 50 miles north of Miami. They are now faced with a daunting choice: Tear down and rebuild, or move out and be stuck with a mortgage and a home they cannot sell.

"We are particularly concerned about the safety and well-being of our children," said Holly Krulik, who lives down the street from Mary Ann Schultheis.

She and her husband, Doug, are suffering sinus problems and respiratory ailments, and their young daughter has repeated nose bleeds.

"If a shiny copper coil can turn absolutely black within a matter of months, it certainly can't be good for human beings," Krulik said.

Neighbor John Willis is moving out, even though he can hardly afford to walk away from a house he's owned for just three years. He cries as he speaks of his 3-year-old son's respiratory infection, which eventually required surgery.

"They basically took out a substance that looked like rubber cement out of my 3-year-old son's sinuses," he said. "My wife and I are now faced with the choice between our children's health and our financial health. My children are always going to win on that."

The subdivision's builder, WCI Communities, is in Chapter 11 bankruptcy restructuring and can do little more than log complaints, said spokeswoman Connie Boyd.

The federal government does not regulate the chemical ingredients of imported drywall.

Plasterboard Tianjin said it has been making drywall for 10 years in accordance with U.S. and international standards.

Another Chinese company facing lawsuits, Taishan Gypsum Ltd., also insists that it meets all U.S. standards.

Determining what is causing the problems could take months. Researchers will try to recreate in a lab the conditions that caused the sulfur compounds normally found in drywall to give off noxious gases.

Meanwhile, people like Lisa Sich, 43, are left with more questions than answers. Sich has not felt well since moving into the Henderson, Nev., apartment she rents less than a year ago, and her silverware quickly tarnished.

"I can hear myself wheezing," said Sich, who is having environmental experts test the apartment, built in 2007. "My eyes are constantly itchy, extreme fatigue."

And while Sich is not even certain she's got the bad wallboard, she has not felt like herself in months. She's missed five weeks of work just since Thanksgiving.

"I'm just tired all the time," she said. "It doesn't make sense."

Associated Press Writer Joe McDonald in Beijing contributed to this report. Burdeau reported from New Orleans.

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