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

Copies of Chinese customs reports obtained by The Times, along with interviews, indicate that drywall made with phosphogypsum was shipped to the U.S. in 2006 by at least four Chinese-based manufacturers and trading firms.

The health risk of phosphogypsum is uncertain, but industry specialists say they are troubled by its widespread use and the possibility it was exported, especially in light of recent incidents in which other 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," he added, "there are no complaints [in China] because most people don't know much about gypsum board and there are no standards against it."

The Times contacted about 20 Chinese-based companies involved in making or selling drywall. Most of them declined to comment about their overseas business. Others offered conflicting answers or said they didn't know whether their drywall contained the banned substance.

One exception was Beijing Building Materials Import & Export Co. It was among at least 10 trading firms that exported drywall to the U.S. in 2006, according to the customs reports, which were provided by an industry source and confirmed by the Chinese government's statistics compiler in Hong Kong.

The reports show that Beijing Building Materials shipped nearly 38 million pounds of drywall to the U.S. in 2006. Sun Yong, the company's vice president, said it didn't matter whether the wallboard was made with mined gypsum or phosphogypsum.

"From China's customs side, there is no special inspection of exported drywall," he said.

Health concerns, inconclusive tests

Phosphogypsum contains radium, prolonged exposure to which can lead to a higher risk of lung cancer, and that is why the EPA banned phosphogypsum for use in construction in 1989.

Dr. Paul Papanek, a board member of the Western Occupational and Environmental Medical Assn., 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.

For similar reasons, a top manager at Tai'an Single Mechanical & Electrical Technology Co., a supplier of gypsum-processing equipment in Shandong province, also suspects phosphogypsum as a root cause.

The manager, who spoke on the condition of anonymity, estimates that 80% of Chinese drywall makers use phosphogypsum because it is cheap and there are no government restrictions.

Gypsum drywall in China is used largely for businesses, and industry associations say there have been few complaints in China like those from American homeowners.

So far, tests in the U.S. 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 said the rotten-egg-like odor and corrosion are associated with volatile sulfuric acids, and 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.

Knauf, a German company that has a joint-venture operation in China and has been a primary target of lawsuits over bad drywall, has acknowledged using gypsum from that area.

Which watchdog is responsible?

Ding, the geoscientist who has worked as a consultant in China's gypsum industry for about 30 years, said some manufacturers began experimenting with phosphogypsum at the start of the decade. He said some drywall plants in China were now using 50% phosphogypsum as a base for plasterboard, others as much as 100%.

Some people in China liken the practice of mixing phosphogypsum in drywall to the recent scandals involving melamine, the industrial chemical that contaminated Chinese baby formula and animal feed.

Amid such heightened product-health concerns, officials at China's quality watchdog agency have been investigating complaints about Chinese-made drywall in the U.S., demanding that manufacturers submit samples for analysis, according to company executives.

But the agency has not issued any public statement on the probe, and officials did not respond to repeated requests for interviews.

In the U.S., federal authorities said there was no one authority responsible for ensuring that imported drywall meets American standards.

Consumer Product Safety Commission spokesman Joe Martyak said 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.

In interviews, officials with U.S. Customs, the Environmental Protection Agency and the Department of Commerce all said their agencies were not responsible for testing drywall. That may reflect the fact that imported drywall is a relatively new phenomenon -- the U.S. used to have more than enough from its own sources, made with gypsum from mines or synthetic gypsum derived from coal combustion.

Waste byproduct is available, cheap

But in China, businesses began turning to phosphogypsum, in part because local governments were eager to get rid of bulging stockpiles of the waste byproduct.

Huge phosphogypsum dump sites can be seen in all corners of China. Near the banks of the Yangtze River in central China's Wuhan area, raw phosphogypsum is spread over 20 acres and packed 65 feet deep into the ground. The smell of sulfur permeates the air. Workers at the site said the material was given away to anyone willing to pay the transportation costs, a mere $1.75 per ton.

No one knows how much phosphogypsum board from China was shipped abroad. But in 2006, Chinese exports of drywall to the U.S. totaled a record 503 million pounds valued at more than $25 million, according to Chinese customs' statistics. That's enough for 32,000 homes.

(With the subsequent American housing market collapse, Chinese drywall exports to the U.S. fell to just $507,000 in 2008 and are expected to drop even more this year.)

Among the exporters in 2006 was Taishan Gypsum Co., a large producer of phosphogypsum wallboard based in eastern Shandong province. Customs reports show that Taishan sent about 10 million pounds of drywall to the U.S. that year.

In interviews, Taishan executives said they were unclear about the raw ingredient in the exported drywall.

At another drywall maker, Yunnan Waste Use Building Materials Co., office director Zhang Wanwei acknowledged that his company focused on making wallboard with phosphogypsum. But he said his firm "was one of the few that bought the most advanced equipment to process phosphogypsum . . . because if not handled properly, the quality of these boards may not be so good. And they could contain materials that are bad for health."

Zhang declined to say whether the company exported gypsum boards to the U.S.

"I don't know exactly how much phosphogypsum we use, but in total we process several hundred tons of all kinds of gypsum every year," he said. "We can get raw materials for our products at very low prices because they are mainly industrial waste."

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