BEIJING (AP) -- More than one in 10 Chinese children sickened by contaminated milk showed signs of kidney damage six months afterward, researchers have found, raising concerns about the long-term effects of the country's massive food safety scandal.

At least six children died and nearly 300,000 children fell ill two years ago after consuming infant formula deliberately contaminated with the industrial chemical melamine in order to fool inspectors testing for protein.

The study's results point to the possibility that as many as 30,000 children could have suffered health complications for months after drinking the contaminated milk. It also shows that the long-term health effects remain little understood, even as China makes plodding progress in cleaning up a food supply strewn with tainted products. More milk products contaminated with melamine resurfaced in markets in recent months.

Researchers from Peking University studying ultrasound images of infants who fell ill in the 2008 contamination found that while most children in a rural Chinese area recovered, 12 percent still showed kidney abnormalities six months later, according to the report published Monday in the Canadian Medical Association Journal.

"The potential for long-term complications after exposure to melamine remains a serious concern," the report said. "Our results suggest a need for further follow-up of affected children to evaluate the possible long-term impact on health, including renal function."

The researchers conducted ultrasound screenings of 7,933 children under 3 years of age living in rural areas near the headquarters of Sanlu Group Co., the dairy at the center of the scandal, in the northern city of Shijiazhuang in September 2008.

The initial screening found that 48 children suffered from kidney stones or swollen kidneys, the report said. The researchers monitored most of these children at intervals of one, three and six months and found that "renal abnormalities" remained in 12 percent of the children.

The results could have implications for the broader population of children who had exposure to melamine, one of the study's researchers said.

"Among the 300,000 affected children, although they don't have symptoms, maybe 12 percent will have abnormal ultrasound images after six months," said Dr. Liu Jianmeng, deputy director of Peking University's Institute of Reproductive and Child Health, one of the researchers.

Liu added, however, that some limitations in the study could have led to a slight overestimation on the prevalence of such abnormalities.

Dr. Peter Ben Embarek, a World Health Organization food safety expert based in Beijing who was not involved in the study, said "there is a need for these types of follow-up studies to better understand what the long-term effect is of high exposure to melamine."

"The good news from this study is that the vast majority of those infants who were affected at that time recovered by themselves, within a few months," Ben Embarek said.

The scandal came to light in September 2008 when reports of babies suffering from kidney stones appeared in the Chinese media, prompting a Chinese dairy to recall hundreds of tons of baby formula and the government to launch an investigation.

With so many children affected, many parents blamed the government for certifying the contaminated milk powder as safe. To try to defuse public anger, the government offered payouts and free health screenings and medical treatment.

Melamine, which can cause kidney stones and kidney failure, was added to watered-down milk to fool inspectors testing for protein content and increase profits. The chemical, which is used to make plastics and fertilizers, has also been found added to pet food and animal feed.

Despite tightened regulations and increased inspections on producers, melamine-tainted milk products have recently shown up repackaged in several places around the country, exposing weaknesses in China's promises to better police the food chain.

The government launched a 10-day emergency crackdown and said earlier this month that most of the contaminated products that resurfaced have been recalled and destroyed.

In recent days, Beijing also issued a draft regulation aimed at strengthening the supervision of animal feed. The regulation stipulates fines for companies found producing, using or selling illegal additives for feed.

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