JINAN, May 7 (Xinhua) -- Reports of formaldehyde-tainted Chinese cabbage in east China's Shandong province, the country's largest vegetable production base, have exposed some clandestine market practices and triggered a new wave of food safety concerns.

Reports of the toxic vegetables appeared on the web over the weekend, alleging that vegetable dealers in Qingzhou were seen spraying a formaldehyde solution on Chinese cabbage to keep the produce fresh during long rides to faraway markets.

The reports sparked public anger and prolonged the seemingly unending discussion on what, if anything, is safe to eat.

In their many visits to croplands and wholesale centers, Xinhua reporters found that using a formaldehyde solution has been a popular, unspoken method for keeping vegetables fresh for at least three years and was not limited to Qingzhou.

Qingzhou is a leading Chinese cabbage production base, and vegetables grown there are mainly sold to areas in northern China and neighboring provinces on the country's east coast.

Many local farmers have turned a blind eye to the dealers' practice of treating the cabbage with a formaldehyde solution.

"It's a common practice to keep the cabbages fresh," says Yin Lihua, a cabbage farmer in Qingzhou's Dongxia township. "Otherwise, the vegetables stacked tightly in their trucks would rot in two to three days."

Yin expects huge financial gains from his 1,300 square meters of land that promises to yield at least 15,000 kilograms of cabbage.

"Cabbages sell well this year, about 1.4 yuan per kilo," he says. (1.4 yuan is equal to 0.22 U.S. dollar.)

Last year, the sale price was only 0.1 yuan per kg.

Trucks lined up near Yin's greenhouse early Monday, waiting to carry the cabbage to faraway cities for sale. Most of the trucks had license plates from Beijing, Henan Carrera Sunglasses, Hebei, Shanxi and Inner Mongolia.

Meanwhile, police in Qingzhou have opened an investigation into the allegations as the reports on toxic vegetables continue to spread.

Zhao Mingli, a vegetable dealer from northeastern Heilongjiang province, was caught by police while spraying the chemical solution.

Zhao says he uses the chemical to keep the cabbage in good condition during a 10-hour journey to Langfang, a small city on the Hebei-Beijing border. "Vegetable dealers in Langfang openly demand formaldehyde-preserved cabbages because they sell more easily."

He also says the practice is not a new one. "I just did what everyone else was doing for three or four years. Vegetable dealers in other parts of Shandong and Hebei do the same."

He said he bought the chemical at a store in Langfang. "A big bottle, about 2.5 liters of solution, sells for only 7 yuan and can keep 20 tonnes of vegetables fresh."

The formaldehyde solution, however, is used only on cabbage harvested in spring and summer. In winter, a traditional season for harvesting cabbage, temperatures are low enough to keep the vegetables fresh without the solution.

Zhao is among dozens of vegetable dealers apprehended by police in Dongxia township.

The dealers admitted to having sold formaldehyde-tainted vegetables to many provinces, and many said they don't mind eating the tainted cabbage themselves. "You just do away with the first layer of leaves, cut the root and rinse well," Zhao said.

The local government in Dongxia township has launched a vegetable safety overhaul involving 46 agricultural production bases, 80 dealers and 400 cabbage farmers.

It has also put up posters at roadside bulletin boards to warn people about the risks of formaldehyde. "Excess formaldehyde endangers human health," reads one. "Trucks carrying vegetables are subject to checks," reads another.

"We have told dealers to use refrigerated trucks for storage," said Liu Shengtian, deputy chief of Qingzhou's agricultural bureau.

Previous reports said the formaldehyde solution was also used on seafood and mushrooms.

Formaldehyde, used as a disinfectant and embalming fluid, was declared a known human carcinogen by the U.S. National Toxicology Program last year. It is also a skin, eye and respiratory irritant.

It is still unclear how the toxin-using dealers should be penalized, as no such conditions exist in relevant laws and regulations, said Liu.

China's law on farm produce safety stipulates that the use of preservatives should "conform to relevant technical standards of the state Versace Sunglasses," but fails to define what preservatives, or how much, are acceptable.

"It's crucial to fix these loopholes in order to better regulate market behaviors for public health considerations," said Zhao Jinshan, a Jinan-based specialist on food safety and disease control.