Minute traces of radiation "not harmful to human health" from the crippled Japanese nuclear plant have been detected in vegetables planted on the Chinese mainland, the Ministry of Health said on Wednesday in an online statement.

A vendor sorts spinach at a vegetable market in Beijing, April 6, 2011. Sample inspections conducted on Tuesday found low levels of radioactive iodine in spinach planted in Beijing, Tianjin and Henan province. [Asianewsphoto]

It is the first report that home-grown food has been contaminated by radioactivity, largely Iodine-131, since the ministry ordered radiation tests on food and water at the end of March in 14 mainland regions including Beijing, Tianjin, and some costal provinces.

Sample inspections conducted on Tuesday found low levels of radioactive iodine in spinach planted in Beijing, Tianjin and Henan province -- about 1-3 becquerels per kilogram (Bq/kg), the statement said. 

"The contamination level detected is too low to be harmful to public health," it said.

According to health experts, radioactive iodine can accumulate in humans once ingested in high concentrations and increases the risk of thyroid cancer. But it decays naturally within weeks.

Leafy vegetables grown in the open like spinach, lettuce and leek are among the first foods to be tainted by radioactive deposits.

Raw milk is also susceptible to radioactive contamination as livestock feed on grass.

Tests carried out in March showed spinach and milk taken from farms near Japan's stricken nuclear plant had exceeded government-set safety limits for radiation.