A vast carpet of more than 100 million porcelain "seeds" in the Tate Modern contain traces of lead, an investigation into the exhibition has discovered.
The installation was declared out of bounds to art lovers only two days after it opened because it poses a health threat due to dust.
But further analysis of the seeds and dust has shown they contain traces of the poisonous metal.
Visitors to the London gallery were initially allowed to walk on the imitation sunflower seeds - which cover 1,000 square metres of its Turbine Hall - but were banned shortly after the piece opened in October.
A Tate spokeswoman insisted that the exhibition poses "no health risk". She said: "Tate did testing on the seeds to check their robustness before the work was installed in the Turbine Hall. Tate also undertook further testing on the dust generated by the enthusiastic interaction of the public in the first days of opening.
"We were advised following the second test, the dust could be damaging to health following prolonged exposure. The tests show that traces of lead are present in the material of the seeds and the dust that resulted from the interaction with the work by visitors.
"Specialist advice confirmed that due to the limited length of time with the work their exposure to the dust has not led to a health risk. The results showed that exposure to the dust during the period when the work could still be walked on was below the relevant Workplace Exposure Limits. The installation as currently seen in the Turbine Hall poses no health risk."
She added that staff involved with the project were also informed about the contents of the dust.
The installation can still be viewed from a bridge in the gallery.
The seeds, which were individually handcrafted by skilled artisans, are the idea of Chinese artist Ai Weiwei. The ceramic seeds were moulded, fired at soaring temperatures, hand-painted and then fired again over the course of two years.