One of the most striking exhibitions at the Tate Modern for years is at the centre of a new health scare.
The BBC has learned that the installation of 100 million porcelain "sunflower seeds" was made in China using paint containing lead.
People were initially able to walk on and pick up the "seeds" but this was soon banned after concerns over the amount of ceramic dust created.
The Tate says the installation poses no health risk in its present form.
The work created by Chinese artist Ai Weiwei in the vast Turbine Hall for the Tate's annual Unilever Series was cordoned off from the public just days after opening in October because of the concerns about dust.
BBC arts editor Will Gompertz says the latest revelation will raise more questions about possible health risks, and whether the Tate carried out enough safety checks.
The Tate said in a statement: "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.
"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.