Visitors to Tate Modern have been banned from walking on more than 100 million porcelain "sunflower seeds" that an artist is exhibiting.
The seeds were created by Chinese artist Ai Weiwei to fill the central London gallery's turbine hall.
When it opened on 11 October he urged guests to stomp on the seeds and roll in them.
But concern has now been raised over "dust" being created and people must now look at the work from above.
It was not immediately clear whether this is because the seeds are cracking or otherwise deteriorating.
A Tate Modern spokeswoman said: "Sunflower Seeds is made up of over 100 million individually handmade porcelain replicas of seeds.
"Although porcelain is very robust, the enthusiastic interaction of visitors has resulted in a greater than expected level of dust in the hall.
"Tate has been advised that this dust could be damaging to health following repeated inhalation over a long period of time."
The spokeswoman continued: "In consequence, Tate, in consultation with the artist, has decided not to allow visitors to walk across the sculpture."