I have discovered the following piece about the "Sunflower Seeds" sculpture at Tate Modern, whilst surfing the internet:
When an installation by the Chinese conceptual artist Ai Weiwei opened on Tuesday at Turbine Hall, the cavernous entrance space of the Tate Modern in London, visitors were encouraged to touch and even walk on the 100 million hand-crafted porcelain sunflower seeds that gave the piece its name. But on Thursday this kind of intimate interaction stopped. Officials at the museum closed off access to “Sunflower Seeds,” and is now only allowing visitors to see it from a walkway above the hall.
The movement of the crowds released a “greater than expected level” of ceramic dust, according to a statement released by the Tate, which added that it had “been advised that this dust could be damaging to health following repeated inhalation over a long period of time.” As a result, the museum, in consultation with the artist, “decided not to allow visitors to walk across the sculpture.’’