Sierra losing ability to surprise

You never know what to expect from Santiago Sierra, the Spanish-born conceptual artist whose actions include locking critics out of a gallery and paying prostitutes for their next heroin fix in exchange for tattooing their backs. The first of those was guaranteed to get journalists' backs up, the second led to accusations of exploitation, but both were underpinned by a political message.

For his latest exhibition at the Lisson's two sites, Sierra brings together a series of projects originally realised in India, Mexico and Venezuela. In one, 21 black rectangular slabs of rock emerge from 21 wooden cases - except they are not slabs of rock but plasticised slabs of human faeces. Collected from sites in New Delhi and Jaipur, these are intended to highlight the estimated 1million manual workers in India whose work it is to unblock latrines.

Across the road, you're greeted by the atonal humming of long plastic tubes diverting the exhaust fumes produced outside by running car engines. These are intended to highlight the issue of pollution by, yes, producing more pollution.

Sierra's work has had, in the past, the power to discomfort and provoke. But remove the human element and not only does his work not pack a punch, it's also losing its ability to surprise: increasingly, viewers are confronted by a case of diminishing returns.

Until Jan 19, Lisson Gallery, 29 & 52-54 Bell Street NW1, Mon to Fri 10am to 6pm, Sat 11am to 5pm, free. Tel: 020 7724 2739. www.lissongallery.com Tube: Marylebone/ Edgware Road

Santiago Sierra: Death Counter
Lisson Gallery
Bell Street, NW1 5DA