BRUSSELS — The man in the respirator mask sits in the window, impassive on his chair, breathing as evenly as he can into the aluminum foil balloon he cradles on his knees . After six hours he has filled more than 20 of them , piling them up behind him like a cloud.

“In the end they’ll be about 100,” said Will Lunn, the director of the London-based Copperfield gallery, explaining “Exhaust,” a marathon performance piece by the British conceptual artist David Rickard, whom he represents. Over 24 hours, Mr. Rickard turns the air a human being requires for one day into an enormous shimmering sculpture.

First seen at the Goethe-Institut in London in 2008, this demanding work (the artist isn’t allowed to eat or drink, and the carbon dioxide levels in his blood must be regularly checked) was the storefront display at the second annual edition of A Performance Affair, a fair exclusively devoted to the sale of performance art.

The four-day fair, which finished Sunday, occupied two floors of the Vanderborght Building, an Art Deco former department store, and brought together more than 30 artists hoping to attract the attention of Belgium’s famously discerning and risk-taking contemporary art collectors.