As part of Bologna's Frontier project, Italian artist Andreco has created a 59-foot mural which he hopes will clean up the city's air. Representing a huge "Tree of Knowledge," the artwork is made of photocatalytic paint, a special substance which, according to manufacturers, breaks down mono-nitrogen oxides, harmful chemicals produced by cars and other motor vehicles. One company claims that "every square meter painted is like taking eight cars off the road."

But do these claims stand up to scrutiny? According to a 2010 advisory note issued by the UK's Department for Environment, Food, and Rural Affairs (DEFRA), the jury is still out on whether photocatalytic paint actually has a significant effect on air pollution. While studies in China and Andreco's native Italy have reported up to 50 percent reductions in outdoor mono-nitrogen oxide concentrations, results from a number of trials in the UK have been inconclusive.

When used indoors, photocatalytic paint may actually create problems of its own. According to a 2008 paper published in the journal Atmospheric Environment, the reaction of certain "indoor air impurities" with the paint can produce dangerous side-products, including acetone, acetaldehyde, and formaldehyde — hardly what you want floating around your living room.

While Andreco's project in well-intentioned, we'll have to wait and see whether it really helps the city of Bologna. In the meantime, manufacturers might want to go easy on the hyperbolic claims.