Carrara marble samples have been exposed in Milan and Aosta to verify the correlation between decay and pollution. Gravimetric, colorimetric and chemical analyses have been carried out every 6 months. The deposition of particulate matter causes a decay both in Milan and Aosta, despite the difference in the amount of deposition. A decay parameter has been calculated from the results of chemical and gravimetrical analyses: it is a useful parameter to evaluate the effects of the environment on the stone.