As a consequence of a monitoring program of a new municipal waste incinerator initiated in 1998, a large data-base of dioxin and furan concentrations in the atmosphere of the metropolitan area of Porto, in northern Portugal, has been collected. The existence of this data coincides with the shutdown in January 2001 of two medical waste incinerators that were under operation in the inner city of Porto. Dioxin emissions from these facilities were measured indicating emissions 100 to 1000 times larger than recent European Union directive limits. Data show that the shutdown of these two units had a clear effect on the improvement of air quality in the region that was observed either on the overall level of dioxins and furans or as in subtle alterations of the homolog pattern of these compounds in the atmosphere.