The invention consists in a cover for a chimney, the cover comprising: a housing adapted to surround at least an upper end of a chimney, the housing having an expansion area for receiving emissions from the upper end of the chimney; a diffuser for diffusing the emissions from the upper end of the chimney into the expansion area; at least one air inlet for introducing air into the expansion chamber; at least one outlet for enabling air or emissions to be exhausted from the expansion chamber; wherein the housing is adapted such that emissions exiting the upper end of the chimney are diffused and expanded into the expansion chamber and a reduced pressure is created within the housing for drawing emissions from the upper end of the chimney into the expansion chamber.