Pollutants in the exhaust from wood burning in a fireplace are removed by positioning reticulated foam between the smoke chamber and the flue. The reticulated foam is a three dimensional latticework of interconnected ligaments forming a porous, open-celled structure with a large internal surface area. The internal surfaces are coated with a catalytic material which converts pollutants in the exhaust to harmless compounds. Heat from the exhaust, when the fire is burning, maintains the catalyst at a temperature for efficient conversion operation. Thermostatically controlled heaters are embedded in the foam, or thermostatically controlled electrical current is applied directly to the catalyst, to raise the catalyst temperature when the fire is growing or smoldering, periods during which the exhaust temperature is insufficient for good conversion operation.