Abstract: An air pollutant and/or fire combustion sensing apparatus includes a pair of sensing electrodes separated by a free space to define a capacitance unit. At least one of the electrodes includes a corrosion-resistant, conductive material which interacts with air borne products to alter the permittivity and/or charge transfer characteristic of the unit. The material is in the form of a highly corrosion-resistant metal selected from a metal or metal alloy including at least one of the metals selected from the Group VIII, periods 4, 5, and 6 of the periodic table, particularly rhodium and stainless steel. Carbon and copper respond to produce a detectible output. An amplifying and alarm circuit is connected to the capacitance unit, and responds to either an increase or a decrease in the capacitance as a result of the interaction with the sensing electrode material.