A system comprising: a self-testing fire sensing device (100, 200, 300, 400), and wherein the self-testing fire sensing device (100, 200, 300, 400) comprises: an adjustable particle generator (102, 302) configured to generate particles; a variable airflow generator (116, 216, 316) configured to generate an aerosol density level by mixing the particles generated by the adjustable particle generator (102, 302) into a controlled aerosol density level within the self-testing fire sensing device (100, 200, 300, 400); an optical scatter chamber (104, 204, 304) configured to measure a rate at which the aerosol density level decreases after the aerosol density level has been generated; and a controller configured to: compare the measured rate at which the aerosol density level decreases with a baseline rate at which the aerosol density level decreases, and determine whether the self-testing fire sensing device (100, 200, 300, 400) requires maintenance based on the comparison of the measured rate at which the aerosol density level decreases and the baseline rate, characterized by the system further comprising a sensor configured to measure ambient airflow outside of the self-testing fire sensing device (100, 200, 300, 400), wherein the baseline rate to compare the measured rate to is determined using said measured ambient airflow. a self-testing fire sensing device (100, 200, 300, 400), and wherein the self-testing fire sensing device (100, 200, 300, 400) comprises: an adjustable particle generator (102, 302) configured to generate particles; a variable airflow generator (116, 216, 316) configured to generate an aerosol density level by mixing the particles generated by the adjustable particle generator (102, 302) into a controlled aerosol density level within the self-testing fire sensing device (100, 200, 300, 400); an optical scatter chamber (104, 204, 304) configured to measure a rate at which the aerosol density level decreases after the aerosol density level has been generated; and a controller configured to: compare the measured rate at which the aerosol density level decreases with a baseline rate at which the aerosol density level decreases, and determine whether the self-testing fire sensing device (100, 200, 300, 400) requires maintenance based on the comparison of the measured rate at which the aerosol density level decreases and the baseline rate, an adjustable particle generator (102, 302) configured to generate particles; a variable airflow generator (116, 216, 316) configured to generate an aerosol density level by mixing the particles generated by the adjustable particle generator (102, 302) into a controlled aerosol density level within the self-testing fire sensing device (100, 200, 300, 400); an optical scatter chamber (104, 204, 304) configured to measure a rate at which the aerosol density level decreases after the aerosol density level has been generated; and a controller configured to: compare the measured rate at which the aerosol density level decreases with a baseline rate at which the aerosol density level decreases, and determine whether the self-testing fire sensing device (100, 200, 300, 400) requires maintenance based on the comparison of the measured rate at which the aerosol density level decreases and the baseline rate, compare the measured rate at which the aerosol density level decreases with a baseline rate at which the aerosol density level decreases, and determine whether the self-testing fire sensing device (100, 200, 300, 400) requires maintenance based on the comparison of the measured rate at which the aerosol density level decreases and the baseline rate,