A breath actuated, handheld nebulizer device for delivering a fluid as an ejected stream of droplets to the pulmonary system of a subject, the device comprising: a housing; a mouthpiece positioned at an airflow exit of the device , the mouthpiece including an exhalation valve ; an air inlet flow element positioned in the airflow at an airflow entrance of the device ; a fluid reservoir disposed in or in fluid communication with the housing for receiving a volume of fluid; an ejector mechanism in fluid communication with the reservoir and configured to generate the ejected stream of droplets; at least one differential pressure sensor positioned within the housing, the at least one differential pressure sensor configured to activate the ejector mechanism upon sensing a pre-determined pressure change within the mouthpiece to thereby generate the ejected stream of droplets; the ejector mechanism comprising a piezoelectric actuator and an aperture plate, the aperture plate having a plurality of openings formed through its thickness and the piezoelectric actuator operable to oscillate the aperture plate at a frequency to thereby generate the ejected stream of droplets; wherein the housing, air inlet flow element , and mouthpiece are configured to facilitate non-turbulent airflow across an exit side of the aperture plate and to provide sufficient airflow through the housing during use; and wherein the ejector mechanism is configured to generate the ejected stream of droplets wherein at least about 50% of the droplets have an average ejected droplet diameter of less than about 6 microns, such that at least about 50% of the mass of the ejected stream of droplets is delivered in a respirable range to the pulmonary system of the subject during use. a housing; a mouthpiece positioned at an airflow exit of the device , the mouthpiece including an exhalation valve ; an air inlet flow element positioned in the airflow at an airflow entrance of the device ; a fluid reservoir disposed in or in fluid communication with the housing for receiving a volume of fluid; an ejector mechanism in fluid communication with the reservoir and configured to generate the ejected stream of droplets; at least one differential pressure sensor positioned within the housing, the at least one differential pressure sensor configured to activate the ejector mechanism upon sensing a pre-determined pressure change within the mouthpiece to thereby generate the ejected stream of droplets; the ejector mechanism comprising a piezoelectric actuator and an aperture plate, the aperture plate having a plurality of openings formed through its thickness and the piezoelectric actuator operable to oscillate the aperture plate at a frequency to thereby generate the ejected stream of droplets; wherein the housing, air inlet flow element , and mouthpiece are configured to facilitate non-turbulent airflow across an exit side of the aperture plate and to provide sufficient airflow through the housing during use; and wherein the ejector mechanism is configured to generate the ejected stream of droplets wherein at least about 50% of the droplets have an average ejected droplet diameter of less than about 6 microns, such that at least about 50% of the mass of the ejected stream of droplets is delivered in a respirable range to the pulmonary system of the subject during use. <EOS>