Disclosed herein is a nebulizer comprising of a medicine cup reservoir containing an aqueous pirfenidone solution a medicine cup reservoir cap, an aerosol generator, an aerosol mixing chamber to which freshly generated aerosol resides until inhaled, a one-way inhalation valve, a mouthpiece and a one-way exhalation valve. The invention allows atmospheric pressure to be maintained inside the medicine cup reservoir during nebulization and optimizes the volume of the aerosol mixing chamber to minimize freshly generated aerosol inter-droplet collision, impaction of aerosol to the aerosol mixing chamber wall, droplet growth and/or condensation during exhalation, prior to inhalation, or during inhalation. The larger aerosol mixing chamber volume also allows the aerosol to accumulate during the exhalation phase. Despite venting producing a larger generated aerosol droplet population mean compared to the non-vented aerosol generator, The combined effect of the invention increases device output rate of respirable aerosol droplets, increases pirfenidone Cmax and AUC to improve treatment or prevention of various diseases, including disease associated with the lung, heart and kidney, including fibrosis, inflammatory conditions and transplant rejection.