A mixer-heater device provides controllable reduction in aerosol droplet size. Additionally, an intermittent delivery mode for administering an aerosol to a patient may take into account patient expiration and reduce aerosol losses without prolonging treatment time. Depositional losses in aerosol delivery systems may be reduced by streamlining the three dimensional geometry of conduits which change stream direction or flow path diameter. Ventilation systems may also benefit from streamlined components, in particular Y-connectors, with resulting advantages such as reduced rebreathed CO2.