An aerosol delivery system has a nebulizer and a humidifier providing a gas flow to the nebulizer. A controller varies humidity level of the gas flow to the nebulizer so that if the nebulizer is not operating it has about 100% humidity and it is operating the value is less to allow for the humidification effect of the nebulizer. The control may be achieved by dynamically varying proportions of flow through a dry branch and a humidification branch.