What is a primary characteristic of passive humidifiers?

A primary characteristic of passive humidifiers is that they recycle exhaled heat and humidity. This process involves capturing the moisture and thermal energy from the patient's exhaled breath, which is then applied to the incoming dry gas (like oxygen) the patient inhales. This method is particularly advantageous in respiratory therapy as it helps maintain the appropriate humidity levels to prevent the drying of airways and ensures patient comfort.

Conversely, the other options highlight mechanisms that are not associated with passive humidifiers. For instance, a power source is generally necessary for active humidifiers that use electricity to add moisture to the airflow. Active humidifiers would also be responsible for directly adding moisture to the inhaled air as opposed to merely recycling what is already present. Similarly, producing aerosolized particles is a function attributed to nebulizers or certain types of active humidifiers, which disperse liquid into small droplets, rather than a characteristic of passive humidifiers. Thus, the correct choice emphasizes the unique function of passive humidifiers in managing humidity through a process of recycling.