A ductwork loop was constructed to simulate a home, forced air, heating system in order to test the transmission of naturally-occurring, airborne micro-organisms and spiked enterobacter cloaceae cells. Three common types of furnace mount humidifiers were operated for 90 day periods under controlled furnace plenum temperature and airflow. Air samples were collected but no evidence was found of organism transmission originating from the humidifiers under the conditions of the experiment.

KEYWORDS: humidifiers, particles, microbiology, measuring, air pollution, experiment, warm air heating, domestic, housing.