A sensing system and method for detecting particles in an air volume includes a main airflow path including an inlet from the air volume. Also included is a collection site within the main airflow path. The collection site is for drawing an air sample from the main airflow path. Additionally the system includes a means to induce a localized increase in particle speed at the collection site relative to air speed along the remainder of the main airflow path. The means to induce a localized increase includes an auxiliary airflow path from the main airflow path. The auxiliary airflow path has an exit upstream of the collection site. In an alternative form of the invention, the means to induce the localized increase in particle speed at the collection site comprises a venturi in the main airflow path with the collection site being disposed along the venturi.