HEPA filter efficiency is measured in units of __________.

HEPA filters are designed to capture airborne particles, and their efficiency is measured in microns. Specifically, a HEPA filter is defined as one that can trap 99.97% of particles that are 0.3 microns in diameter. This measurement is crucial because it indicates the filter's ability to capture a wide range of particulate matter, including dust, pollen, pet dander, and other pollutants.

The choice of microns as the unit of measurement is based on the size of the particles that HEPA filters are most effective at capturing. Particles larger than 0.3 microns are easily trapped, while those smaller can be more challenging due to their behavior in airflow. Thus, the focus on microns provides a clear standard of measurement for the filter's filtration capabilities and performance.

In contrast, the other units like millimeters, nanometers, and centimeters do not correspond to the standard measurement used in evaluating HEPA filter efficiency. Therefore, the correct answer highlighting microns reflects the established convention in assessing the effectiveness of such filters.