Weller MG130 - Fume Extraction Unit

Filters are classified based on the size of the particle that they can keep from returning back into the environment. The smaller the particles are, the deeper they penetrate into the respiratory system. The smallest particles travel via alveoli into the bloodstream. The top graph shows filter classes and the size of the particles they can efficiently filter. Particles are measured in microns, which is one millionth of a meter.

Each filter class is assigned an efficiency rating based on the ability to filter MPPS (Most Penetrating Particle Size). HEPA filters have efficiencies of at least 99.95% at 0.3 microns. The most penetrating size is a function of the structure of the filter medium, the velocity of the airflow through the filter, and the physical and chemical nature of the particles.