Airborne particle monitoring measures the number and size of particles at a single point. This is done either with a particle counter, or by collecting the airborne particles on a filter or a deposition surface so that the collected particles can be characterized by microscopic observation. The particle counter is usually an optical particle counter or a condensation nucleus counter. Particulate sampling involves determining the number and size of particles in a specific volume of air. The sample volume of air is pumped through a capillary sensing chamber where a beam of light illuminates an individual particle. Measurements of light scattered off the particle provides information on the optical size of the particle. The numbers and sizes of particles are collected and stored in the instrument memory. Results are displayed in terms of particles per unit volume of air in selected size ranges.