The Composition of Air

Air is defined as the gas mixture of the earth’s atmosphere. The exact composition of gaseous, solid and liquid components (particles) dispersed in the air varies greatly depending on time and location. Dry, particle-free air is mainly composed of two gases: nitrogen (around 78%) and oxygen (around 21%). There are also traces of argon, carbon dioxide and other gases, as well as varying amounts of water vapour, which is typically in the range of several grams per cubic meter of air

Figure 1: General guide to particle size distribution of common atmospheric
contaminants

Figure 1 compares different types of air contamination with the respective particle size ranges. The scale shows that the various contaminants cover a size range of several orders of magnitude. This is a major reason why so many different filters are used in a wide variety of classes and designs. The common technical classification of fine dust fractions is also based on the classification of particle sizes according to the areas they are deposited in the human respiratory system. The classes are PM10 (particles with an aerodynamic diameter of ‹ 10 μm), PM2.5 (‹ 2.5 μm) and PM1 (‹ 1 μm).

PM stands for particulate matter. There has been a recent addition to the established particle size classes: particle fractions with a diameter of ‹ 100 nm. Particles of this size are referred to as ultrafine particles (UFP) or nanoparticles. The mechanisms for the separation of particles and gaseous contaminants from the air are fundamentally different. Particles are separated by mechanical or electromechanical effects whereas gases are generally separated by adsorption or absorption.

Source: Eurovent Guidebook. Air Filters for General Ventilation
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