What is the definition of outside air? Outside air is the air located outside of a given building or structure, or outside of any enclosed space. It is the air that generally exists in the atmosphere, which is the layer of gases that surrounds the Earth and provides it with the necessary conditions for life. Outside air can be either still or in motion, depending on the current weather conditions. It is typically composed of 78 percent nitrogen, 21 percent oxygen, and 1 percent other gases. Outside air can also contain varying amounts of pollutants and other harmful substances, depending on the location and environmental conditions. For example, in heavily polluted cities, the outside air can contain a significant amount of airborne particles, such as PM10, nitrogen dioxide, and sulfur dioxide. These substances can cause health problems if they are inhaled in large concentrations. In order to maintain healthy indoor air quality, it is important to use air filters, ventilation systems, and air conditioning systems to ensure that the air indoors is as clean