Compressed air is air that has been compressed and stored at a pressure higher than the surrounding atmosphere, effectively increasing the density of the air molecules. This common utility is used across diverse industries, from powering pneumatic tools to operating complex machinery and medical devices. The composition of compressed air originates directly from the ambient air drawn into the compressor, but the mechanical process fundamentally alters the concentration and nature of certain components.
The Components of Atmospheric Air
The air surrounding us serves as the baseline for compressed air, consisting primarily of two gases. Approximately 78% of dry atmospheric air is diatomic Nitrogen (N2), which is an inert gas that passes through the compression process unchanged in its chemical state. The next largest component is Oxygen (O2), making up about 21% of the total volume. These two gases account for nearly 99% of the air inhaled by a compressor.
The remaining 1% is composed of several other gases and vapors. Argon, an inert noble gas, constitutes about 0.93% of the atmosphere. Carbon Dioxide (CO2), a trace gas, is present at about 0.04%, with other noble gases like Neon and Helium making up the rest.
Impurities Introduced During Compression
The act of compression introduces and concentrates several non-gaseous elements, transforming clean ambient air into a mixture that can be harmful to equipment and end products. The most prevalent contaminant is water, which enters the system as vapor drawn in from the atmosphere. When air is compressed, its temperature increases, but when it cools in the piping, the water vapor becomes supersaturated and condenses into liquid water droplets. This liquid moisture is a major source of rust and corrosion within the air distribution network.
The concentration of water vapor is often quantified by the pressure dew point, which is the temperature at which water condenses into liquid at a given pressure. A compressor drawing in air at 80°F and 70% relative humidity can produce several gallons of liquid water per hour in a typical industrial setting. Beyond liquid water, the compression process itself is a major source of oil contamination, especially in lubricated compressors. These machines use oil to lubricate, seal, and cool the internal components, and some of this lubricant aerosolizes, becoming entrained in the air stream as oil vapor and fine droplets.
Even in oil-free compressors, the air can still become contaminated with oil vapor drawn from the atmosphere of a manufacturing environment. The system also concentrates solid particulates that bypass the intake filter, such as dust, pollen, and airborne dirt. These particles combine with internal contaminants like rust and pipe scale that flake off the air distribution pipes. Furthermore, the presence of liquid water and the warmer temperatures within the system create a suitable environment for the proliferation of microorganisms, including bacteria and mold, which become biological contaminants carried by the compressed air.
Purity Grades and Specialized Uses
The presence of these impurities—water, oil, and particulates—necessitates purification before the air can be used for most applications. Different industrial and medical processes require varying degrees of air purity to prevent product spoilage, equipment damage, or health risks. The International Organization for Standardization (ISO) established the ISO 8573-1 standard to uniformly classify compressed air quality based on the concentration of the three main contaminants.
This standard uses a three-digit code to define the maximum allowed concentration of solid particles, water, and total oil, respectively. For instance, air used for general utility purposes, such as powering basic pneumatic tools, may only require a mid-range purity classification.
Conversely, specialized uses demand extremely high purity levels, often requiring filtration and drying equipment to achieve near-zero contamination. The food and beverage industry, which uses compressed air for packaging or direct product contact, typically requires air free from all liquid oil and moisture. Similarly, medical applications, such as supplying air for hospital ventilators, or pharmaceutical manufacturing processes require the highest standards of cleanliness to protect patient health and product integrity.