Industrial Utility Efficiency

ISO 8573.1 – Contaminants and Purity Classes

Industry standards serve a very important purpose for the end users of compressed air equipment. If the standards are well written, they can help to promote the equipment that they govern, as long as the equipment manufacturers properly apply and promote the standards. One of the most widely used standards in use today in the compressed air industry is ISO8573. It is a multipart standard that seeks to establish a method of classifying the purity of compressed air in part 1, then gives us the tools for measuring and quantifying that purity in parts 2 through 9.

ISO8573 is arranged as follows:

  • Part 1: Contaminants and Purity Classes
  • Part 2: Test methods for oil aerosol content
  • Part 3: Test methods for measurement of humidity
  • Part 4: Test methods for solid particle content
  • Part 5: Determination of oil vapor and organic solvents content
  • Part 6: Determination of content of gaseous contaminants
  • Part 7: Test methods for viable microbiological contaminant content
  • Part 8: Test methods for solid particle content by mass concentration
  • Part 9: Test methods for determining liquid water content

In this article I will focus on Part 1 of ISO 8573, and describe why the standard was developed, how it should be used, and what the future holds for this standard in the compressed air industry. But before we look at Part 1 of ISO8573, we need to take a look at what causes compressed air to be “impure”.

How successfully compressed air stream cleanliness requirements are met, can have a dramatic impact on overall plant operating costs. Excessive contamination shortens the life of components and systems, adversely affects product quality, can result in excessive maintenance costs, and can even create health and safety problems.

Contaminants in the form of solid particulates, oil aerosols and vapor, water aerosols and vapor, and even unwanted gaseous vapors can be introduced from the plant environment, ingested by the compressors, or created by the air compressor and distribution system.

While many compressed air applications require a high degree of purity, all compressed air applications work better if the air is clean and dry. However, when the air leaves a compressor, it is anything but clean and dry.

ISO Chart
Chart taken from ISO8573.1 : 2001

Sources of Contamination

Contaminants in compressed air systems have three possible points of origin. They can come from the air drawn into the compressor, from internal compressor mechanisms, and from the compressed air distribution system. Compressors draw in virtually all particles, vapors, and gases in the air within a six-foot radius of the inlet. Smaller particles, less than 10 microns in size, can be drawn in from a larger radius. The compressor inlet filter is designed to stop larger particles that could cause rapid wear of compressor parts. This design prevents excessively frequent replacements of the air intake filter element, but it does little to protect sensitive applications downstream of the compressor. Most of the airborne particles smaller than 10 microns can enter the compressor. Also, any gases and vapors around the intake will enter the compressor, and become part of the compressed air supply. These include combustion by-products such as carbon dioxide, carbon monoxide, nitrous oxides, or sulfur dioxides.

Another factor affecting air contamination is that during compression to 100 PSI, the air volume is reduced by a factor of seven, meaning seven cubic feet of ambient air becomes one cubic foot of compressed air. The result is an increase in the concentration of airborne particles in the compressed air stream. After compression, some of the most common airborne contaminants include dirt & pollen particles, iron oxide (rust) particles, microorganisms, unburned hydrocarbons, liquid water, water aerosols and water vapor, and oil aerosols and vapor.

Now that we know what the contaminants are made up of, we can take a look at how the ISO standard is used to classify the type and amount of contamination in compressed air.

The Purity Classes

The current version of ISO8573 Part 1 was published in 2001, although it is currently in the process of being revised. Every 5 years ISO standards are reviewed to determine whether they are still timely, accurate, and useful to the industries that they serve. If the Working Group, which is made up of volunteer industry experts, decides that the standard requires no revision, then nothing is done to change the standard, and it retains its current publication date. If the standard is revised, then a new publication date is assigned to it once the revision has completed the required balloting procedure. When referring to an ISO standard, it’s common practice to include the publication date, so you may see Part 1 of this standard referred to as ISO8573.1 : 2001.

There are three categories of contaminants that have been assigned classes in ISO8573.1 : 2001. The first category is solid particulates. The second category is made up of a combination of liquid water and water vapor. The third category is called oil, and it too consists of the sum of the liquid oil (in aerosol or liquid droplet form) and oil vapor. The chart below summarizes the three categories of contaminants, and shows the limits of contamination that are required to differentiate one purity class from another.

The purity classes range from the cleanest, class 0, to the most impure, class 9. Note that not all of the categories have the full range of classes; only the water category does. Also, notice that class 0 does not have any numbers associated with it in any of the categories. In the text of ISO8473.1 : 2001 class 0 is defined by stating “As specified by the equipment user or supplier and more stringent than class 1”. It is very important to understand that class 0 does not imply that there are no contaminants present; it simply means that there are fewer contaminants than in class one.

Solid Particulates

There are eight possible classes for solid particulates, from class 0 to class 7. Class 0 is the most pure, but it is numerically undefined, other than to say that it must be more pure (fewer particles in each size range) than class 1. Classes 0 through 5 are defined by the number of particles in a particular size range, in one cubic meter of compressed air. Measurement methods are described in Part 4 of ISO8573 for classes 0 through 5.

Classes 6 and 7 are used to describe compressed air that is typically too “dirty” to be measured with a particle counter. Instead, mass measurements are used to determine the amount of particulate contamination in the compressed air, according to Part 8 of ISO8573.


There are ten possible classes for water contamination, from class 0 to class 9. Class 0 is the driest, but it is numerically undefined, other than to say that it must be drier (a lower pressure dew point) than class 1. Classes 0 through 6 are defined by the pressure dew point of the compressed air. Pressure dew point is defined as the temperature at which moisture begins to condense in the pipes and storage tanks of a compressed air system while it is operating, and hence, under pressure. Pressure dew point is a useful method of describing the humidity in compressed air because it tells us that we must keep the ambient temperature that surrounds the compressed air distribution system above the pressure dew point in order to prevent liquid water from condensing inside the piping. Pressure dew point measurements are described in Part 3 of ISO8573.

Classes 7 through 9 are used to describe compressed air that contains liquid water. As mentioned, liquid water appears in the distribution piping and storage when the pressure dew point of the compressed air is higher than the temperature of the ambient air, and it means that the compressed air contains as much water vapor as is possible for it to contain. This condition is usually called “saturated” air. When liquid water is present in the compressed air line, we use the methods described in ISO8573 Part 9 to measure the amount.


There are only five classes for oil in the standard, but they describe a wide range of concentrations. Again, class 0 is the most pure, and according to the standard, it describes compressed air that must be more pure than class 1. Classes 1 through 4 cover the range from less than 0.01 mg of oil content per cubic meter of compressed air to less than 5 mg per cubic meter.

It is very important to understand that the oil classes can only be determined by adding the contribution from a.) any liquid oil in the compressed air, b.) the oil aerosols in the compressed air (typically generated by the reciprocal or rotary motion in lubricated compressors), and c.) oil vapors that can come from the oil in the compressor crankcase or sump, or from ingestion at the inlet of the compressor. Liquid oil and oil aerosols are measured using the techniques in ISO8573 Part 2, and the oil vapors are measured using the methods in Part 5.

Reporting the Purity Classes

According to the standard, the purity classes of compressed air shall be expressed by stating the standard reference number and part, the date of issue, and the three class designations in a specific order: Particulate Water Oil. For example, if the compressed air purity of an audited air system was expressed as ISO8573.1 : 2001 1 2 1, the Particulate Class would be 1, the Water Class would be 2, and the Oil Class would be 1. If the class for a particular category is omitted, then a hyphen is used in its place.

Many manufacturers of equipment powered by compressed air are now using this standard to express the purity level of the compressed air supply required in order to keep their tool or process running smoothly and in control. Air tool manufacturers and paint and powder coating suppliers are just two examples of entities that are using ISO8573 to improve their customer’s satisfaction with their products.

For more information please contact Dan Ryan, Division Engineering Manager, Parker Hannifin Corporation, Industrial Gas Filtration & Generation Division. Tel: (716) 686-6463, email:,