What are the combustible dust classifications?

Combustible dust is defined as fine, dry material that can become a potential fire or explosion hazard when suspended in the air at sufficiently high concentrations. The hazard of the material can be identified by looking at the combustible dust classification.

Knowing what type of dust you are working with is important because it determines the safety procedures necessary to protect employees.

Combustible dust explosions

For a fire to occur, we know that there needs to be a “fire triangle”:

1. Fuel
2. Oxygen
3. Source of ignition

For a dust explosion to occur, there has to be a “dust pentagon”, as the NFPA calls it.

4. Dispersion of dust particles
5. Confinement of the dust cloud

Explosions are an extreme hazard because they can cause serious injuries or death. They can occur when combustible dust is present in areas that are not properly ventilated. When a source of ignition, such as a spark from static electricity or friction ignites the dust cloud, it creates an explosion. The explosion then spreads through the entire area where the dust is located.

There are two types of explosions: deflagration and detonation. Deflagration is a fire that burns at a subsonic speed with flame visible throughout its length. Detonation is an explosion that occurs without preliminary decomposition and without flame propagation; it results in pressures so high that structures are destroyed by shock waves before ignition takes place.

Examples of combustible dust

  • Wood dust
  • Metallurgical dust (Metals)
  • Plastics, rubber, and synthetic materials
  • Chemical combustible dust
  • Food and agricultural products
  • Waste, by-products, and recyclables

Combustible dust classifications

There are different parameters to consider when it comes to combustible dust classification. For instance, Pmax and Kst are among the most commonly used measurements of explosion severity and combustible dust safety.


The term Pmax refers to the maximum amount of pressure and damage combustible dust can cause in a confined space. This measurement allows you to understand the potential damage your dust can create in a confined space.


Kst refers to the rate of pressure increase as a dust explosion occurs. Determining which “St Class” your material falls in provides valuable data to help mitigate dust explosion hazards.

St 0 – KSt value 0 | no explosion
St 1 – KSt >0 and ≤ 200 | weak explosion
St 2 – KSt >200 and ≤ 300 | strong explosion
St 3 – KSt >300 | very strong explosion

combustible dust classification chart

Class II Locations

Combustible dust is also covered under NEC’s (National Electrical Code) Hazardous Locations Classification.

These hazardous locations are defined as “an area at risk of fire or explosion due to:

Class I – Flammable gases or vapors
Class II – Combustible dust
Class III – Fiber or flyings.”

Class II locations mean that there is a sufficient amount of combustible dust in the air which can be explosive or ignitable. Class II is divided up into Division 1 and Division 2.

Division 1 is classified as an area where combustible dust can exist under normal, everyday operating conditions. While Division II is classified as an area where combustible dust mentioned above is not likely to exist during regular operation.

Determining the Pmax and Kst of the material along with the Class and Division of the work area can help understand the impact of a potential hazard while providing insight into the elimination or substitution of the risk. That’s why it’s critical that combustible dust classification is administered in the workplace.

If you’re looking for training materials on this topic, access our free Combustible Dust Explosion safety training course!

Ceren Dulgar

Ceren is a marketing enthusiast who is fascinated by the expansive world of EHS. You can catch her reading about the latest EHS news or advocating workplace safety.