Combustible Dust and Explosion Protection

Is your dust combustible? If so, how can you protect your facility from dust explosions? This article covers the Deflagration Index and dust safety standards.

Friday, May 10, 2024

What do you think these foodstuffs have in common?

  • Cocoa powder?
  • Flour?
  • Coffee?
  • Sugar?

You got it - they are all combustible and can be explosive! Any powder bulk handling company or facility involved in the production, handling, processing, packaging, and storage of dry particulate matter or bulk materials is likely to be at risk of a deflagration or dust explosion.

The Deflagration Index

It is crucial to determine whether your dust is combustible, and if your application requires deflagration venting. The Deflagration Index measures the explosiveness of a particular dust in comparison to other dust types, denoting it with a Kst value. Kst values depend on a range of factors, including particle size, dust cloud concentration, and particle agglomeration. OSHA considers a Kst of 1.5 bar-m/sto be the minimum threshold for an explosion. However, any combustible dust with a Kst value greater than St0 could be subject to dust deflagration and must be treated accordingly.

Four Kst Classes identify the severity of the explosion produced by the dust:

OSHA and NFPA publish data on the Kst classes of different dust types. However, the actual combustibility of a dust can be influenced by local conditions within facilities and processes. Additionally, no two dusts will have the same composition and characteristics. It is therefore essential to test the combustibility of the dust present in your facility and ensure appropriate safety measures are in place.

How to Test for Combustible Dust

Follow these six steps to test the dust in your facility for combustibility. The test results will help you identify the measures required to protect the facility from dust explosions.

  1. Find a certified test lab: OSHA’s Salt Lake Technical Center or another licensed laboratory using recognized testing methods should be selected. The lab should adhere to standards such as ASTM (American Society for Testing and Materials) or ISO (International Organization for Standardization), and follow guidance provided by OSHA and NFPA.
  2. Collect a sample: Collect a representative sample of your dust for testing. OSHA recommends collecting a full 1-liter bottle of sample whenever possible, though your testing center will advise on their preferred specifications.
  3. Ship your sample: Contact your chosen lab for instructions on safely packaging and shipping your dust sample.
  4. Lab testing: The lab performs tests to measure your dust’s Kst value. This involves dispersing the dust sample in a chamber and igniting it to precisely measure the rate of pressure rise.
  5. Results: The lab will send you a report with the Kst value.
  6. Implement safety measures: Use the Kst value and other findings to introduce appropriate safety measures in your facility to protect against dust fires, deflagrations, and explosions.

Safety Standards for Combustible Dust

The OSHA Technical Manual, Section IV, Chapter 6, Combustible Dusts, provides an extensive list of relevant safety references and standards. A few key standards are listed below.

OSHA Standards

The Code of Federal Regulations Title 29, Part 1910 - Occupational Safety And Health Standards (29 CFR 1910) contains sections relevant to combustible dusts. Areas covered include housekeeping, equipment, electrical hazards, and inspections.

NFPA 652

NFPA 652, Standard on the Fundamentals of Combustible Dust, outlines the processes that employers should follow to ensure that equipment, systems, process buildings, and other structures containing combustible dusts are designed, installed, operated, and maintained in a manner that protects workers.

NFPA Commodity-Specific Standards

Following on from NFPA 652, there are several Commodity-Specific Standards applicable to different dust types, including:

  • NFPA 61 – Agricultural and food processing dusts
  • NFPA 484 – Metal and metal alloy dusts
  • NFPA 654 – Combustible particulate solids
  • NFPA 655 – Bulk and liquid sulfur
  • NFPA 664 – Wood working and wood processing dusts
NFPA 660

From 2025, NFPA 652 and the Commodity-Specific Standards listed above will be combined into a new standard, NFPA 660. This is part of the Combustible Dust Document Consolidation Plan (consolidation plan) as approved by the NFPA Standards Council.  


NFPA 68, Standard on Explosion Protection by Deflagration Venting, applies to “devices and systems that vent the combustion gases and pressures resulting from a deflagration to minimize damage.”


NFPA 69, Standard on Explosion Prevention Systems, applies to systems for the prevention and control(protection) of explosions by methods other than those covered in NFPA 68.

We recommend you consult OSHA and local guidance to identify all the regulations and standards that may apply to your facility and processes. Prevention is better than cure, and many explosion hazards can be mitigated by good housekeeping and preventive measures.