How this combustible dust reference works
This combustible dust reference gives you three tools in one box. First, it screens your process against the five sides of the dust explosion pentagon. Second, it turns a lab Kst value into an explosion class. Third, it lists typical classes for common materials, and a search box filters the list.
The screening is a starting point. It tells you whether a formal dust hazard analysis (DHA) deserves your attention. It cannot replace the DHA or lab testing.
What NFPA 660 is and what it replaced
NFPA 660, Standard for Combustible Dusts and Particulate Solids, is the newest national standard for this hazard. The 2025 edition was approved on December 6, 2024. It pulled six older standards into one document. Those were NFPA 61 for agriculture and food, NFPA 484 for combustible metals, NFPA 652 for fundamentals, NFPA 654 for chemicals and other materials, NFPA 655 for sulfur, and NFPA 664 for wood processing.
| Chapters | Topic | Replaces |
|---|---|---|
| 1 to 10 | Fundamentals, including definitions in Chapter 3 and the dust hazards analysis in Chapter 7 | NFPA 652 (Chapters 1 to 9 align) |
| 21 | Agricultural and food processing | NFPA 61 |
| 22 | Combustible metals | NFPA 484 |
| 23 | Sulfur | NFPA 655 |
| 24 | Wood processing | NFPA 664 |
| 25 | Other materials, such as chemicals and plastics | NFPA 654 |
Chapters 11 to 20 are reserved for future topics. Plan to see items such as fire protection and additive manufacturing there later.
The dust explosion pentagon
A dust explosion needs five things at once. Fuel is a combustible dust. Oxygen comes from normal air. Ignition can be a spark, a hot bearing, static, or a cutting torch. Dispersion means the dust floats in a cloud. Confinement, such as a collector, duct, silo, or closed room, lets pressure build.
The first three sides make up the old fire triangle. Dispersion and confinement turn a fire into an explosion. Most prevention plans work on removing one of the five sides.
What counts as combustible dust
Combustible dust is a fine solid that can burn fast when it is suspended in air. Metals, wood, grain, sugar, plastics, rubber, and many chemicals qualify. For years, NFPA documents used a particle size of 420 microns, or smaller than a No. 40 sieve, as a rough reference. Finer dust burns faster. Dust that is smaller than 75 microns is the main concern in most plants.
Do not assume a material is safe because you have not seen a fire. Test it. Properties depend on particle size, moisture, and shape.
Lab tests that define the hazard
A lab measures how a sample behaves in a closed chamber. These tests drive protection design.
| Property | What it tells you | Test method |
|---|---|---|
| Pmax and Kst | Maximum pressure and rate of pressure rise | ASTM E1226 |
| Minimum explosible concentration (MEC) | Lowest cloud density that burns | ASTM E1515 |
| Minimum ignition energy (MIE) | Smallest spark that lights a cloud | ASTM E2019 |
| Minimum autoignition temperature of a cloud | Lowest hot surface that lights a cloud | ASTM E1491 |
| Layer ignition temperature | Lowest hot surface that lights a settled layer | ASTM E2021 |
Kst feeds directly into explosion vent and suppression sizing, which NFPA 68 and NFPA 69 cover.
Reading Kst and the St classes
Kst measures how fast pressure builds in an explosion. The classes are St0 for no explosion, St1 for weak up to 200 bar·m/s, St2 for strong from 200 to 300, and St3 for very strong above 300. Corn starch, sugar, and coal sit in St1. Cellulose and wood flour sit in St2. Aluminum and magnesium sit in St3.
Class alone does not make a dust safe. An St1 dust still explodes. It just does so with less violence than an St3 dust.
Dust hazard analysis and housekeeping
NFPA 660 expects facilities that handle combustible dust to complete a dust hazard analysis. Chapter 7 sets the rules. The older NFPA 652 asked owners to review the analysis at least every five years. Check Chapter 7 of the edition your fire marshal enforces for the current interval.
Housekeeping cuts the fuel for secondary explosions. OSHA's combustible dust program has used 1/32 inch of dust, about the thickness of a paper clip, as a screening point for floors and surfaces. Clean with vacuums rated for dust, not with compressed air.
The U.S. Chemical Safety Board counted 281 combustible dust incidents between 1980 and 2005. They killed 119 people and injured 718. The 2008 Imperial Sugar blast in Georgia killed 14 workers.
Worked example for a woodshop
A cabinet shop in Lubbock sands hardwood and pulls the dust into a bag collector. Wood dust is combustible, so fuel is present. Sanding makes fine dust, and the collector keeps it airborne. Motors and fans supply ignition. The collector is closed, so confinement exists too.
All five sides are present at the collector. The screening gives 5 of 5. The shop should complete a DHA under NFPA 660 Chapter 24, and move the collector outside or add the protections the DHA calls for. Typical steps include spark detection, explosion venting with a flame-arresting design, and isolation devices on the duct.
Texas context and who to call
Texas has many grain elevators, feed mills, sawmills, and chemical plants. Local fire marshals enforce the fire code, and many adopt NFPA 660 or its parent standards by reference. OSHA also covers combustible dust under its general duty clause and housekeeping rules.
Start with a qualified dust hazard analysis team. Then use licensed fire protection contractors for suppression and detection. Our Texas fire protection services directory can help you find them.
Frequently asked questions
What is NFPA 660?
It is the Standard for Combustible Dusts and Particulate Solids. The 2025 edition combines NFPA 61, 484, 652, 654, 655, and 664 into one document.
What does Kst mean?
Kst is a measure of how fast pressure rises in a dust explosion, in bar·m/s. A higher number means a faster, stronger blast.
What are the dust explosion classes?
St1 is weak, up to 200 bar·m/s. St2 is strong, from 200 to 300. St3 is very strong, above 300. St0 means no explosion under test conditions.
Is my dust combustible?
If it is organic, a plastic, a chemical, or a metal and is fine enough to float in air, assume yes until a lab test says otherwise. Do not guess from the material name alone.
What is a dust hazard analysis?
A DHA is a written review of where dust can burn or explode in your process. It lists the hazards, the safeguards, and the actions needed. NFPA 660 Chapter 7 covers it.
Does a closed collector make dust safe?
No. A closed collector adds confinement, one of the five pentagon sides. That can turn a flash fire into an explosion. The DHA should call for venting, isolation, or suppression.
Related Texas fire safety guides
Sources and standards
These references shaped the numbers and rules on this page. Always read the edition your local fire marshal has adopted.
- NFPA 660, Standard for Combustible Dusts and Particulate Solids
- U.S. Chemical Safety Board, Combustible dust hazard investigation
- OSHA, Combustible dust
Last reviewed October 13, 2026.