Frequently Asked Questions

A Dust Hazard Analysis (DHA) is a systematic review to identify and evaluate the potential fire, flash fire, and explosion hazards from combustible dust in a facility.

Required by NFPA 660 (formerly NFPA 652), a DHA examines:

• Materials handled and their combustibility properties
• Processes and operations that generate or handle dust
• Equipment where dust hazards may exist
• Existing safeguards and their adequacy
• Potential ignition sources

The DHA results in documented findings and recommendations for managing combustible dust hazards. DHAs must be performed or led by a qualified person and revalidated at least every 5 years.

You likely need a DHA if your facility handles, processes, generates, or stores combustible dust or particulate solids. Approximately 70% of industrial dusts are combustible.

Industries that typically require DHAs:

• Food and grain processing
• Woodworking and lumber
• Metal fabrication
• Pharmaceutical manufacturing
• Plastics and rubber
• Chemical processing

Exemptions may apply for:

• Retail operations with consumer quantities
• Sealed warehouse storage (not opening containers)
• Certain on-premises agricultural operations

Use our free screening tool to determine your specific requirements in under 2 minutes.

Per NFPA 660, a DHA must be revalidated at least every 5 years. However, review should occur sooner if:

• Significant process changes are made
• New combustible materials are introduced
• Equipment is modified or replaced
• A fire, explosion, or near-miss occurs
• Regulatory requirements change

The 5-year cycle is a maximum interval, not a target. Many facilities review their DHA annually as part of their safety management system.

Per NFPA 660 Section 7.2.2, a DHA must be performed or led by a qualified person who:

• Is familiar with combustible dust hazards
• Understands applicable codes and standards
• Knows dust testing parameters and their meaning
• Understands fire and explosion protection systems

The DHA team should also include personnel familiar with the facility: operations, maintenance, engineering, and safety staff.

While internal personnel with proper training can lead DHAs, many facilities engage external consultants for specialized expertise. Find qualified DHA consultants in our directory.

DHA costs vary significantly:

• Internal DHAs: Cost of time plus testing fees
• Consultant-led DHAs: $5,000 - $50,000+ depending on facility scope
• Dust testing: $500 - $3,000 per material for comprehensive characterization

Factors affecting cost:

• Number of process areas/nodes
• Number of materials requiring testing
• Complexity of equipment and systems
• Depth of analysis required

Free tools like our DHA documentation tool can help organize your analysis and potentially reduce consulting time.

DHA.Support offers free online tools for combustible dust compliance:

• DHA Screening Tool: 8-question assessment to determine if you need a DHA, covering facility type, materials, equipment, and hazard indicators
• DHA Documentation Tool: 7-section framework for documenting DHAs per NFPA 660, including team info, standards, materials, scenarios, protection, risk assessment, and action tracking

Both tools are completely free, require no signup or email, save automatically to your browser, and include PDF/Excel export capabilities.

NFPA 660: Standard for Combustible Dusts and Particulate Solids became effective December 6, 2024.

It consolidates and replaces six previous NFPA standards:

• NFPA 652 - Fundamentals of Combustible Dust
• NFPA 61 - Agricultural and Food Processing
• NFPA 484 - Combustible Metals
• NFPA 654 - General Manufacturing
• NFPA 655 - Sulfur
• NFPA 664 - Wood Processing

Structure:

• Chapters 1-10: Fundamentals (all facilities)
• Chapter 21: Agricultural & Food Processing
• Chapter 22: Combustible Metals
• Chapter 23: Sulfur
• Chapter 24: Wood Processing
• Chapter 25: Other Dusts & Particulates

NFPA 652 was the Standard on the Fundamentals of Combustible Dust, which established baseline DHA requirements.

NFPA 660 (effective December 2024) replaced NFPA 652 and consolidated it with five other industry-specific standards into one comprehensive document.

Key differences:

• 660 is a single consolidated document vs. multiple separate standards
• 660 includes industry-specific chapters (21-25)
• 660 has updated technical requirements
• 660 provides clearer guidance on implementation

Facilities previously complying with NFPA 652 should now reference NFPA 660 for current requirements. The core DHA requirements remain similar.

The dust explosion pentagon identifies five elements that must ALL be present simultaneously for a dust explosion:

• 🔥 Fuel: Combustible dust in sufficient quantity
• 💨 Oxygen: Typically from ambient air
• ⚡ Ignition source: Spark, hot surface, static discharge, etc.
• 🌀 Dispersion: Dust suspended in air at the right concentration
• 📦 Confinement: Enclosed or semi-enclosed space

Remove ANY one element = No explosion.

This is why DHAs focus on identifying locations where all five elements could exist together, and implementing controls to eliminate at least one element.

Kst (deflagration index) measures explosion severity, expressed in bar·m/s. It indicates how fast pressure rises during an explosion.

Dust Classifications:

• St 0: Kst = 0 — Non-explosible
• St 1: Kst 1-200 — Weak explosion (most organic dusts)
• St 2: Kst 201-300 — Strong explosion
• St 3: Kst >300 — Very strong explosion (aluminum, magnesium)

Why it matters: Kst is critical for sizing explosion protection systems including vents, suppression systems, and isolation devices. Higher Kst requires larger vents or faster-acting suppression.

Kst is determined through standardized testing per ASTM E1226.

NFPA 660 requires dust properties be determined through testing or representative published data.

Key tests include:

• Go/No-Go Screening (ASTM E1226): Determines if dust is combustible
• Kst and Pmax (ASTM E1226): Explosion severity for protection system design
• MIE - Minimum Ignition Energy (ASTM E2019): Sensitivity to sparks and static
• MIT Cloud (ASTM E1491): Hot surface ignition temperature for dust clouds
• MIT Layer (ASTM E2021): Hot surface ignition for dust layers
• MEC (ASTM E1515): Minimum explosible concentration (lower explosive limit)
• LOC (ASTM E2931): Limiting oxygen concentration for inerting

Testing should be performed by accredited laboratories using samples representative of actual process conditions.

Common ignition sources identified in DHAs include:

• Static electricity: From material transfer, pneumatic conveying, pouring
• Hot surfaces: Overheated bearings, dryers, heaters
• Mechanical sparks: Metal-on-metal contact, tramp metal
• Electrical equipment: Non-rated motors, switches, lighting
• Friction: Belt slippage, misaligned components
• Hot work: Welding, cutting, grinding
• Self-heating: Oxidation, biological activity in organic materials
• Open flames: Pilot lights, smoking

A thorough DHA identifies all potential ignition sources in each process area and evaluates controls to eliminate or manage them.

The historical guideline is that dust accumulation exceeding 1/32 inch (0.8 mm) over 5% or more of a room's floor area indicates a potential deflagration hazard requiring immediate cleaning.

However, NFPA 660 takes a more nuanced approach:

• Cleaning frequencies should be based on actual accumulation rates
• Material properties (Kst, MIE) affect hazard severity
• Hidden areas require special attention
• Never use compressed air for cleaning

Best practice: Establish cleaning schedules that prevent accumulation from reaching hazardous levels, rather than waiting for a threshold to be exceeded.

Explosion protection methods include:

• Deflagration Venting (NFPA 68): Relief panels that open during explosion to release pressure to a safe area
• Explosion Suppression: Systems that detect and chemically suppress explosions within milliseconds
• Explosion Containment: Equipment designed to withstand full explosion pressure
• Explosion Isolation: Devices (chemical barriers, fast-acting valves, rotary valves) that prevent propagation between equipment
• Inerting: Reducing oxygen below levels that support combustion

The appropriate protection depends on equipment type, location, dust properties (Kst, Pmax), and personnel proximity. Most facilities use a combination of methods.