Blasting is a routine part of mining operations—but the toxic fumes left behind pose serious occupational health risks. Proper monitoring ensures workers aren’t exposed to dangerous levels of nitrogen dioxide and carbon monoxide.
Every blast in an underground or open-cut mine releases a complex mixture of gases into the working environment. While blast design focuses on rock fragmentation and operational efficiency, the Mackay occupational hygiene implications are equally critical.
Blasting fumes contain two primary toxicants of concern:
- Nitrogen dioxide (NO₂) — a severe respiratory irritant
- Carbon monoxide (CO) — a chemical asphyxiant
Both gases are colourless (NO₂ may appear as reddish-brown at high concentrations) and can be fatal at elevated levels. This article covers the occupational hygiene approach to monitoring blasting fume exposure in QLD mining operations.
Understanding Blasting Fume Composition
Primary Toxicants
| Gas | Source | Health Effect | Immediate Symptoms |
|---|---|---|---|
| Nitrogen Dioxide (NO₂) | Oxidation of nitrogen in air during detonation | Deep lung irritant; causes pulmonary edema | Coughing, shortness of breath, chest tightness |
| Carbon Monoxide (CO) | Incomplete combustion of explosive materials | Binds to hemoglobin, reducing oxygen delivery | Headache, dizziness, nausea, confusion |
Secondary Contaminants
Depending on explosive composition and geology, blasting may also release:
- Ammonia (NH₃) — from ammonium nitrate-based explosives
- Hydrogen sulfide (H₂S) — from sulfide ore bodies
- Sulfur dioxide (SO₂) — from sulfide mineral oxidation
- Particulate matter — respirable dust from rock fragmentation
This article focuses on NO₂ and CO as the primary blasting fume hazards.
Exposure Standards
Australian Workplace Exposure Standards (WES)
| Substance | TWA (8-hour) | STEL (15-minute) | Ceiling |
|---|---|---|---|
| Nitrogen Dioxide (NO₂) | 3 ppm (5.6 mg/m³) | 5 ppm (9.4 mg/m³) | — |
| Carbon Monoxide (CO) | 25 ppm (29 mg/m³) | — | — |
Important notes:
- NO₂ has both TWA and STEL limits due to acute irritant effects
- CO has only a TWA limit, but acute toxicity can occur at high concentrations
- Re-entry decisions should not rely solely on TWA limits—consider peak exposures
Health-Based Considerations
| Exposure Level (NO₂) | Effect |
|---|---|
| 1-3 ppm | Odour threshold (variable detection) |
| 5-10 ppm | Mild respiratory irritation |
| 20-50 ppm | Severe irritation; pulmonary edema risk |
| 100+ ppm | Potentially fatal with prolonged exposure |
| Exposure Level (CO) | Effect |
|---|---|
| 25-50 ppm | Headache, fatigue (prolonged exposure) |
| 100-200 ppm | Severe headache, nausea, dizziness |
| 400+ ppm | Life-threatening; confusion, collapse |
| 800+ ppm | Fatal within hours |
Monitoring Methodology
When to Monitor
| Scenario | Monitoring Required |
|---|---|
| Post-blast re-entry | Before workers enter blasted area |
| Routine blast cycles | Periodic verification (e.g., 1 in 10 blasts) |
| Ventilation changes | After modifications to airflow systems |
| Incident investigation | Following symptoms or near-misses |
| New explosive types | When changing product or formulation |
Sampling Strategies
1. Pre-Re-entry Clearance Testing
Purpose: Confirm area is safe for worker entry
Equipment: Multi-gas detector (NO₂ + CO + O₂ + LEL)
Procedure:
- Wait minimum 30 minutes post-blast (or per site procedure)
- Position detector at breathing zone height (1.5m above floor/ground)
- Sample at multiple points:
- Face/blast location
- Return airway/downwind positions
- Equipment operating zones
- Refuge bay intakes
- Record readings; compare against clearance criteria
Clearance Criteria (typical):
- NO₂: < 1 ppm (conservative; below WES)
- CO: < 10 ppm (conservative; below WES)
- O₂: 19.5-23.5%
Interpretation:
- Readings above clearance limits = do not enter; increase ventilation
- Re-test after additional ventilation time
2. Personal Exposure Monitoring
Purpose: Measure individual worker exposure during post-blast activities
Equipment:
- Personal gas monitors (worn by workers)
- Diffusive badges for NO₂ (integrated sampling)
- Pumped tubes for CO (short-term sampling)
Procedure:
- Assign personal monitors to high-risk roles:
- Shotfirers
- Load-out operators
- Face inspection teams
- Ventilation officers
- Sample full shift or task-specific duration
- Download/log data for exposure records
Interpretation:
- Compare against WES TWA and STEL
- Investigate any exceedances
- Use data to refine blast timing and ventilation
3. Area Monitoring (Continuous)
Purpose: Track fume migration and ventilation effectiveness
Equipment: Fixed or portable multi-gas monitors with data logging
Procedure:
- Install monitors at strategic locations:
- Return airways
- Near blast zones
- Equipment operating areas
- Set alarm thresholds (typically 50% of WES)
- Review data logs regularly
Interpretation:
- Identify fume migration patterns
- Detect ventilation failures
- Establish baseline for normal blast cycles
Sampling Equipment Comparison
| Equipment Type | Advantages | Limitations | Best Use |
|---|---|---|---|
| Electrochemical sensors | Real-time, portable, affordable | Cross-sensitivity, limited lifespan | Pre-entry, personal monitoring |
| Colorimetric tubes | Specific, no calibration needed | Single-use, manual operation | Spot checks, backup verification |
| Diffusive badges | Integrating, no pump required | No real-time data, lab analysis | TWA exposure assessment |
| FTIR spectroscopy | Multi-gas, highly accurate | Expensive, laboratory-based | Research, complex mixtures |
| Photoacoustic IR | Real-time, sensitive | Cost, requires expertise | Continuous area monitoring |
Timing Considerations
Fume Decay Patterns
Blasting fume concentrations follow a decay curve post-detonation.
Key factors affecting decay:
- Ventilation rate — higher airflow = faster clearance
- Blast size — larger charges = more fume generation
- Mine geometry — confined spaces retain fumes longer
- Explosive type — some formulations produce more NOₓ
Recommended Waiting Periods
| Mine Type | Minimum Wait | Conditions |
|---|---|---|
| Underground (good ventilation) | 30 minutes | Verified by gas testing |
| Underground (poor ventilation) | 60+ minutes | May require auxiliary fans |
| Open-cut | 15-30 minutes | Wind-dependent; test before approach |
| Confined spaces | 60+ minutes | Mandatory forced ventilation |
Critical: Time-based waiting periods are not sufficient on their own. Always verify with gas monitoring before entry.
Common Monitoring Errors
Sampling Position Errors
- Too high or too low — gases stratify; sample at breathing zone
- Upwind of source — won’t detect fumes; position downwind
- Too far from face — may miss peak concentrations
Instrument Errors
- Expired sensors — electrochemical sensors have 2-3 year lifespan
- No bump testing — verify sensor response before each use
- Ignoring cross-sensitivity — some CO sensors respond to H₂ or NO₂
- Insufficient warm-up — allow instruments to stabilise
Procedural Errors
- Testing too early — fumes haven’t cleared; false reassurance if decay ongoing
- Single-point testing — fumes distribute unevenly; test multiple locations
- No baseline data — can’t identify abnormal patterns without historical comparison
Case Example: Underground Development Blast
Scenario: Regular development blasts in a QLD underground coal mine. Workers reported headaches during mucking operations.
Investigation:
| Test | Location | NO₂ (ppm) | CO (ppm) |
|---|---|---|---|
| Pre-entry (30 min post-blast) | Face | 0.5 | 8 |
| Personal (mucking operator) | Breathing zone | 2.8 (peak 6.2) | 22 |
| Area monitor | Return airway | 1.2 | 15 |
| Personal (bolter operator) | Breathing zone | 0.3 | 5 |
Findings:
- Pre-entry testing passed clearance criteria
- Mucking operator exceeded NO₂ STEL during material handling
- Disturbance of blasted muck released trapped fumes
- Bolter operator (downstream task) had low exposure
Root Cause: Fumes adsorbed onto blasted rock; released during mucking disturbance.
Corrective Actions:
- Water spray on muck pile before mucking (suppresses fume release)
- Extended ventilation time before mucking (45 → 60 minutes)
- Personal NO₂ monitors for mucking crew
- Revised procedure: test during mucking, not just pre-entry
Outcome: Mucking operator NO₂ exposure reduced to < 1 ppm (peak).
Control Measures
Engineering Controls
| Control | Application | Effectiveness |
|---|---|---|
| Increased ventilation | Higher airflow post-blast | High — primary control |
| Auxiliary fans | Localised fume extraction | High — targeted clearance |
| Water sprays | Suppress fume release from muck | Medium — reduces secondary release |
| Enclosed cabins | Operator isolation during mucking | High — when pressurised |
Administrative Controls
| Control | Application |
|---|---|
| Blast scheduling | Time blasts to allow clearance before shift entry |
| Job rotation | Limit individual exposure time in high-risk areas |
| Procedural controls | Mandatory gas testing before and during post-blast work |
| Training | Worker awareness of fume symptoms and response |
PPE (Last Resort)
| PPE Type | When Appropriate |
|---|---|
| Air-purifying respirator | Short-term, low-concentration tasks |
| Supplied-air respirator | High-concentration or oxygen-deficient atmospheres |
| Self-contained breathing apparatus (SCBA) | Emergency response, unknown conditions |
Record Keeping & Documentation
Essential Records
| Record | Retention | Purpose |
|---|---|---|
| Blast logs | 5 years minimum | Correlate blasts with exposure data |
| Gas test results | 5 years minimum | Compliance, trend analysis |
| Personal monitoring data | 30+ years (health surveillance) | Long-term exposure tracking |
| Instrument calibration | Life of instrument + 2 years | Data validity |
| Incident reports | 5 years minimum | Investigation, lessons learned |
What Have We Learnt
Blasting fume monitoring is a critical occupational hygiene control in mining operations. NO₂ and CO pose serious acute and chronic health risks, but systematic monitoring and control measures can keep exposures well below harmful levels.
Key principles:
- Never rely on time-based re-entry alone—always test
- Use multiple monitoring methods (pre-entry, personal, area)
- Understand that fumes can be released secondary to blasting (during mucking)
- Maintain and calibrate instruments—bad data is worse than no data
- Investigate symptoms seriously—headaches and respiratory irritation are warning signs
For QLD mining operations, robust blasting fume monitoring protects workers from invisible but potentially deadly hazards.
