Excavation Safety – Hazardous Atmospheres and Prevention
The Problem
When we think of excavation hazards, cave-ins and falling objects are usually top of mind. But hazardous atmospheres can be just as dangerous—if not more so—because they often go undetected. Toxic gases, oxygen-deficient environments, or flammable vapours can quickly overwhelm workers, leading to serious illness, unconsciousness, or even death. Today, we’ll look at what makes an atmosphere hazardous, how to identify the risks, and what steps we need to take to work safely.
Identify the Hazards
Methane Gas
- Source: Often found in excavations near landfills.
- Risk: Methane is colourless, odourless, and highly flammable. It can displace oxygen and create an unsafe breathing environment.
- Impact: Can lead to dizziness, unconsciousness, or suffocation.
Hydrogen Sulphide (H₂S)
- Source: Produced by decomposing organic matter, commonly found in sewer systems and wastewater lines.
- Risk: Initially smells like rotten eggs, but ongoing exposure can desensitise your sense of smell.
- Impact: Can cause respiratory failure and is fatal at high concentrations.
Carbon Monoxide (CO)
- Source: Emitted by internal combustion engines (generators, welders, diggers).
- Risk: CO is colourless, odourless, and deadly in enclosed or poorly ventilated spaces.
- Impact: Can cause nausea, confusion, unconsciousness, and death.
Contaminated Soil
- Source: Underground fuel tanks or leaking pipelines may release chemicals that vaporise during excavation.
- Risk: Can lead to toxic or explosive air mixtures.
- Impact: Inhalation or ignition of vapours may cause poisoning, burns, or explosions.
Safety Practices and Preventative Measures
Atmospheric Testing
- A Competent Person must test the air in any excavation where harmful gases are suspected—especially near landfills, pipelines, or where combustion engines are in use.
- Use a multi-gas detector to check oxygen levels and detect flammable or toxic gases.
Ventilation
- If gas is detected, ventilate the trench immediately using mechanical ventilation systems to introduce fresh air and dilute harmful gases.
- Work cannot resume until the air has been confirmed as safe.
Proper Work Procedures
- Always follow your organisation’s safe work method statements (SWMS) and permit-to-work systems.
- Use portable gas monitors when working in at-risk locations.
- Avoid running engines inside excavations unless there's proper ventilation and controls in place.
Report Suspicious Conditions
- Alert the Competent Person or Supervisor immediately if you notice unusual smells, visible vapours, or signs of oxygen deficiency.
- Don’t ignore symptoms or assume it’s nothing—early reporting saves lives.
Use Protective Equipment
- Wear suitable PPE, such as respirators, where required.
- Make sure there’s a rescue plan and emergency gear ready if atmospheric hazards are suspected.
Real-Life Case or Scenario
Last year, a work crew entered an excavation near an old landfill without testing the atmosphere. Unknown to them, methane gas had displaced the oxygen. One worker collapsed within minutes due to oxygen deficiency. The quick actions of his team saved his life—but the incident could have been completely avoided with proper gas testing.
Employee Responsibilities
- Inspect your work area for signs of hazardous atmospheres.
- Report any suspicious conditions or unusual odours straight away.
- Follow the Competent Person’s direction regarding testing and ventilation.
- Use PPE and gas monitors when working in high-risk zones.
Employer Responsibilities
- Provide and maintain appropriate gas detection and ventilation equipment.
- Ensure atmospheric testing is conducted by a Competent Person where risks are present.
- Train all workers on the risks of hazardous atmospheres and how to respond.
- Establish emergency procedures and rescue plans for atmospheric emergencies.
Discussion & Questions
- “Have you ever noticed strange odours or signs of gas while working in a trench or excavation?”
- “Do you know what to do if you suspect hazardous gases are present?”
Invite team members to share their experiences, raise questions, or clarify their understanding.
Commitment/Action Plan
Let’s commit today to reporting suspicious conditions, following safety procedures, and never entering an excavation without atmospheric testing if there’s any chance of gas or oxygen deficiency. If in doubt, stop and check.
Wrap-Up
Hazardous atmospheres are invisible killers. With the right testing, ventilation, and a safety-first mindset, we can eliminate these risks. Speak up, follow the rules, and never assume the air is safe. Your health and your life depend on it. Thank you for making safety a shared responsibility.
Standards Overview
Under the Health and Safety at Work (General Risk and Workplace Management) Regulations 2016, PCBUs must:
- Identify and manage risks associated with hazardous atmospheres, particularly in confined spaces and excavations.
- Conduct air monitoring when hazardous substances or gas build-up is possible.
- Implement appropriate ventilation and control measures.
- Provide training and emergency response procedures for hazardous atmospheric conditions.
Disclaimer
These toolbox talks are provided for informational and educational purposes only and are not a substitute for professional legal, health and safety, or regulatory advice.
The content reflects recognised best practices and general industry standards as at the date of publication. However, it may not be applicable to all workplace environments or specific operational contexts.
It is the responsibility of the Person Conducting a Business or Undertaking (PCBU), as well as officers, workers, and other duty holders under the Health and Safety at Work Act 2015 (HSWA), to ensure compliance with all relevant laws, regulations, and WorkSafe New Zealand guidance.
The authors, presenters, and distributors of this toolbox talk disclaim all liability for any loss, damage, or injury arising from the use or reliance on the information provided. Users should seek advice from a suitably qualified health and safety professional or legal advisor for guidance tailored to their specific circumstances.
Always follow your organisation’s policies, procedures, and safe work practices to manage risks and maintain a safe and healthy workplace.
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