Exposure Monitoring 101: How Often Should You Test Workplace Hazards?

Exposure monitoring is a critical part of protecting worker health, but it’s also one of the most misunderstood aspects of occupational health and safety. Many employers ask the same question:

How often do we actually need to test workplace hazards?

The answer isn’t always straightforward. In Canada, exposure monitoring requirements depend on the hazard, the work being performed, and whether existing controls are effective. For safety professionals and project managers, understanding when and how often to conduct exposure monitoring is essential for compliance, risk management, and prevention.

What Is Exposure Monitoring?

Exposure monitoring involves measuring how much of a hazardous agent—such as dust, fumes, vapours, noise, or biological contaminants—workers are exposed to during their workday.

Monitoring data is used to:

Compare exposures against occupational exposure limits (OELs)
Evaluate the effectiveness of existing controls
Identify trends or emerging risks
Support regulatory compliance and due diligence

Exposure monitoring is a key component of occupational hygiene and should be part of any comprehensive hazard management program.

Occupational Exposure Limits in Canada

In Canada, OELs are established under provincial and territorial OHS regulations, with most jurisdictions adopting or referencing limits based on ACGIH Threshold Limit Values (TLVs®).

Examples include:

Alberta OHS Code – Part 4
Ontario Regulation 833
WorkSafeBC Occupational Exposure Limits

Employers are legally required to ensure worker exposures do not exceed these limits and to take corrective action if they do.

When Is Exposure Monitoring Required?

Exposure monitoring is typically required or strongly recommended when:

A hazardous substance or agent is present in the workplace
Workers report symptoms consistent with exposure
New materials, processes, or equipment are introduced
Engineering or administrative controls are modified
Regulatory limits may be approached or exceeded
There is insufficient data to demonstrate exposures are controlled

If you cannot confidently demonstrate that exposures are below OELs, monitoring is often the next step.

How Often Should You Conduct Exposure Monitoring?

There is no single universal schedule—but best practices and Canadian regulatory guidance point to several key triggers.

Baseline Monitoring

Baseline testing should be conducted:

When a new process, chemical, or task is introduced
During early project phases or commissioning
When exposure potential is unknown

Baseline data helps establish whether controls are adequate from the outset.

Routine or Periodic Monitoring

Ongoing monitoring may be required when:

Exposures are close to OELs
Workers perform high-risk or variable tasks
Conditions change frequently (e.g., construction sites)

Frequency may range from annually to every few years, depending on risk level, stability of processes, and previous results.

Monitoring After Changes

Exposure monitoring should be repeated when:

Production rates increase
Materials or chemicals are substituted
Ventilation systems are modified
Work practices change

Even small changes can significantly affect exposure levels.

Triggered Monitoring

Immediate monitoring may be necessary when:

Workers report health symptoms
Odours, dust, or visible emissions increase
Equipment fails or controls are compromised
Incidents or near-misses occur

Triggered monitoring helps identify whether exposures are contributing to reported concerns.

Air Sampling Frequency: Risk-Based, Not Calendar-Based

One of the most common mistakes employers make is relying on fixed schedules rather than risk-based decision-making.

Air sampling frequency should consider:

Toxicity of the substance
Proximity to OELs
Duration and frequency of exposure
Number of workers affected
Reliability of control measures

Higher-risk environments typically require more frequent monitoring, while stable, well-controlled processes may justify less frequent testing.

What Types of Hazards Require Monitoring?

Common workplace hazards that often require exposure monitoring include:

Airborne contaminants (silica, welding fumes, solvents, diesel exhaust)
Noise
Biological agents (mold, bacteria in healthcare or remediation work)
Thermal stress (heat and cold exposure)

Each hazard has its own monitoring methods and evaluation criteria.

Documenting and Using Monitoring Results

Exposure monitoring data should never sit on a shelf.

Results should be used to:

Confirm regulatory compliance
Improve engineering or administrative controls
Inform PPE selection and respiratory protection programs
Communicate risks and controls to workers
Support ongoing health and safety planning

Documentation is also critical for demonstrating due diligence during inspections or investigations.

When to Involve an Industrial Hygienist

While some basic monitoring can be conducted internally, complex or high-risk exposures often require specialized expertise.

An industrial hygienist can help:

Design appropriate sampling strategies
Select the right monitoring methods
Interpret results against Canadian OELs
Recommend practical, effective control measures

Engaging qualified expertise ensures monitoring results are accurate, defensible, and meaningful.

Final Thoughts

Exposure monitoring isn’t about checking a regulatory box – it’s about understanding real-world risks and ensuring workers are protected over the long term.

By taking a risk-based approach to air sampling frequency, aligning with Canadian exposure limits, and responding proactively to change, organizations can build stronger, more effective occupational health programs.

Knowing when to test – and when to test again – is a key part of preventing occupational illness before it starts.