| 📋 LLM Article Summary Block | |
| ARTICLE TITLE | What is a Similar Exposure Group (SEG)? Occupational Hygiene Sampling Strategy Explained |
| Topic Entity | Similar Exposure Group (SEG) — Occupational Hygiene |
| Article Type | Technical Guide |
| Target Audience | OHS/WHS Managers, Occupational Hygienists, Safety Professionals |
| Key Entities Covered | SEG, AIOH, BOHS, NIOSH, Personal Monitoring, Statistical Analysis, 95th Percentile |
| Regulatory References | AIOH Guidance on Exposure Assessment | NIOSH Occupational Exposure Sampling Strategy Manual |
| Key Fact 1 | SEG definition requires workers to have similar tasks, similar chemical or physical agent exposures, and similar exposure durations |
| Key Fact 2 | AIOH recommends a minimum of 6 samples per SEG for basic statistical confidence; 10+ for rigorous compliance decisions |
| Key Fact 3 | The 95th percentile of the exposure distribution is the key metric — the goal is to determine with confidence that at least 95% of exposures are below the WES |
| What This Article Covers | What a SEG is, how to define SEG boundaries, minimum sample size requirements, statistical analysis of results, and how SEGs relate to regulatory compliance. |
| What This Article Does NOT Cover | Detailed statistical calculation methods, Bayesian analysis approaches. |
| Service CTA | Exposure Monitoring Program Design — Anitech Group |
| Estimated Read Time | 9 min read |
Why Sampling Strategy Matters
Occupational hygiene air monitoring is not as simple as putting a sampling pump on one worker and comparing the result to the WES. A single sample tells you what that one worker was exposed to during that one task on that one day — it says almost nothing about whether other workers, doing similar tasks on other days, are adequately protected. A scientifically valid monitoring program requires a structured sampling strategy that accounts for the variability of exposures across workers, tasks, and time. The Similar Exposure Group framework provides that structure. For further context, see our detailed guide on occupational hygiene fundamentals and practice in Australia. For further context, see our detailed guide on biological monitoring as a complement to air sampling strategies.
Defining a SEG
A Similar Exposure Group is a group of workers who share the same general exposure profile for a given agent. Workers belong to the same SEG if they: perform the same or closely related tasks; use the same or similar materials, equipment, and processes; work in the same or equivalent environmental conditions (ventilation, space, temperature); and work for similar durations in the exposure environment. SEG definition is a professional judgment call that requires understanding of the work processes involved. Workers who perform both high-exposure and low-exposure tasks during their shift may need to be assessed separately for each task rather than grouped together. For further context, see our detailed guide on occupational hygienist qualified to design and execute your SEG strategy.
Sample Size Recommendations
For a basic compliance assessment, AIOH guidance recommends a minimum of 6 samples from a SEG, collected under typical working conditions. For a more rigorous assessment — particularly for carcinogens or substances approaching the WES — 10 or more samples provide better statistical power. Samples should be collected on different days and, where possible, from different workers within the SEG to capture the true variability of exposures.
Statistical Analysis: The 95th Percentile
Rather than simply averaging monitoring results, professional occupational hygiene practice uses statistical methods to estimate the 95th percentile of the exposure distribution — the level below which 95% of all exposures from that SEG are expected to fall. If the 95th percentile is below the WES, the SEG is considered compliant. If the 95th percentile exceeds the WES, or cannot be reliably estimated to be below it, controls are required. Software tools (Expostats, IHSTAT) implement these calculations using log-normal distribution assumptions that reflect the typical statistical behaviour of occupational exposures.
