| 📋 Article Summary — Similar Exposure Groups for Noise | |
|---|---|
| Key Fact 1 | A SEG is a group of workers with similar work patterns, tasks, and noise exposures — once characterised, results apply to all members without measuring each individual |
| Key Fact 2 | AIOH and Safe Work Australia guidance recommends a minimum of 6 measurements per SEG for statistically valid characterisation |
| Key Fact 3 | Workers at the upper end of exposure within a SEG should be included in audiometric monitoring even if the group average is below the action level |
| Read Time | 8 min |
Why SEGs Are Essential for Efficient Noise Monitoring
Large workforces with noise-exposed workers present a sampling challenge: it is not practical to measure every worker individually every time a noise assessment is required. Similar Exposure Groups (SEGs) — a concept borrowed from industrial hygiene air monitoring strategy — provide a statistically valid approach to characterising noise exposure across large populations efficiently.
A SEG is a defined group of workers with similar work activities, noise sources, and exposure patterns. Once a SEG is characterised by sufficient measurements to estimate the exposure distribution, the results can be applied to all members of the SEG — without measuring each individual worker separately. This is the cornerstone of efficient occupational noise monitoring program design.
Defining SEGs for Noise
SEG definition requires understanding which workers perform similar jobs in similar noise environments. Key factors that determine SEG membership:
- Job title and role: Workers in the same role performing the same tasks are the starting point
- Noise sources encountered: Workers exposed to the same machines, tools, and environmental noise sources
- Time allocation: Similar proportions of time spent in different noise zones or performing different tasks
- Work shift patterns: Different shifts may have different noise profiles (e.g., more or less equipment running on afternoon vs. day shift)
- Physical location: Workers in the same area or bay of a facility, where ambient noise levels are similar
Workers who appear to have the same job title but work in noticeably different noise environments should be in separate SEGs. Conversely, workers with different titles but identical noise exposure patterns may be grouped together.
How Many Measurements per SEG?
The AIOH Guidance on Occupational Hygiene Sampling (aligned with OSHA Technical Manual statistical methodology) recommends a minimum of:
- 6 measurements for adequate characterisation of a SEG where normal noise variability is expected
- More measurements where high day-to-day variability is observed (e.g., noise levels that depend heavily on production rates or the specific tasks performed)
- Measurements should be on random workers within the SEG, not always the same individual, and on random working days, not selected for particular production conditions
For noise specifically, variability within a well-defined SEG (same task, same machines, same location) is often lower than for chemical exposure SEGs — meaning fewer measurements may adequately characterise the group. However, the minimum of 6 measurements is a reasonable starting point for confident statistical characterisation.
Statistical Interpretation of SEG Noise Data
With adequate measurement data, the occupational hygienist calculates:
- The group geometric mean (GM) — the central tendency of the noise dose distribution
- The geometric standard deviation (GSD) — the variability of exposure within the SEG
- The 95th percentile exposure — the exposure expected to be exceeded by only 5% of days/workers. This is the key value for compliance assessment: if the 95th percentile exposure is below the WES, the group is likely compliant with adequate confidence.
Where the 95th percentile exceeds the action level (85 dB(A) LAeq,8h), the SEG is characterised as a high-exposure group requiring hearing conservation measures. Where the 95th percentile is below the action level, the SEG is considered low-risk and may not require hearing protection or audiometric monitoring (though good practice includes monitoring anyway).
Audiometric Monitoring Strategy Based on SEGs
SEG characterisation informs audiometric monitoring in two ways:
- All workers in SEGs with 95th percentile exposure above the action level should be enrolled in audiometric monitoring
- Within a high-exposure SEG, focus audiometric surveillance on workers with the highest individual exposures (those measured at the top of the SEG exposure distribution), as they are at greatest risk
Frequently Asked Questions
If I measure one worker in a SEG and they are below the action level, can I assume the whole group is safe?
No. A single measurement characterises one worker on one day — it cannot reliably represent the full exposure distribution within a SEG. Statistical characterisation requires multiple measurements across multiple workers and days. A single compliant measurement may simply reflect a quiet day, a particularly low-exposure worker, or measurement error — it does not reliably demonstrate group-level compliance.