Air Sampling Guide for Toxic Substances Monitoring in Singapore
A practical guide for planning, conducting and reporting workplace air sampling for toxic substances monitoring in Singapore, based on the Ministry of Manpower’s guidance on sampling strategies and toxic substances monitoring reports.
Air sampling must be planned around worker exposure, process variability and the purpose of the assessment.
Toxic substances monitoring is not only about collecting air samples. A good sampling strategy should identify the highest expected exposure groups, select representative workers, use suitable sampling methods and ensure that the results can be compared meaningfully with the applicable Permissible Exposure Levels.
Plan the sampling strategy
Understand the work process, toxic substances used, exposed job groups, process duration, exposure variation and existing controls before monitoring starts.
Monitor personal exposure
Personal monitoring should generally represent the worker’s exposure, with the sampler placed as close as practicable to the breathing zone.
Report clearly and promptly
Monitoring reports should document the sampling method, process, exposure duration, control measures, measured concentration, TWA result and recommendations where required.
A practical sequence for toxic substances air monitoring.
The workflow below converts the MOM guidance into a practical planning sequence that can be used by employers, occupational hygienists, WSH professionals and service providers.
Review the process
Obtain information on work process, substances, exposed workers, duration, controls and expected exposure pattern.
Select subjects
Prioritise occupations or job groups with the highest expected exposure and select representative workers for monitoring.
Choose the method
Use suitable sampling and analytical methods, such as recognised NIOSH methods or other validated procedures.
Conduct sampling
Calibrate equipment, collect samples correctly, observe field conditions and check pump flow rates during monitoring.
Report and act
Calculate exposure, compare against PELs, provide recommendations where needed and submit reports within the required timeframe.
Air Sampling Planning and Field Checklist
Use this searchable checklist to review key requirements and precautions before, during and after toxic substances air monitoring.
| Stage | Checklist Item | Practical Guidance | Why It Matters |
|---|---|---|---|
| Planning | Select representative workers | Monitor job groups or occupations with the highest expected exposure first, and select representative subjects for sampling. | This improves the chance that monitoring results reflect meaningful workplace exposure risk. |
| Sampling | Prefer personal exposure monitoring | Attach the sampling device close to the worker’s breathing zone unless another type of monitoring is specifically required. | Personal samples are more relevant than area samples for assessing worker exposure. |
| Sampling | Collect enough sampling time | For stable exposure conditions, collect at least 6 hours for an 8-hour shift or 8 hours for a 12-hour shift, excluding meal and break periods. | A short or unrepresentative sample can underestimate or misrepresent the full-shift TWA exposure. |
| Planning | Use partial-period sampling only where appropriate | If exposure occurs for less than 6 hours, sampling may cover the exposure period, with unsampled non-exposure time clearly treated and stated. | Transparent assumptions are essential when calculating and interpreting TWA exposure. |
| Equipment | Calibrate before and after sampling | Calibrate sampling equipment using suitable standard calibration methods before and after monitoring. | Flow rate errors directly affect sampled air volume and calculated concentration. |
| Sampling | Check flow rate during the survey | Check flow rate after the first 15-30 minutes and at intervals of about 2 hours during the sampling period. | This helps identify blocked filters, pump faults, tubing problems or tampering. |
| Sampling Media | Prepare filters correctly | For total particulate monitoring, condition membrane filters before weighing according to the required procedure. | Moisture and filter handling can affect gravimetric results. |
| Sampling Media | Keep sorbent tubes vertical | Position sorbent tubes vertically during sampling and cap both ends after collection. | Incorrect orientation can reduce collection efficiency and affect analytical results. |
| Sampling Media | Store sorbent samples properly | After sampling, store sorbent tubes at low temperature where required and send samples for analysis promptly. | Poor storage can lead to sample loss, degradation or unreliable results. |
| Sampling | Use validated passive samplers | Confirm that passive samplers are suitable for the contaminant and exposure scenario, and check supplier performance documentation. | Not all passive samplers have adequate precision and accuracy for every application. |
| Sampling | Avoid stagnant air for passive area sampling | When passive samplers are used for static monitoring, confirm that local air movement meets the manufacturer’s requirements. | Insufficient air movement may produce falsely low concentrations. |
| Sampling | Keep cyclones vertical | Use cyclones at the specified design flow rate and maintain the correct orientation during sampling. | Cyclone performance depends on orientation and flow rate, especially for respirable aerosol sampling. |
| Sampling | Do not invert cyclones | Avoid inverting a cyclone during or after sampling. | Large particles from the grit pot may fall onto the filter and cause falsely high respirable dust results. |
| Equipment | Use constant-flow pumps | Use pumps that maintain stable flow when resistance changes due to filter loading or tubing restrictions. | Stable flow reduces error in the estimated air volume. |
| Sampling | Avoid using grab samples for 8-hour exposure | Use long-term integrating samples for full-shift exposure assessment. Grab samples are more suitable for short-term exposure checks. | Grab samples may not represent the full work shift exposure profile. |
| Reporting | Document sampling details | Record subject details, sampling location, method, process, duration, control measures, concentration and TWA result clearly. | Incomplete documentation weakens the reliability and traceability of the monitoring report. |
| Reporting | Attach a layout plan | Include a layout showing sampling points or the work area of mobile workers where relevant. | Sampling location context helps reviewers interpret the exposure assessment. |
| Reporting | Provide recommendations where exposure is significant | Where TWA concentration exceeds 50% of the PEL, include appropriate exposure control recommendations in the report. | The 50% PEL action level indicates that exposure reduction should be considered. |
Sampling duration should match the exposure pattern and assessment purpose.
Full-shift sampling is generally required for evaluating time-weighted average exposure. Shorter or partial-period sampling may be acceptable only where the exposure pattern and assumptions are clearly documented.
Full-shift exposure assessment
For processes with small concentration variations, the MOM guide states that a minimum sampling duration of 6 hours is required for evaluating an 8-hour shift exposure, and 8 hours for a 12-hour shift. Meal and break periods should not be counted as part of the minimum sampling time.
Partial-period exposure assessment
If a worker is exposed for less than 6 hours, sampling may cover the exposure period. The period not sampled may be assumed to have zero exposure, but this assumption must be stated clearly in the monitoring report.
Recommended number of samples
- At least 3 to 5 samples for each job classification or exposure group.
- For groups of 10 or more workers, sample approximately 25% to 50% of the group.
- For grab or spot sampling when full-shift sampling is not possible, 4 to 7 randomly timed samples may be used.
Important field observations
- Record periods of high exposure caused by specific activities or process cycles.
- Check for signs of filter or media overloading.
- Observe whether sampling devices are tampered with during monitoring.
- Document process conditions, worker activities and control measures during sampling.
Air monitoring frequency depends on the measured exposure level relative to the PEL.
The MOM guide provides a frequency framework based on the percentage of the Permissible Exposure Level. Exposure at or above 50% of the PEL is treated as an action level and should, as far as practicable, be reduced below 50% of the PEL.
| Exposure Level | Recommended Monitoring Frequency | Practical Meaning |
|---|---|---|
| Less than 10% of the PEL | At least once every 3 years | Exposure appears low, but periodic review remains necessary. |
| 10% to less than 50% of the PEL | At least once every year | Exposure is present and should be reviewed regularly. |
| 50% to 100% of the PEL | At least once every 6 months | Exposure has reached the action level and control improvements should be considered. |
| More than 100% of the PEL | At least once every 3 months | Exposure exceeds the limit and urgent control action is required. |
Reliable air monitoring depends on method selection, equipment control and sample integrity.
The MOM guide highlights many common sources of sampling error. These issues can affect the quality of results and should be controlled as part of fieldwork planning and quality assurance.
Sampling method
Use methods sensitive enough to quantify the exposure level of interest. Long-term sampling is used for long-term PEL comparison, while short-term sampling is needed for short-term PEL assessment.
Physical state
Consider whether the contaminant exists as particulate, vapour, gas or a combination. Some contaminants may require media that collect more than one phase.
Sample volume
Collect sufficient air volume for reliable laboratory quantification, but avoid overloading filters, sorbent tubes or other collection media.
Sample handling
Assemble, disassemble and pack media in clean areas. Supply blanks to the laboratory and maintain proper chain-of-custody documentation.
Sampling pumps
Use suitable pumps, calibrate them with the sampling train in-line, and consider possible electromagnetic or radio frequency interference in the work environment.
Cyclones and filters
Clean sampling devices before use, keep cyclones vertical, use the correct flow rate and do not reuse disposable plastic filter cassettes.
Passive samplers
Use validated passive samplers and avoid stagnant air conditions, particularly when passive samplers are used for static or area monitoring.
Laboratory analysis
Use an accredited laboratory or a laboratory with an effective quality assurance programme. Analytical methods should be able to detect levels below 10% of the PEL.
The monitoring report should be complete, traceable and useful for exposure control decisions.
Toxic substances monitoring reports should include workplace details, authorised personnel details, workplace exposure assessment information, analytical results, layout plans, findings and recommendations.
Key report contents
- Authorised personnel and organisation conducting the assessment.
- Workplace details, monitoring date and workplace representative.
- Toxic substances monitored and number of persons exposed.
- Sampling method, process, process duration and monitoring duration.
- Existing control measures and PEL standard used for comparison.
- Measured concentration, TWA concentration and person monitored.
- Location or label of monitoring point with layout plan attached.
Submission and follow-up points
- Analytical results should be submitted to the customer within the stated timeframe after receipt of samples.
- Monitoring reports should be provided after analytical results are received from the laboratory.
- Where the same organisation conducts monitoring and analysis, the complete report should be issued within the specified overall timeframe.
- The workplace occupier should submit monitoring results to MOM within the required period after receiving the results.
- If a sorbent tube breakthrough is identified, it should be reported and re-sampling should be conducted for affected points.
Air sampling should be designed by competent occupational hygiene professionals.
The quality of air monitoring depends heavily on the sampling strategy, method selection, field execution, laboratory analysis and interpretation of results. IEH can support employers in planning and conducting toxic substances monitoring, interpreting results and improving exposure controls.
Sampling strategy
Identify exposed groups, representative workers, sampling duration, number of samples, sample type and relevant PEL comparison.
Field monitoring
Conduct personal and area sampling using suitable media, calibrated equipment, recognised methods and appropriate quality control.
Interpretation and control
Compare results with exposure limits, identify significant exposure, recommend controls and support workplace health risk management.
Primary source used for this page.
This page is a practical web adaptation of the official guidance and should be read together with current Singapore workplace safety and health legislation, MOM guidance and applicable sampling and analytical methods.
Primary reference: Ministry of Manpower. Guidelines on Sampling Strategies and Submission of Toxic Substances Monitoring Report . Second Revision, 2022.
Key sections adapted: Sampling strategies; precautions in sampling and monitoring; toxic substances monitoring and sample analysis report; hygiene toxic substances monitoring report format; workplace exposure assessment requirements; sampling point layout plan example.
Professional note: This IEH page summarises and explains the guide in practical language. It does not replace the official guide, legal requirements, competent occupational hygiene judgement or site-specific exposure assessment.
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