In brief
Annex 1 of the Regulation on Chemical Agents gives, for each substance, an 8-hour time-weighted average (TWA) and a 15-minute short-term (STEL) limit value in mg/m³ and ppm; for some substances the "Skin" notation indicates absorption through the skin. When comparing a measurement result with these values, the shift length, short-term peaks and other chemicals used at the same time are taken into account. When a limit value is exceeded, the employer takes protective and preventive measures to remedy the situation as soon as possible; the risk assessment and health surveillance are updated.
What is an occupational exposure limit value?
Imagine a measurement report that says "toluene: 157 mg/m³". Is this value good or bad? The answer depends on which limit value, over which period and under which conditions the report compares it with. An occupational exposure limit value is defined in Art. 4 of the Regulation on Health and Safety Measures in Work with Chemical Agents as the upper limit of the time-weighted average concentration of a chemical in the air within the worker's breathing zone over a specified reference period (TWA, STEL), or the upper limit that must not be exceeded at any moment of the working time (CEILING).
The breathing zone in the definition is also defined in the regulation: the front half of a sphere with a radius of 30 cm centred on the midpoint of the line joining the two ears. In other words, the limit value is based not on the average air in the area but on the air the worker breathes. For the general framework of the regulation, see our article on the Regulation on Chemical Agents.
How to read the Annex 1 table
Annex 1 of the regulation, renewed on 20 October 2023, gives one substance per row with the following columns:
| Column | Meaning |
|---|---|
| EINECS and CAS | Numbers that identify the substance precisely; match them with the CAS number on the safety data sheet |
| TWA (8 hours) | The time-weighted average measured or calculated for an 8-hour reference period |
| STEL (15 minutes) | The upper value not to be exceeded over a 15-minute period unless another period is specified |
| CEILING (ceiling value) | The value that must not be exceeded at any moment of the working time |
| mg/m³ and ppm | Two units for the same limit; the mg/m³ value is given for 20 °C and 101.3 kPa |
| Special notation "Skin" | Indicates that significant amounts of the substance can also enter the body through the skin |
Some examples from the table (values taken from Annex 1):
| Substance | TWA (8 hours) | STEL (15 minutes) | Special notation |
|---|---|---|---|
| Toluene | 192 mg/m³ – 50 ppm | 384 mg/m³ – 100 ppm | Skin |
| Xylene (mixed isomers) | 221 mg/m³ – 50 ppm | 442 mg/m³ – 100 ppm | Skin |
| Methanol | 260 mg/m³ – 200 ppm | Not given | Skin |
| Ammonia (anhydrous) | 14 mg/m³ – 20 ppm | 36 mg/m³ – 50 ppm | None |
| Hydrogen sulphide | 7 mg/m³ – 5 ppm | 14 mg/m³ – 10 ppm | None |
| Chlorine | Not given | 1.5 mg/m³ – 0.5 ppm | None |
Two rows are particularly instructive. Methanol has no short-term value; the assessment is made using the 8-hour average. Chlorine, conversely, has only a 15-minute value, because chlorine does harm through short-term peak exposures. A value missing from a substance's row does not mean "unlimited" for that period; the assessment is made using the value given in the table.
How to convert ppm to mg/m3
Instruments measuring gases and vapours usually report in ppm, while laboratory reports mostly give mg/m³. Since the mg/m³ values in Annex 1 are calculated for 20 °C and 101.3 kPa, the volume of one mole of gas under these conditions is approximately 24.04 litres and the ppm to mg/m3 conversion uses the following formula:
- mg/m³ = ppm × molecular weight / 24.04
- ppm = mg/m³ × 24.04 / molecular weight
Examples:
- Toluene (molecular weight 92.14): 50 ppm × 92.14 / 24.04 ≈ 191.6 mg/m³. This matches the 192 mg/m³ in the table.
- Ammonia (17.03): 20 ppm × 17.03 / 24.04 ≈ 14.2 mg/m³. The value in the table is 14 mg/m³.
- Hydrogen sulphide (34.08): 5 ppm × 34.08 / 24.04 ≈ 7.1 mg/m³. The value in the table is 7 mg/m³.
Some instruments and sources use 25 °C; the divisor then becomes 24.45 and the result changes slightly. So use the unit in the table directly for comparison, and if you convert the unit on the instrument, state in the report which temperature you have used.
8-hour limit value and short-term exposure limit: a worked example
The 8-hour limit value is the average exposure over the whole shift; periods without exposure are counted as zero and the total is always divided by 8 hours. Let the toluene exposure of a paint mixing operator be as follows (values are illustrative):
| Period | Duration | Measured toluene |
|---|---|---|
| Mixing | 2 hours | 240 mg/m³ |
| Can filling | 4 hours | 150 mg/m³ |
| Meal break | 1 hour | 0 |
| Tank cleaning | 2 hours | 90 mg/m³ |
8-hour TWA = (2 × 240 + 4 × 150 + 1 × 0 + 2 × 90) / 8 = 1,260 / 8 = 157.5 mg/m³. This value is below 192 mg/m³, but has reached about 82 per cent of the limit.
If, on the same day, a 15-minute measurement taken when the lid was opened during mixing came out at 420 mg/m³, the short-term exposure limit of 384 mg/m³ has been exceeded. So a satisfactory daily average does not cancel out a short-term exceedance; both values must be met separately.
Take care if the shift is longer than 8 hours: for a worker exposed to 160 mg/m³ of toluene for 10 hours, the value is (10 × 160) / 8 = 200 mg/m³ and the limit is exceeded, even though each individual measurement result is below 192. This calculation makes the effect of extended working hours on chemical exposure visible.
Points to watch when comparing a measurement result
Art. 6 of the regulation requires the risk assessment to take into account the type, level and duration of exposure determined by a laboratory authorised under the Regulation on Occupational Hygiene Measurements, Tests and Analyses; Art. 7 also requires measurements to be carried out regularly and repeated when conditions affecting exposure change. We plan measurements with authorised laboratories and healthcare facilities and enter the results into the risk assessment. When reading the report, check the following:
- Personal or area sampling? What is compared with the limit value is the personal measurement taken from the breathing zone with a sampler clipped to the worker's collar.
- Representativeness: Was the measurement taken on the day and shift when the work is most intensive? For the measurement strategy, the TS EN 689 standard provides for grouping workers with similar exposure and deciding on the basis of more than one measurement.
- More than one chemical: Art. 6 of the regulation requires each of the substances used together, and their interactions, to be assessed. For substances with similar effects, the sum of the ratios is used in practice: adding the toluene in the example (157.5 / 192 ≈ 0.82) to the xylene in the area (66 / 221 ≈ 0.30) gives a result above 1.
- The "Skin" notation: For substances such as toluene, xylene and methanol, air measurement may understate total exposure; glove selection and, where necessary, biological monitoring become important.
- Foreign values: The safety data sheet may also show values from other countries or bodies. The value applied in Türkiye is the one in Annex 1; Annex 2 of the regulation contains a biological limit value for lead. For how to read the sheet, see our article on the safety data sheet.
What to do when a limit value is exceeded
Art. 7 of the regulation is clear: in every case where a limit value is exceeded, the employer takes protective and preventive measures to remedy the situation as soon as possible. In practice, follow this order:
- Immediately: Identify the work and the people affected by the exceedance; until a permanent measure is in place, introduce suitable respiratory protection and an arrangement that shortens exposure time.
- At source: Consider replacing the substance with a less hazardous one, using a closed system and reducing the time lids are left open. Our article on the hierarchy of controls guides prioritisation.
- Ventilation: Install local exhaust ventilation close to the source or have the flow rate of the existing system measured.
- Health surveillance: The workplace physician assesses the exposed workers; Art. 12 of the regulation requires the risk assessment to be reviewed and other exposed workers to be checked if a biological limit value is exceeded or a health effect related to the substance is observed.
- Re-measurement: The measurement is repeated after the measure and the result is entered into the file.
- Risk assessment: Measurement results are among the situations requiring the risk assessment to be renewed regardless of its validity period.
The 2026 basic administrative fine for failing to have the required measurements and inspections carried out is 33,326 TL; for details see our OHS administrative fines tool. For common situations in work involving solvents, see solvent vapour exposure, and for sector-specific topics, our chemicals and plastics page.
Frequently Asked Questions
What is the difference between TWA and STEL?
TWA is the time-weighted average over an 8-hour shift. STEL is the upper value not to be exceeded, usually over a 15-minute period. Even if the average is satisfactory, short-term peaks can exceed the STEL; both values must be met separately.
How is a ppm value converted to mg/m³?
Since Annex 1 is based on 20 °C and 101.3 kPa, the formula mg/m³ = ppm × molecular weight / 24.04 is used. For example, 50 ppm of toluene is about 192 mg/m³.
Does the limit value change if the shift is 12 hours?
The limit value does not change, but because total exposure is divided by 8 in the 8-hour average calculation, the result rises with a long shift. Even if each measurement is below the limit, the average over a long shift can exceed it.
Is action needed if the measurement is just below the limit value?
Yes. Measurement results carry uncertainty and exposure varies from day to day. For results close to the limit, the right approach is to plan measures that reduce the source and to repeat the measurement.
A risk assessment that identifies and scores hazards on site, links each measure to an owner and a date, and has a clear renewal schedule.
Bursa SİSTEMER OSGB Technical Team — written from the field experience of our OHS specialists and workplace physicians. Legal information is valid as of the publication/update date; contact us for an assessment specific to your business.