Sulfuric Acid Exposure Limits: EU IOELV, TRGS 900 and UK WEL Compared. Sulfuric acid (CAS 7664-93-9, H₂SO₄, molecular weight 98.08 g/mol) is processed in alkylation units, metal pickling lines, anodising tanks, and battery formation. The concentrated grade has a density of approximately 1.84 g/cm³ and a boiling point near 337 °C. Exposure in normal ambient conditions is predominantly to acid mist or aerosol rather than vapour, because the vapour pressure of sulfuric acid is low. The three regulatory exposure limits compared here differ not only in numerical value but also in particle size-selective convention and short-term excursion rules.

Strong inorganic acid mists containing sulfuric acid are classified by the International Agency for Research on Cancer as Group 1. The respiratory endpoint is local irritation and corrosion of the upper airways and bronchial epithelium. The EU indicative occupational exposure limit value and the UK Workplace Exposure Limit are expressed against the thoracic fraction defined in EN 481:1993, while the German TRGS 900 Arbeitsplatzgrenzwert is expressed as the inhalable fraction with notation E. The thoracic convention has a 50% penetration at an aerodynamic diameter of 10 µm; the inhalable convention has a 50% penetration at 100 µm. Direct numeric comparison between a 0.05 mg/m³ thoracic limit and a 0.1 mg/m³ E limit is therefore not technically valid without knowledge of the workplace aerosol size distribution.

EU IOELV and UK WEL Align at 0.05 mg/m³ Thoracic Fraction

Commission Directive 2009/161/EU, implementing Council Directive 98/24/EC, lists sulfuric acid mist with an 8-hour time-weighted average of 0.05 mg/m³ measured as the thoracic fraction and a 15-minute short-term limit of 0.1 mg/m³ measured as the thoracic fraction. The EU value is indicative and functions as a minimum harmonised benchmark. The UK Health and Safety Executive publishes the same numerical values in EH40/2005 under the Control of Substances Hazardous to Health Regulations 2002. The UK 8-hour TWA is 0.05 mg/m³ thoracic fraction and the 15-minute STEL is 0.1 mg/m³ thoracic fraction. In both EU and UK frameworks, the short-term value is twice the 8-hour value. A process averaging 0.04 mg/m³ may still fail if a 15-minute task such as manual transfer from a hot acid bath exceeds 0.1 mg/m³.

The thoracic-fraction basis reflects the deposition zone for acid aerosol injury: the larynx, trachea, and bronchial tree. A thoracic sampler excludes extrathoracic material that deposits in the nasal passages but does not contribute to lower-airway damage. An inhalable sampler includes that extrathoracic material. In acid mist generated by gas evolution at anodes or by hot bath agitation, the droplet size is polydisperse and shifts with bath temperature, current density, acid concentration, and surface tension. The relationship between inhalable and thoracic mass is therefore site-specific. A fixed conversion factor between E and thoracic fractions can misclassify compliance.

Why Does the TRGS 900 AGW Use the Inhalable E Fraction?

The German TRGS 900 Arbeitsplatzgrenzwert for sulfuric acid is published as 0.1 mg/m³ E, where E denotes the einatembare Fraktion defined in EN 481:1993. The value is binding under the Gefahrstoffverordnung. The peak limitation Kategorie I with excursion factor 1 applies because the endpoint is local irritation. The 15-minute average concentration must not exceed 0.1 mg/m³ E. Since the numerical value is twice the EU/UK thoracic-fraction TWA, the apparent difference is partly a sampling convention artifact. For acid mist below 10 µm aerodynamic diameter, the inhalable and thoracic fractions converge, and the German AGW then permits approximately twice the thoracic mass concentration allowed in the EU and UK. For coarse droplets above 10 µm, the inhalable sampler collects substantially more mass than a thoracic sampler, and the numeric gap may narrow or disappear.

Production-scale electrolytic and pickling operations illustrate this interaction. Anode gas bubble bursting in electrowinning cells can release fine acid mist below 10 µm, where inhalable and thoracic sample masses are close. Open steel pickling at acid temperatures above 60 °C produces larger droplets through splashing and bubble collapse, so the inhalable sample can exceed the thoracic sample by a site-specific factor. Local exhaust ventilation designed according to the ACGIH Industrial Ventilation Manual should maintain tank-edge capture velocity between 0.25 m/s and 0.5 m/s for open surface tanks. Failure to hold that boundary during manual rack transfer can generate personal exposures above the EU/UK STEL even when the full-shift average remains below 0.05 mg/m³.

Sampling and Analytical Standards for Compliance Assessment

Compliance demonstration under all three frameworks should follow EN 689:2018 for exposure measurement strategy, including definition of similar exposure groups, basic characterisation, and re-measurement. Sample collection for sulfuric acid mist uses a sampling pump connected to the correct particle size-selective inlet: a thoracic sampler for EU/UK assessment and an inhalable sampler for TRGS 900 assessment. Common personal inlets operate at flow rates near 2.0 L/min. An 8-hour sample at 2.0 L/min yields 0.96 m³ of air; at 0.05 mg/m³, the collected acid mass is 48 µg. This mass is readily quantifiable by ion chromatography, but filter extraction must be controlled to avoid sulfate salt artifacts.

ISO 21438-1:2007 specifies the determination of non-volatile inorganic acids, including sulfuric acid, in workplace air by ion chromatography. The sample filter is extracted in deionised water and analysed with suppressed conductivity detection. Method reporting limits below 1 µg sulfate per sample are achievable. Interferences from phosphate, chromate, and nitrate are resolved chromatographically. EN 482:2021 sets general performance requirements for measurement procedures used to evaluate chemical agent exposure; for comparison against an 8-hour limit value, the relative expanded uncertainty should not exceed 30%. In the United Kingdom, MDHS 14/4 provides gravimetric sampling procedures for inhalable aerosols at a flow rate of 2.0 L/min.

STEL sampling must be collected over 15 minutes during the highest expected exposure period identified from process observation. Sulfuric acid mist concentrations can spike during tank charging, rack withdrawal, filter press cleaning, and transfer of hot acid. These tasks must be sampled separately rather than averaged over a full shift. The STEL value of 0.1 mg/m³ applies to the thoracic fraction in the EU and UK; in Germany the peak limitation is a 15-minute average of 0.1 mg/m³ E. A short-duration sample collected for less than 15 minutes must be expressed as a 15-minute average before comparison with the short-term limit.

Where acid mist is generated in electroplating and metal pickling, continuous exposure records from personal samplers have shown that fugitive emissions at tank edges are the dominant source of exceedance of the 0.05 mg/m³ thoracic fraction criterion. In these operations, the EU/UK STEL of 0.1 mg/m³ is often the binding constraint for manual crane or hoist transfer because the 15-minute task average can exceed the limit even when the full-shift average remains below 0.05 mg/m³.