Aluminium OEL: Why 'as Al' Is Missing

In 2010, the Dutch Health Council (Gezondheidsraad) established a health-based recommended occupational exposure limit of 0.05 mg/m³ (inhalable dust, 8-hour TWA) for aluminium chlorohydrate. A clear number — until you try to apply it to a different aluminium compound. The recommended value is expressed as mg compound/m³, not as mg Al/m³. That distinction is nowhere made explicit, and it has direct consequences for your risk assessment.

What the Health Council report says

In report GR2010/05OSH, the GBBS Committee proposes a health-based recommended OEL of 0.05 mg/m³ (inhalable dust, 8-hour TWA) for aluminium chlorohydrate (CAS 12042-91-0). The toxicological endpoint: chronic inflammation of lung tissue.

The recommended value was derived from inhalation studies in rats and guinea pigs, in which animals inhaled the soluble salt aluminium chlorohydrate as respirable dust (Mass Median Aerodynamic Diameter 1.5 µm). In contact with water, aluminium chlorohydrate forms complex gels of aluminium oxide hydroxide that slowly convert to insoluble aluminium hydroxide. The observed toxic effects occurred primarily in alveolar lung tissue. This mechanism — a soluble salt that forms a gel in the lung and mobilises Al³⁺ — is specific to this class of compound.

For metallic aluminium and other aluminium compounds, the committee could not derive a separate health-based OEL. The report does include supplementary considerations on using the aluminium chlorohydrate value as a read-across for aluminium forms that are insoluble or poorly soluble in water — a precautionary approach as a starting point where substance-specific data are lacking.

The problem: mg compound or mg aluminium (Al)?

The recommended value of 0.05 mg/m³ was derived from studies with aluminium chlorohydrate (Al₂Cl(OH)₅). The aluminium fraction in that compound is 24.5% (Table G.1, p. 208, first study). This means the recommended value implicitly corresponds to:

0.05 × 0.245 = 0.0123 mg Al/m³

This is stated nowhere in the report. There is no “as Al” qualifier next to the limit value, nor is the conversion made explicit. That becomes a problem as soon as you want to apply the value to a different aluminium compound.

Example: aluminium oxide (Al₂O₃)

Take aluminium oxide (CAS 1344-28-1), a compound commonly encountered in workplaces. Its molecular mass is 101.96 g/mol, and the aluminium fraction is 52.9%.

If you apply the read-across value of 0.0123 mg Al/m³ and convert back to aluminium oxide:

0.0123 / 0.529 = 0.023 mg Al₂O₃/m³

That is more than a factor 2 lower than the original 0.05 mg/m³ for the compound. If you were to adopt 0.05 directly as the limit for aluminium oxide — without the conversion — you would be using a value more than twice as high as the protection level the Health Council intended.

And this applies to every aluminium compound: the conversion factor differs per molecule. There is no single standard factor.

A caveat for Al₂O₃ specifically

For aluminium oxide in particular, caution is warranted. The difference between the soluble, gel-forming chlorohydrate on which the advisory rests and a truly insoluble salt like Al₂O₃ is mechanistically large: Al₂O₃ cannot form an aluminium gel in lung tissue at physiological pH (7.0–7.4), so the critical effect underpinning the 0.05 mg/m³ value likely does not occur with Al₂O₃ in this form. The arithmetic conversion to 0.023 mg Al₂O₃/m³ is therefore primarily illustrative of the “as Al” issue — not a validated limit value for aluminium oxide.

Why this is extra difficult in practice

In the workplace, aluminium rarely appears as a single pure compound. Welders, grinders and foundry workers are exposed to mixtures of metallic aluminium, aluminium oxide, and other compounds. Air sampling therefore typically analyses total aluminium (ICP-MS or ICP-OES after digestion), not individual compounds — gravimetry is impractical at a limit of 0.0123 mg/m³ in combination with the detection limit requirements of EN 482.

This makes the “as Al” interpretation not only scientifically more logical, but also practically the only workable approach: you measure Al, you benchmark against a limit expressed as Al.

Practical recommendation

Always convert the recommended value to mg Al/m³. The implication of the Health Council’s guidance is that the protective limit is 0.0123 mg Al/m³ (inhalable dust, 8-hour TWA). Use this value as your benchmark when measuring total aluminium.
Use the read-across as a starting point, not an endpoint — and be sceptical for truly insoluble compounds. The Health Council explicitly states that the aluminium chlorohydrate value can serve as a precautionary approach for poorly soluble aluminium compounds. That is a useful starting point where substance-specific OELs are absent — but be aware of the assumptions. For truly insoluble compounds such as aluminium oxide, which cannot form an aluminium gel at physiological pH, the underlying mechanism of the chlorohydrate studies may not apply at all, and the read-across may not be appropriate. Other mechanisms (such as lung overload from fine dust) may still be relevant and require substance-specific assessment.
If useful, convert back to a per-compound limit. This can help when using exposure models that assume exposure to the molecule. Converting the 0.0123 mg Al/m³ limit (inhalable dust, 8-hour TWA) back to a per-compound limit lets you benchmark the model’s output (concentration in mg compound/m³) directly against that limit.
Document your conversion. Record in your risk assessment which limit value you are using, what it is based on, and how you converted. This is essential for defensibility during inspections.

Reference

Health Council of the Netherlands. Aluminium and aluminium compounds — Health-based recommended occupational exposure limit. The Hague: Health Council of the Netherlands, 2010; publication no. 2010/05OSH. PDF available via gezondheidsraad.nl