Aluminium OEL: Why 'as Al' Is Missing — and Why It Matters

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 Al₂Cl(OH)₅/m³, not as mg Al/m³.

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: pulmonary effects.

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 Health Council guidance

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

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.

How to deal with this in practice

In professional and industrial workplaces, aluminium and its compounds rarely appear as a single pure substance. Welders, grinders and foundry workers are exposed to mixtures of metallic aluminium, aluminium oxide, and other compounds. Air sampling therefore typically analyses elemental 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 of 0.0123 mg Al/m³ as a starting point, not an endpoint. 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. The toxicological basis (pulmonary effects) applies primarily to soluble compounds; for insoluble compounds, other mechanisms may also be relevant (such as different toxicokinetics or lung overload from fine dust).
Document your reasoning. Record in your risk assessment which limit value you are using and what it is based on. This is essential for defensibility during inspections and occupational disease claims.

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