What W3 Is — and Isn't

On the most misunderstood concept in welding fume filtration.

Some technical specifications include a line like: "the fume extraction unit must include a W3-certified filter." That sentence sounds precise, but it contains two separate errors: W3 is not a filter class, and there is no single, universally uniform "W3 certificate" in the way the phrase is often used. This article explains what W3 actually means, what standard it's based on, and what to ask a supplier who says their unit "is W3."

Where the W Classes Come From

Welding-fume separation classes are defined in the ISO 21904 series (formerly EN ISO 15012 — some in the market still refer to that older number). The standard defines three classes by separation efficiency: W1 at least 95%, W2 at least 98%, W3 at least 99%.

What Determines Which Class Is Needed

The standard itself doesn't dictate which class must be used for which material — it only defines and tests the classes. In practice, the choice of class is driven by national workplace regulation, the employer's own risk assessment, or contractual specification, and it typically tracks the hazard classification of the fume involved. Fumes from high-chromium-nickel stainless steel contain hexavalent chromium and nickel oxide compounds, both recognised carcinogens, which is why W3 is commonly specified or required by regulation for that work — but that requirement comes from hazard-classification and workplace-exposure rules, not from the ISO 21904 series mandating a class by material.

The Real Distinction: Filter Media Efficiency ≠ System Efficiency

A common but incorrect claim is: "our unit has a 99.95%-efficient H13 filter, so it's W3." Filter media efficiency is the performance of the filter material tested alone in a lab. The W class is the separation efficiency of the entire unit, measured per ISO 21904-2 — gasket, cover, housing, and body joints included. Fume can leak past any of those points without ever passing through the filter. An analogy: installing the highest-insulation-class glass in a window doesn't help if the frame is mounted with a gap around it — the standard tests the whole window, not just the glass.

What "W3 Certification" Actually Means

Two different pathways exist in practice. One runs through IFA (DGUV) in Germany and results in a recognised test mark. The other is a measurement performed by an accredited test laboratory under ISO 21904-2, resulting in an accredited test report. Both are based on the same underlying standard and measurement; the difference is who issues the documentation and how it's recognised across markets. On the buying side, it's more useful to ask for the test report itself and check three things: is the issuing laboratory accredited, is the tested model the same one being offered, and does the report cover the filter media alone or the complete unit.

Conclusion

W3 is a documented, accredited-laboratory-tested system class — not a cartridge's theoretical capacity, a vendor's brochure claim, or a requirement written into the ISO standard itself by material type. The reference that matters is the test report issued for that specific configuration, and which class is actually required for a given job is a separate question governed by hazard classification and applicable regulation.

Frequently Asked Questions

I fitted a W3-labelled cartridge — is my unit now W3? — No; the classification rests on an accredited test report for the whole system. Does the ISO standard require W3 for stainless steel? — Not directly; the standard defines and tests the classes, while the requirement to use W3 for a given material typically comes from workplace regulation or risk assessment.

References

●        ISO 21904-1:2020 and ISO 21904-2:2020.

●        EN ISO 15012 (predecessor standard family, referenced for context).

●        IARC Monographs — hexavalent chromium and nickel compounds (carcinogen classification, referenced for why W3 is typically specified for stainless steel work).