What is W3, and what isn't it?

On the most misunderstood concept in welding fume filtration.

Some technical specifications include the statement: “The fume extraction unit must include a W3 certified filter.”

This sentence sounds technical at first, but it contains two separate errors.

Firstly, W3 is not a filter class. Secondly, there is no single, uniform “W3 certification” in the sense everyone understands it. These two errors are so common that we see purchasing teams with years of experience in the industry, and even some manufacturers, falling into the same trap. In this article, we will explain what W3 actually means, what standard it is based on, and what exactly you should ask when a supplier tells you “our unit is W3.”

Where do W classes come from?

Welding fume separation classes are defined in the ISO 21904 standard series, which is currently in effect. (Formerly known as EN ISO 15012 — many still refer to it by this number in the market; it's an earlier version of the same family.)

The standard defines three classes based on welding fume separation efficiency: W1 at least 95%, W2 at least 98%, and W3 at least 99%.

So when is which class required?

This is determined by the material being welded. For unalloyed and low-alloy steels, W1 is considered sufficient. As the alloy content increases, W2 is used. For stainless steels with high chromium-nickel content, W3 is mandatory; because the fumes from these materials contain hexavalent chromium (Cr(VI)) and nickel oxide compounds, which are known to be carcinogenic.

So W3 doesn't mean "better filtration"; it means "if you are working with this material, less is not acceptable."

The real critical distinction: Filter efficiency ≠ System efficiency

Now let's get to the most confusing part. You often hear statements like: "Our unit has a 99.95% efficient H13 filter, therefore it's a W3." This conclusion is incorrect, and we insist on this distinction because the difference is small on paper but very large in practice.

Filter media efficiency is the performance of the filter material tested alone in the laboratory. The W class, on the other hand, is the separation efficiency of the entire unit measured according to ISO 21904-2. The unit contains a gasket, a cover, a filter housing, and body joints. It is possible for smoke to leak through any of these points instead of passing through the filter and reach the clean air side. Even if you install the best filter in the world, a unit with an improperly tightened gasket will fail the test. Let's make an analogy: You can install the highest insulation class glass in your house window. But if the window frame is mounted to the wall with a three-centimeter gap, the insulation value of the glass becomes irrelevant. That's why the W test measures the window as a whole, not just the glass.

What does "W3 certification" mean?

There's a conceptual confusion in the industry here as well. In practice, two different paths emerge. The first is the process conducted through IFA (DGUV) in Germany, which ultimately results in the awarding of the famous test mark. The second is a measurement performed in an accredited test laboratory according to ISO 21904-2 and the resulting accredited test report. Both are based on the same measurement of the same standard; the differences are who issues the certificate and in which markets it is recognized in what form. If you are on the purchasing side, our suggestion is this: instead of asking "Do you have a W3 certificate?", ask for the test report itself and look at three things. Is the laboratory that issued the report accredited? Is the model tested the same as the model offered to you? And does the report cover the filter media or the entire unit? The answers to these three questions say much more than a glossy brochure page.

W3 label

The standard doesn't just measure efficiency.

One final point. ISO 21904-1 imposes other requirements besides separation efficiency that most people don't think of. One of these is a mechanism that warns the user when the airflow falls below the minimum value. Because a unit operating with a clogged filter, even if it appears to be working, cannot capture the smoke. Another important aspect is that plastic parts such as hoses and hoods must meet certain flame resistance criteria. You will agree that there is no need to explain why this is important in an environment where welding sparks enter the suction line.

So W3 is not just a single filter label; it is the name of a whole system that extends from the design of the unit to the material selection, from the gasket to the warning system. In our next article, we will return to the question at the heart of this whole system: Why do ePTFE membrane cartridges, which provide the same efficiency in a single stage, make a difference compared to the classic two-stage architecture?