What makes a quality pm2.5 filter?

To be effective, a carbon PM2.5 filter needs the right ratio of high-quality materials.

PM2.5 filters come in several shapes and forms, but understanding what differentiates a high quality certified pm 2.5 filter from a low quality pm 2.5 carbon filter requires a close look of both the material used as well as the filtration capabilities. Let's start with the material, and then talk about filtration. In the world of 2.5 filtration, size matters, so we will also discuss why the name "pm2.5" refers to micrometers, despite the filters being able to block much smaller particles, as small as the particles stopped by the "N"-class respirators.

So what is a pm2.5 filter made of?

A quality carbon PM2.5 filter is made up of three non-woven materials: melt-blown fabric, spun-bond fabric, and activated carbon fabric. Melt-blown fabric is the most important part of a filter. Non-woven fibers act as a physical barrier against microscopic particulates while also providing an electrostatic barrier to stop aerosolized nano-particles. Spun-bond is a non-woven fabric that filters out larger particles, while providing structure to the inner melt-blown layers. Activated carbon fabric traps particulates via a process called adsorption, and also serves as an antibacterial and deodorizer. The most effective pm2.5 filters are about 40% melt-blown, 20% activated carbon, and 40% spun-bond.

What kind of materials do you see in low-quality filters?

The surest sign of a shoddy pm2.5 filter is one that replaces the melt-blown fabric with spun-bond. Spun-bond fabric is great for structure and moisture resistance, and provides a limited degree of filtration efficiency, but it doesn't have the same electrostatic charge and highly dense structure of melt-blown. You can recognize spun-bond by the fact that it's somewhat shiny, adentured (pocketed), and a bit stiff. Melt-blown, on the other hand, is almost like highly condensed cotton candy in fabric form. It's soft to the touch, almost like tissue paper but much more dense, matte in appearance, and malleable. Melt-blown also absorbs moisture much more quickly than spun-bond, so applying a bit of water and watching how fast it gets absorbed is a good way to tell the difference.

Some filters will also skip the activated carbon layer. This doesn't necessarily mean the filtration is worse, but you will lose the benefit of the anti-bacterial properties the carbon provides, along with the odor-fighting power. We've heard that activated carbon also acts as a filter of its own for the plastic micro-particles that escape from the melt-blown layers, although with a structure that encases the melt-blown in spun-bond a similar function may be achieved (we're really in geek-land now, aren't we?).

How are the materials arranged?

As Ella Fitzgerald says, "it's not [entirely] what you do, it's the way that you do it." As you can see from the picture above, a pm2.5 filter should encase the inner melt-blown fabric in layers of stiff spun-bond. This improves the structure of the filter, while also providing a barrier between your lungs and loose melt-blown threads that might separate from the inner material. The carbon layer typically sits between the two spun-bond layers. If your filter is darker on one side, we recommend using that side toward your mouth (due to the anti-microbial properties we mentioned above).

Why are they called "PM 2.5" filters?

The PM2.5 terminology for the filters refers to the fact that they were introduced as a mitigant against air pollution, and since the smallest term to refer to air pollutants is PM2.5 (2.5 micrometers) that's the name they ended up with. This causes some people to erroneously assume that a pm2.5 filter is ineffective at protecting against viral transmission, since virus particles and the fine-aerosols they tend to travel with are measured in nanometers (a nanometer is 1/1000 of a micrometer). 

How do PM2.5 filters compare to medical-grade respirators?

In fact, pm2.5 filters are tested to the same size particulates as medical respirators like the n95, kn94 and kn95 (the testing media is around 75 nanometers, the same standard used by the new CDC/ASTM F3502 standards). Of course, a pm2.5 filter without a mask isn't going to do you much good, so you have to look at how they perform with the type of cloth "pm2.5" masks for which they're designed.

When used with a fitted cloth face mask made to pm2.5 specifications, a carbon pm2.5 filter is capable of achieving up to 90% filtration in the critical fine-aerosol range. This is just a few percentage points away from a medical-grade respirator, but with far less breathing resistance (learn more).

How long can I use a carbon filter before it loses effectiveness?

Remember, a certified pm2.5 filter provides both mechanical and electrostatic protection. Static energy requires space to be effective, so the filters don't lose effectiveness so much as become clogged over time. We recommend using them for a total period of between 16-24 hours of active use, and around 40 hours of seated use. Store unused filters in a dry, dark place to maximize shelf life.

Questions about your filter? Contact info@cleanairresources.com and we'll do our best to answer them. Be well and stay healthy!