I am looking at home air purifiers and I realize I have no idea how to properly rate them. How do I tell what I am looking at?
Also, any recommendations?
Are you refering to whole home purifiers, or portable room only ones? The problem with portables is they are easily overwhelmed by the volume of air that a furnace moves. Also having dozens of openings in a home does not help any.
Consuler Reports has consistently said that all HEPA air filters are good and that you should get one that’s the correct size for the room it’s in.
Do you mean Consumer Reports? Because they apparently just did a story on them not too long ago. BTW, the “ozone emitting” air purifiers are about to be banned by California because they do more harm than good.
This is a problem I’ve been interested in for a long time, having about 17 years experience in industrial and cleanroom air filtration, ad/absorption, and contamination control, and also having chronic pulmonary disease and allergies. Plus, 4 big air purifiers installed in my house and the ductwork set from the movie Brazil.
Air purifiers that have a particle filter will remove particles. If you multiply their air flow by their capture efficiency over 100, you will get the decay rate of particles in air volume per time. Many of these devices use HEPA filters, which have efficiencies of 99.97% for particles that are 0.3 um in size (which is a predominant size in smoke and is much smaller than pollen or lint or fibers from clothing). Their efficiency is higher for larger particles. Interestingly, their efficiency is also higher for particles smaller than 0.3 um, because these particles diffuse around so rapidly and always stick to whatever they touch. The story of air filtration particle capture is a fascinating one - look around for impaction, interception, and diffusion as the three big capture mechanisms. An excellent book is Aerosol Technology by W. C. Hinds (“aerosol” in this context means the scientific term, particles suspended in air, and is not a reference to propellant can packaging of products - they stole our word). I had a great deal of involvement in this world years ago, and in fact wrote part of the “Recommended Practice” for testing ULPA filters (the next step up from HEPAs, being 99.999% at 0.1 um), published by the Institute of Environmental Sciences.
Many of these air cleaners also have absorbent filters, typically using charcoal to sorb a broad spectrum of nuisance gasses. If you use one of these, the air will smell better. But you want a few pounds of charcoal in there - ten or so. Many devices put a tiny bit of charcoal on some kind of nonwoven backing, and its capacity is exhausted in minutes or maybe hours. You can also get other kinds of sorbents to grab things the charcoal misses. A good choice to add to charcoal is potassium permanganate on an alumina substrate. This is good at absorbing amines, for example, which are common irritants that charcoal isn’t so good at.
There are other nifties, too, like ultraviolet lamps to kill bacteria. I suspect these are overkill because filters are already going to stop the bacteria.
I have bought several filters from AllerAir, especially from their 5000 series. These are big and well made and have large HEPA and carbon filters. They cost about $800. I have no ties whatsoever with them - I just buy and like their products.
The field of knowledge that involves these devices is “indoor air quality”, and there are a few good books out there. Also, the Harvard School of Public Health has excellent courses, as does the Mechanical Engineering school of the University of Minnesota, especially those taught by Ben Liu’s group - they’re pretty widely acknowledged to be the world leaders in aerosol technology. There have also been good courses offered by the University of Arizona’s Electrical Engineering department (because contamination control is critical to the manufacture of integrated circiuts). Also, the Lawrence Livermore Laboratories have had critical expertise in the area in the past, having to do with processing nuclear materials safely. I’ve visited and worked with all these groups. There’s lots of stuff out there.
Hope this is useful…
Thanks. What about those ‘ionic’ things?
Tit’s on a boar, in terms of effectiveness.
What’s the word about the electrostatic precipitator types (NOT the ionic breeze types) like the Trion/Fedders ones?
I think that those don’t really work, even though the principle’s sound. IIRC, someone posted in a thread that they use similar technology to that used in cleaning powerplant emissions, but because the units aren’t large enough, they don’t do anything useful. I’ll fully admit that I could be wrong on this, however.
Yep. CR. And they do about an article a year on this. Get a good HEPA unit. I suggest one from your local store that also sells the filters, such as Target.
The manual to my electrostatic precipitator (it calls itself ionic, but it’s not like an ionic breeze) says, essentially, “if the filter gets dirty and becomes ineffective, the charged particles will stick to your furniture and be very hard to clean off.” I thought that was very funny.
I like how my unit works, but I guess it’s possible it’s all due to its traditional filter. I dunno… it’s excellent at cleaning smoke, but it doesn’t say it’s HEPA. The filter is washeable, not one-time-use.
btw, are ozone-emitting purifiers the ones that make air smell like after a thunderstorm? I really want one of those. Are the ozone levels they produce actually harmful, or are people just in a mood for bitching? I’d be surprised if it wasn’t the latter.
The ozone emitters are the ones which claim to make the air smell like it’s post-thunderstorm. If you’ve got any kind of respiratory problems, you have to stay the hell away from them.
Electrostatic precipitators really work, but I don’t think they are a good choice for homes because they are often designed too small for the application, and because they can emit ozone, and because you need to clean them very frequently. I think if you installed quite a big one and ran its plates through the washer daily, and they had managed to keep its ozone emissions quite low, then it could work almost as well as a much cheaper filter.
An interesting beast in between these is the electrically enhanced filter, such as the Filtrete brand electret filters. 3M bought Filtrete technology from, I think, a Dutch company. They would start with polypropylene film. I think it was isotactic polypropylene, meaning that the side CH3 groups are all on the same of the polymer backbone. They would heat and stretch this film while passing it through a strong electric field, so that the molecules would tend to rotate about their axes so most of the CH3 groups in the entire sheet are aimed at one side of the sheet. They’d then cool the sheet while maintaining the field, to freeze this orientation in. Then they shred the sheet to make staple fiber, and turn this into a felt. There are zillions of strong, irregular electric fields in this batting. Charged particles will zip one way or the other in these fields and get caught, by what’s called Coulombic electrophoresis. Even the smaller, uncharged particles get caught, not because there are fields but because they are irregular (that is, not only does the first spatial derivative of potential vary, but so does the second). This is called dipolar electrophoresis. Filtrete filters, and electret filters in general, really work. They take less energy to push air through for the same capture efficiency. That said, their failure mode is not to clog, it’s to let particles through (though this depends on the size distribution they get loaded with).
Negative air ions are said to be very pleasant and helpful, and positive ions (like typify certain winds like the Santa Ana in the American southwest, the Scirocco (in Israel, I think) and the Foehn (in Geneva) are said to be the opposite. (I hope I remember those winds correctly). But this could also be snake oil. I think the evidence is spotty and there are a few folks trying to make money off it, which always muddies things. I don’t know what to think. But the ion effects with particles are cute and interesting. They don’t figure that heavily in mainstream efforts to catch particles, though.