Cheap air filters for the broke?

The reason is the left Dylos “PRO” channel (which they assume are using, although our app also supports the non-PRO version, which has different channel calibrations but should still provide good conversion to US EPA AQI equivalents when used with our app) measure 0.5-2.5 microns, and is intended for PM2.5, while the rightmost channel measures 2.5 microns and up, and, when summed with the left channel, measures PM10. (So above I said they called the PM10 channel the PM2.5 channel. In reality, the right channel is not the PM10 channel, but combining the two channels together mathematically, as our app does, lets you measure PM10.)

Here’s the reason. Most of the traditional government and health data was collected using much older technology, and this measures particles SMALLER than the size using a collection filter. So PM10 is everything SMALLER than the 10 micron collection filter that was used, including PM2.5. The new technology (particle counters like the ones supported by our app) measures everything in a range or LARGER than a certain size, so the 2.5 channel (right Dylos PRO channel) measures everything LARGER than 2.5 microns out to 10 microns, and thus is intended to help measure PM10 (by combing the two channels together to create a 0.5-10 channel). It is not currently cost-effective to go below 0.5 microns (or 0.3 microns, really) with a home laser particle counter, but by measuring 0.5-2.5 you can statistically estimate what 0-0.5 microns looks like from the 0.5-2.5 reading, and thus estimate both PM2.5 and PM10 from the two channels.

If you use our AQcalc iOS app this will becomes clear. Download it here.

The reason this is important, of course, is that PM2.5 is generally considered much more dangerous than PM10. (Most people have a DIY PM10 filter called “their nose” that seems to work OK. 🙂 This is a bit of an exaggeration — PM10 eventually becomes dangerous — but if you mixed up PM2.5 readings and used them as PM10 readings as they do on their webpage and video you would get a false sense of security. )

Looking at their numbers, it becomes pretty clear something is very wrong for Beijing! They show their make-shift duct-tape air purifier supposedly getting the PM2.5 numbers down to zero right away. I assure you this is really, really hard in Beijing! Of course, once you realize they’re actually talking about PM10 when they are saying PM2.5, it becomes clear what is going on.

Defending traditional manufacturers

Let me put in a defense for the “expensive” air purifier manufacturers:

• Most put a cheap “pre-filter” in front of the expensive HEPA filter. This greatly increases the life of the HEPA filter. The pre-filter usually also includes a charcoal filter that helps against VOCs and odors. This is a huge selling point with consumers; you see VOCs and odors being stressed again and again in the marketing for these units. We tend to agree with many consumer testing labs that odors aren’t dangerous and VOCs are really hard to get rid with an air purifier, so the only thing that matters are PM2.5 and PM10. But apparently consumers demand purifiers also eliminate odors.
• Consumer Reports and other testing labs don’t look at unit cost, they look at cost of operation. If you don’t have a pre-filter, your HEPA filter won’t last very long. The cost of this cheap unit is mostly the HEPA filter. So you might have to constantly replace the HEPA filter, resulting in a high cost of operation. Penny wise, pound foolish?
• This is a kit that uses duct-tape. The consumer-ready air purifiers undergo testing by labs such as UL to ensure they won’t catch fire due to the fan overheating and that sort of thing. That’s not possible when you’re selling a kit where the consumer has to supply their own fan.
• Consumer Reports and other testing labs looked at lot of other factors, such as how much air the unit moves, how quiet the unit was, etc. Here they put the Dylos right in front of the unit, but it’s not good if the room air doesn’t get clean quickly. (And in their charts they show it taking a long time if you look at PM2.5, which they’ve mis-labelled “PM0.5”) I suspect Consumer Reports would not give these air purifiers a good grade on anything except their upfront cost.
• Duct tape is a little cheap as a manufacturing process, and the resulting unit might be a little bit fragile even if the fan doesn’t overheat. Most consumers demand better.

We’ve discussed the PM2.5 and PM0.5 mislabeling on their website with the Smart Air folks, and they’ve acknowledged there may be some issues with their blog information. (They admit elsewhere on their blog that they’re just a small start up and aren’t experts in air quality or anything like that. They seem to have sold out of their kits, so it looks like they have a hit product.)

That being said, for people in developing countries who absolutely can’t afford a full-blown air purifier, desperately need protection from PM2.5 pollution, don’t mind risking something that’s not UL safety-tested, and don’t like respirator masks and don’t want to make an air quality fashion statement these cheap air purifiers may be a good solution.

Note: We are not affiliated in any way with Smart Air Filters.

Photo Credits: taxcredits.net/Flickr/CC-BY

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