Ignoring the part about radon, but I assumed an air purifier would filter out decay products from the air. So you're saying instead it will detach decay products or prevent them from being attached? Seems totally counterintuitive to me but if you or anyone else could point me to that research I'd be interested to see it.

Bill, I haven't read the original research, but I believe Dave Wilson's name is on the training material that speaks to an increase in the unattached fraction, and I have discussed this topic with him in the past. If I remember correctly, he has been involved in some RDP filtering research.

Looking forward to the experts' commentary here, but in the meantime, a few items for the discussion:

Here is one study from 1990 that describes the relative effectiveness of air cleaning techniques, but bear in mind the constraints imposed by any experimental conditions lest one generalize too easily to real-world conditions: https://www.osti.gov/servlets/purl/6155991

This document https://www.epa.gov/sites/default/files/2018-07/documents/residential_air_cleaners_-_a_technical_summary_3rd_edition.pdf does include the statement "Note that EPA does not recommend air cleaning to reduce the health risks associated with radon and radon progeny." More detail is provided in that document.

The bottom line appears to be that although some air cleaning methodologies may, under certain conditions, reduce either radon itself (via use of adequate adsorbent media) or the PAEC of its airborne progeny (via HEPA filtration), the degree of reduction cannot be relied upon to be tantamount to effective mitigation via more classical means (e.g., SSD).

And even in those cases where PAEC is reduced somewhat as a result of lowering the concentration of airborne particulate matter (via removing those particles carrying radon progeny--the attached fraction), the result is likely to be an increase in the unattached fraction that has been understood for some time to pose a greater health risk: https://aarst.org/proceedings/1991/1991_02_Overview_of_The_Unattached_Fraction_of_Radon_Progeny.pdf

Kevin I think i agree with what your previous post has said. Let me try to say this in a less precise way.
Hepa filters do not (in my opinion) reduce radon in the air i believe that the radon atom is to small to be trapped by the filter media. What it does do is catch the airborne particles that radon daughters would attach to thereby raising the unattached fraction in the air of radon daughters leading to higher health risk ( more unattached fraction to be breathed in that could attach to airway or lung)
i cannot point to a research paper that can prove this hypothesis, but if not it would make for a great thesis topic.

Yes, I agree. That captures the understanding well, I think.
I don't know the efficiency and capacity of typical adsorbent media (e.g., activated carbon) at removing radon itself, but it would be interesting to see a calculation showing how much would be needed to be reasonably effective over a long period of time. My guess is "lots" and that it would be cost-prohibitive.

The other side of a carbon based capture system is that it will build up with lead 210 and all the other short lived radon daughters (progeny for all you proper people) giving off gamma every time there was a radioactive decay the vessel holding the carbon may become a larger heath risk, from gamma exposure, than the radon health risk from alpha energies.

it would become very difficult to dispose of as well

This is a complicated discussion because of all the variables which is likely the primary reason EPA does not recommend filtration for radon risk reduction. We have a slide in our training course that states radon decay products RDP are 20 times more deadly the radon atoms. So reducing the RDP seems like it would be the most prudent approach. I have measured very low WL (less than 5% equilibrium ratio) in an office building that had an air circulation rate of 5 to 10 times per hour. The health risk of radon in that building was very low even though the radon levels were above guidance levels. I did not measure unattached fraction so I can't factor in that affect. Filters on air handlers will also capture a percentage of the unattached but certainly not as efficiently as air borne particles and water molecules that the filter captures. The health risk from the unattached fraction has been stated by others to be 5 to 30 times more deadly than the attached fraction. If the breathable air has less particles the unattached fraction will rise. I would occasionally joke in my class that if you wanted to reduce your risk from radon,
smoke a cigar in your house and the unattached fraction will be greatly reduced.

So if the WL is reduced in half but the unattached fraction doubles than the health risk may not be reduced. The other compounding factor is the size of the particles and the thoroughness of the filtration. A stand alone air filter may be very effective if it is blowing filter air towards you but it may have very little effect on the other side of the room even though it may be reducing the number of particles in the air. All of these variables and the complicated cost to actually measure WL or even more so the unattached fraction lead the EPA to not recommend filtration as a radon risk reduction method.

As to the comment on using charcoal to gather radon it was tried years ago by a company that filtered air through a large carbon filter and then back flushed the carbon to the outside to try and remove the collected radon atoms. It could not capture enough radon and RDP to emit any significant amount of gamma and it was not able to back flush out the radon efficiently so it turned out to be impractical and expensive to operate.

Not having read the articles Kevin posted, I'll assume for the moment the hypothesis that air filtration may actually *increase* the risk that indoor radon poses is true.

Is it still fair to say that increasing air movement within a space is likely to lower the working level in said space? I've been operating under the assumption that something as simple as ceiling fans or box fans would cause RDP to attach to particles in the air, and thus lower the working level. Would love to hear some feedback on this thought.

I would think that air movement would be more likely to cause plate out on walls and floors, especially if floors were carpeted ( more surface area on carpet fibers than wood or tile floor). back to subject that started all this....... i think the marketing material of filtration system is deceptive and untrue. but anyone who is manufacturing out of country and selling into North American market is unlikely to care.

The question regarding the impact of air cleaning devices has arisen a number of times over the years. The answer to that question has also evolved as the dosimetry models have become better at allocating dose to various particle sizes.

Rather than adding a lot of verbiage into a posting I am attaching a Technical Bulletin that was prepared recently for an agency that was interested in radon decay product reduction in an occupational setting. Although the setting is occupational the substance of the article is transferable to other building types. It is also a bulletin that we had some SMEs review, some who also participate in this ListServe review. I will let them share their opinions should they choose to.

Included in the bulletin is a link to an interview we did with Dr. Hopke who has done much of the research on this topic where we discuss air cleaning as a trim technique for active soil depressurization. I paste it below for convenience.
Dr. Phil Hopke Interview: Hopke Interview

I will say this that air cleaners do not reduce radon, as has been pointed out by others, but they do reduce radon decay products. The reduction of dose is not as linear as is achieved by radon gas reduction via active soil depressurization, but nonetheless is a reduction rather than an increase in dose.

Doug Kladder
Colorado Vintage Companies

That's excellent stuff, Doug; I'm so glad you jumped in here. I highly recommend that readers listen to the excellent interview Doug did with Dr. Hopke. I definitely learned some things that refined what I may have first picked up on this topic as long as 30 years ago.

For me, one part of the interview summarizes the findings at the time very well, as Dr. Hopke was discussing how well (or not) he found different air cleaner methodologies worked: "...we had some cases where there was no reduction in dose because of the changing [particle] size distribution. There was no case [among the units studied, of course] where the dose increased. In most cases, the reduction in dose was of the order of half what the reduction in exposure was." In the cases studied, a common exposure reduction was about 50%, with the corresponding dose reduction being about 25%.

In other words, although the ratio of "unattached fraction to attached fraction" would have increased with application of the air cleaning technique, yielding an altered particle size distribution that may have posed higher risk when measured by the ratio of dose to activity (e.g., mSv/WLM), the fact remains that total activity (WLM) was reduced sufficiently that dose itself never increased, but rather typically decreased.

Finally, I also want to emphasize that Dr. Hopke very strongly recognizes that testing and ASD mitigation is the first line of defense, but I think it could well be that the role of proper air cleaning methodologies as an adjunct or "trim method" may not have been given its due.

P.S. Although I could not locate a copy of this item referenced in Doug's attachment:
Hopke, P. Studies on the Performance of Air Cleaners and their Optimal Reduction of Dose from Radon Progeny, EPA Cooperative Agreement CR-820470, October 5, 1992-1996.
I did find this article by Hopke et al.:
https://www.tandfonline.com/doi/abs/10.1080/02786829308959635

Kevin:

Kudos for taking the time to listen to the interview.

Here is a Dropbox link to the Hopke paper referenced in the Technical Bulletin that you were asking for:
Hopke Paper

I also am providing a Dropbox link to the Technical Bulletin as I have been told that the original link I posted in the Listserve is not working with all browsers.

Technical Bulletin-18


I totally agree with your assessment that air cleaning is a good Trim technique. We have worked with a number of filters and have found that a whole house air cleaning system is preferred rather than console units that treat a single room. It is quite amenable for commercial builingd with large HVAC systems and where better filters would improve other indoor air quality concerns. Also, a HEPA filter is the least desirable as it not only creates too much pressure drop for blower but also has a more dramatic effect on increasing the unattached fraction. Actually, the lower MERV 8-10 filters reduce total WL and with minimal impact on unattached fraction. This is a case where a less expensive filter is preferred. So, for those that may be considering this approach please do not consider “true HEPA filters.”

Doug Kladder

My two cents of knowledge, which is very limited and comes mostly from discussions and training by Doug. We have used whole house filtration a few times in special circumstances where other alternatives were not viable and the initial radon levels were just above the action level. Pre and post testing was performed using an ERPISU device with results showing that we did achieve a reduction in RDP's using whole house filtration.

One challenge comes when the current occupant sells the property and the new owner has a radon test performed, which of course will still show an elevated radon gas result. We have stressed to our client that subsequent radon testing will need to be also be done using an ERPISU monitor in order to determine that the whole house filtration system is still reducing RDP's.

I think that the biggest challenge comes in explaining the science to an owner and if any real estate agents are involved in a transaction. Agents are often very narrow minded and focused on the action level as being the end all for moving forward with a real estate deal. Trying to explain how whole house filtration works to reduce RDP's can be very difficult when so much focus is on radon in general.

Just a few thoughts on the topic from the field.

Ryan asked about running a fan to reduce RDP without I assume changing the unattached fraction which I did not measure in this paper. I would think air agitation might actually cause more RDP to attach to air borne particles.

But the basic result of testing air movement and RDP levels or more relevant the Equilibrium ratio is fans as simple as a casablanca can reduce risk by reducing RDPs but the reduction in this case as we are not including filtration happens only where the air is being agitated .

So a good thing is a casablanca over your bed where you spend 8 hours a day. Here is the link to the tests I did to demonstrate that.

https://www.wpb-radon.com/pdf/RDP%20&%20Ventilation%20changes%2005.pdf

Bill's paper is excellent in showing both the sensitivity of actual RDP levels to environmental conditions and the ease with which one may interfere with RDP measurements.
Given that the majority of folks--e.g., homeowners, purchasers, real estate agents, et al.-- involved in decision-making around radon testing and mitigation are not scientifically trained, it seems best to continue with the primary focus remaining on first reducing radon levels, preferably to below the strongest defensible action level, and only then looking to RDP reduction as a further exposure-reduction intervention.
In most cases, there are simply too many variables associated with RDP control for me to support reliance on it as a technique subsituting for radon gas control--in other words, second-line and supplementing it, yes; first-line or replacing it, no.

Why let Radon into the home and then use air filters, HEPA filters, electrostatic precipitators, paddle fans, etc., to control Radon Progeny. Simply use ASD to prevent the Radon from coming in.

At the risk of steering this discussion off topic, I received a question asking if radon can build up in continuous radon monitors and become a radiation source within the home/office that they are stored in. Example being if there were 5 or 10 monitors in one spot. I understand background checks are important as part of the calibration process because of possible build up (of RDP I am guessing?) but I assume that cannot realistically build up to levels high enough to be a health concern? Taking this concept one step further, the discussion above brought up disposal of filters being a possible concern, so could RDPs build up enough in radon fan blades to a high enough degree that their disposal would ever become an issue, or at least something you wouldn't want sitting in your office/shop? Surely there would be plating out on those fan blades? I am pretty sure I was taught years ago that the pipe and fan would not be a health concern when modifying a very old system, so is it the surface area of the filter media that allows the HEPA filter disposal to be a possible concern? Any correction of my assumptions from the scientific folks in the audience would be appreciated.

John you hit the nail squarely on the head. Basic safety is to 1/ remove contaminate at source 2/ remove it along the pathway 3/ use personal protective equipment to protect the worker.
I don't understand why when number 1 is usually the cheapest path even in commercial buildings long term that people go looking at alternative more expensive less reliable options.
Our workplace safety people here in Ontario have consistently said that they want a people proof solution. i.e. it cannot be solved using outdoor air in HVAC system, unless it is with fixed dilution not a variable dilution because in the next year when building operation costs go up someone will change the program to save money. Now i can see in a military base where there is chain of command and penalties for disobeying direct written orders and a sub slab is very expensive and filtering or bringing in outside air is cheap yes go for it but in a building like a school or office where competition for scarce dollars is high it is not a good solution.
It is not rocket science people! it is just nuclear science and building science put together.

As Yogi Berra would say 'Dejevu all over again'. What's old is new again

1) Stating that air cleaners remove radon because the public does not understand the concept of RDP's does not make it accurate. I had this argument with Dade Moeller and his hassock air cleaner probably at the First Radon Symposium. These days it sounds like an open invitation for a class action lawsuit for fraudulent advertising.

2) When I did presentations back in the day when the Relocation industry bought into only using WL measurements I always got phone calls after this:
If you remove the radon - you cannot physically have decay products. If you test WL and something changes you still have radon and have no idea what the risk is.

Having subsequently worked in Indoor Air as well as radon, it is very clear that not all buildings large and small, particularly schools are not always well maintained. Any risk reduction system for a deadly carcinogen needs to be robust and as foolproof as possible.