How to pick the best air purifier, objectively

Short answer: use Lightwork’s Air Purifier Guide — 100% free; no signup required; no affiliate links, partnerships, or bias; completely data-driven, quantitative, and transparent.

Or, if you want to understand how we at Lightwork Home Health evaluate and select air purifiers (and how we arrived at the algorithm for our tool), read on…

The basics of air purification

There is so much marketing noise in the air purifier space. Companies use and come up with all sorts of buzzwords to suggest that their purifier does something special. It’s shockingly difficult to get straight answers and objectively evaluate performance.

But there are a couple key concepts to understand upfront, before getting into the nitty-gritty of comparisons.

First: what are you filtering? Broadly, there are two categories you might want to cover. The first is particulate matter (or “PM”) — everything from dust to pollen to smoke, mold, pet dander, and more. The second is “VOCs” — volatile organic compounds and their cousins semi-volatile organic compounds (SVOCs) and other gases and chemicals.

We’ll focus most of this post on the former (PM). This is the more common air quality issue that nearly every home would be well-served to address. VOCs can certainly be an issue as well, but it is more nuanced and they are not quite as prevalent. We’ll talk about VOCs at the end of this post.

The mechanisms for filtering PM and VOCs are quite different from each other. Some air purifiers can help with both, but in general, we would strongly recommend filtering them separately.

Once you’ve decided you’re focusing on particulate matter, the question becomes: what makes a good PM air purifier?

Particulate matter ranges from really tiny particulates (perhaps 0.1-0.5 microns in size, like airborne viruses, small bacteria, and ultrafine pollutants like vehicle emissions) to slightly larger ones (around 1 micron is typically larger bacteria and fine dust), or even larger (3-10 microns includes mold and plant spores, pollen, and most dust particles). A good air purifier needs to cover all of these.

So here’s a fun trivia question: in general, do air purifiers struggle the most with filtering:

1. Larger particles
2. Or small particles?

The surprising answer? Neither.

Air purifiers have their worst performance on mid-size particles, typically around 0.3 microns. They are extremely effective at filtering tiny particles and extremely effective at filtering large particles.

How can this be? Well: modern air purifiers (and certainly “HEPA” ones) actually use a combination of a few filtration techniques. One of these techniques (diffusion) is extremely good at small particles and gets worse the larger the particle is. Others (inertial impaction, interception) get more effective the larger the particle size is. (And there is also settling and electrostatic attraction in the mix too.)

And so, combined, the efficacy curve looks like a “U,” as in the diagram below (Christopherson et al. (2020), Otolaryngol Head Neck Surg.). The purifier has the lowest filtration rate around 0.15-0.3 microns. This is referred to in the industry as the “Most Penetrating Particle Size,” or MPPS.

But how bad is low efficacy in this situation? It turns out that any HEPA filter (which is the filter standard all reputable air purifiers use) in the US must filter at least 99.97% of particles at 0.3 microns.

Said another way: when air goes through the unit, it must filter out a minimum of 99.97% of the particles around those it is worst at handling, and more than 99.97% of every other particle size. That is pretty dang good! It means that when air passes through a HEPA air purifier, very few particles come out the other end.

So if all HEPA air purifiers are that good at filtering the air that goes through, what really matters?

How much air goes through.

If your HEPA air purifier is underpowered, it will take a long time to clean a room — or, worse, if new pollution is being introduced (and it usually is) — it will never get the air clean. So, aside from using a HEPA filter in the first place, the most important factor is the throughput.

This is typically measured as Clean Air Delivery Rate, or CADR. CADR is measured in cubic feet per minute, or CFM. It is often broken down by pollutant type, but for the sake of keeping this post somewhat simple, we can just talk about it generally.

So, net of all that, the most important factors for filtering PM are that the filter is a HEPA filter, and that it has sufficient throughput for the room and situation (the more the better!). If you take nothing else away from this post… remember that.

Here’s a little toy visualization of air purifier efficacy (assuming a well-mixed room — another rabbithole we could get into. Good air circulation helps purifiers work well!). I’ve included three levels of throughput, and you can shift the room size and incoming pollution. You’ll see that small purifiers often will just never get a room clean!

Room size 250 sq ft

Incoming pollution None (sealed room) Light (typical) Moderate (cooking, open windows) Heavy (wildfire, construction)

The real-world twist

At Lightwork, we’ve consulted on air filtration with a lot of clients. And in so doing, we’ve realized that — while what I wrote above is all accurate — there are some other real-world, practical considerations. Namely:

1. Quiet matters. If an air purifier is too loud, people simply won’t use it. And an unused air purifier is no good at all.
2. Ease matters. If you have to replace the filters too often, most people will simply not do it. And dirty filters tank the efficacy of a unit.
3. Beauty matters. For many customers, having an ugly air purifier is a non-starter. So finding aesthetically-appealing ones — at least for certain rooms — is a factor.
4. Price matters. On top of all that: people want value. If it’s too expensive, it won’t get purchased in the first place.

Numbers 2 and 3 are easy: we can restrict our search to units with a 12-month or longer filter lifespan, and to aesthetically-appealing ones (that is the only subjective filter we are going to apply!).

(And don’t worry — we also let you disregard any of these limitations if you don’t care.)

Number 4 is also easy: once we get down to a list of candidate purifiers that work, we can sort them by price.

But number 1 — quietness — is trickier. How do we figure out if a purifier has sufficient throughput at a quiet level?

Unfortunately, CADR is typically not reported at the different fan speeds. Manufacturers only report full-speed CADR, and that is usually very loud.

So we came up with a metric that we refer to as “Quiet CADR.” We’ve defined this as “Clean Air Delivery Rate at 41 decibels of volume or less.” We’ve found by talking to clients and evaluating their response to various units that 41 decibelsNote that we will use “decibels” and “dBA” interchangeably throughout this post. dBA specifically refers to sound intensity adjusted to match human ear sensitivity (as opposed to e.g. dBC, dBZ, or plain dB). But since we’re focused on humans here we’ll use the generic “decibels” to refer to dBA. seems about right. It’s about the noise level of a library.

So here’s what we’ve done. It gets a little math-y, so feel free to skip ahead.

We first need to find the noise levels of the various fan speeds for a unit. There are two ways to do this:

1. Manufacturers typically do report the noise level at the lowest fan speed. So we can use this. This may underestimate the unit’s Quiet CADR (that is, if fan speed 2 is still under 41 decibels, we will think the Quiet CADR is lower than it actually is by using fan speed 1. But we’d rather underestimate Quiet CADR than overestimate it!)
2. Independent reviewers like AirPurifierFirst and HouseFresh have tested the noise levels of various fan speeds.

Once we have that data, we find the highest fan speed that is below 41 decibels.

Now, we need power data — how much energy does the unit draw? We can get this from manufacturers, independent reviewers, energy use certification programs, or elsewhere. And specifically, we need the power usage in wattage at the “quiet” fan speed we found, as well as the top fan speed. Lots of data to collect!

Once we have this data, we can use what are called Fan Affinity Laws to make further calculations. The fan affinity laws define various physical relationships between performance and power for pumps and fans in general.

The summary here is that CADR (which we can reasonably treat for these purposes as proportional to airflow) scales with the cube root of power. So if we know:

• max CADR throughput ($C_{max}$)
• power at the “quiet” level ($P_q$)
• power at the max level ($P_{max}$)

We can estimate the CADR at the quiet level ($C_q$) with:

$$C_q = C_{max} \times (P_q / P_{max})^{1/3}$$

(More specifically, we’ve modeled this out based on known measured data from the purifiers for which it is available and found that the real-world scale is closer to $0.344$ than raising the power ratio to $1/3$. Pretty validating that it’s so close! We use the $0.344$ exponent in our recommendations given the empirical validation, but it doesn’t really make a difference.)

So, take a purifier that is rated at a CADR of 200 CFM. Say it draws 50W at max speed and 15W at its quiet speed. The Quiet CADR would be:

$$C_q = 200 \times (15/50)^{1/3} \approx 136 \space\text{CFM}$$

This metric is how we evaluate throughput — because, again, most people won’t leave an air purifier on high. It drives them nuts. So what we care about is sufficient throughput at a sufficiently quiet setting.

Now: how do we define sufficient throughput? Is 136 CFM good?

This has to be defined by the size of the room. Big rooms need more filtration than small ones.

The main way you look at this is “Air Changes per Hour” or ACH. Our general recommendation is that you want to target 5 ACH (AHAM, the Association of Home Appliance Manufacturers, recommends 4.8). You could go lower, and for certain situations (while you’re cooking, or if you’re dealing with wildfires or bad pollution), you may want to go higher.

So, say you have a 200 square foot room with 8 foot ceilings. That’s 1,600 cubic feet of volume. To get to 5 ACH, you’d need to have throughput of:

$$1,600 \times 5 = 8,000 \space \text{cubic feet per hour} = 133 \space \text{cubic feet per minute (CFM)}$$

So, yes! Our 136 CFM Quiet CADR air purifier will do the trick there.

Our algorithm

So, when we put all that together, our objective recommendation “algorithm” for customers for PM filtration ends up looking something like this:

1. Take every possible air purifier and calculate their Quiet CADRs
2. Filter out the ones with <12 month filter lifespans (if they want)
3. Filter out the ones that aren’t premium designs (if they want)
4. Find the square footage of the room in question and its filtration needs in ACH (based on room type/usage, outdoor/indoor conditions, specific concerns, etc.), and use that to calculate a Quiet CADR minimum, and filter out the ones that don’t meet it
5. Make two recommendations: 1) the best value (i.e. cheapest one that meets the above standards), and 2) an upgrade pick (i.e. the one that has the highest throughput up to a 3x multiple on the minimum Quiet CADR)

It comes out with something like this:

The upgrade pick is an option for really clean air (without going overboard). Also, bigger units with higher throughput can usually get even quieter for the necessary performance since they are effectively oversized for the room.

There is a little more nuance to the formula (detailed on the Methodology page of our tool), mostly having to do with allowing for “combos.” Sometimes, it’s cheaper to get multiple smaller air purifiers instead of one bigger one. We adjust for this in a couple ways, including:

1. Noise. Two units make more noise than one. Making some reasonable simplifying acoustic assumptions, two units at 38 decibels each equals 41 decibels total, and three units at 36 decibels each equals 41 decibels total. So when recommending a combo, we need to adjust the Quiet CADR for the combo units to allow for a lower noise ceiling.
2. Slightly penalizing combos (with a 25% “premium” for each added one), since it’s more of a pain to deal with multiple instead of one

As an aside, are you wondering why two units at 38 decibels = 41 decibels total? Decibels are a logarithmic unit (and a pretty odd one at that). Decibels are measured on sound intensity, where with $L$ as the level in decibels, $I$ as intensity of the sound, and $I_0$ as reference intensity:

$$L_{\text{source}} = 10\,\log_{10}\!\left(\frac{I}{I_0}\right)$$

If we’re assuming two equal sources togetherThis is a simplification. In practice, sound waves from two sources at different positions in a room interact in complex ways — they can constructively or destructively interfere depending on frequency and listener position, and reflections off walls add further complexity. But for a rough noise budget, simply adding the intensities is a reasonable and slightly conservative approximation., that means they produce $2I$ intensity, and so:

$$L_{\text{double}} = 10\,\log_{10}\!\left(\frac{2I}{I_0}\right) = 10\,\log_{10} 2 + 10\,\log_{10}\!\left(\frac{I}{I_0}\right) \approx 3.01 + L_{\text{source}}$$

So adding a second equal noise source, regardless of the sources’ actual decibels, always increases the sound level by 3 decibels! (And you can do similar math for 3 equal sources, which results in a ~5 decibel increase)

People often ask us about other features, like ionizers (we don’t recommend them), UV-C lights, plasma generators, “nano” coatings, photocatalytic oxidation, and more. In general, our view is that most of it is marketing and far less impactful than everything written above (and in some cases, potentially harmful). We may write a future post about some of these in particular, but broadly, our advice would be: nail the basics we’ve written about here.

The VOC problem

As discussed up top, VOCs require a fundamentally different filtration approach than particulate matter — they pass straight through HEPA and similar filter types. Instead, you need a medium like activated carbon to adsorb and trap the VOC gas molecules. Over time, the carbon medium saturates — so the more carbon you have, the better it will filter and longer it will last.

Most HEPA filters include a thin carbon pre-filter. This will help with low-level ordinary VOCs, but won’t do anything for more serious or chronic issues (and regardless, it saturates fast). To really deal with VOCs — if they’re an issue for you — you need a purpose-built air purifier.

There are other challenges, too. To push air through a thick carbon media, you need a really strong fan. And a really strong fan means a lot of noise. Plus, few of these purifier units are aesthetically appealing.

There are combination units that combine nontrivial carbon media with a HEPA filter. But at some level, these two facets are battling against each other. The carbon media (if it’s meaningful) is thick and dramatically reduces airflow in the performance of its role. But, as discussed at length above, airflow is the most important factor in a particulate matter purifier doing its job. So the two are at odds.

And on top of all of that, not all carbon media is equal. Aside from considerations about media thickness, density, and granule size (often hard to find details for and compare), different VOC or VOC-like pollutants are adsorbed at different rates by different media.

For example, formaldehyde is poorly captured by plain ol’ carbon. Potassium permanganate or potassium iodide impregnated carbon is significantly more effective for it and its aldehyde siblings.

Many of the top-end VOC-targeted air purifiers let you swap between different filter media blends, or even customize your own to target specific contaminants. But to pick this out, you really need to know what compounds exactly you’re trying to target — either by having someone test specific VOCs in your home, or identifying a source and determining what it is outputting or offgassing.

We go into all this in more detail and share some quantitative model recommendations in the free VOC Guide on Lightwork’s air purifier tool.

So… what do we recommend?

Well, like I said: the easiest way to see our recommendations is to check out Lightwork’s Air Purifier tool — which again is 100% free; no signup required; no affiliate links, partnerships, or bias; and purely quantitative and transparent.

I’m not just trying to get you to use the tool — the recommendations really do vary based on the size of the rooms you’re trying to filter, and your preferences around design, noise levels, filter lifespan, and more.

One of our main goals at Lightwork is to bring science-backed, data-driven rigor to an industry — home health — that has historically been plagued with imprecision, poorly-supported claims, and (at worst) scams. We hope this post has taught you a thing or two about air quality and helps you avoid falling victim to marketing claims that don’t measure up to reality.

Enjoy your clean air!

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