Thread by @PeteUK7

What to look for in a commercial HEPA filter.
It's not easy to compare machines.
A few key considerations:
1. CADR
2. Noise
3. Filter grade
4. Cost
5. Hidden extras / tech
6. Replacement filters
7. Auto mode
9. Carbon filter
10. Final thoughts
11. Resources & tweets
1/🧵

1. CADR pt1
Stands for Clean Air Delivery Rate. It's the volume of air cleaned by the machine per unit time. Often listed in m3/hr or CFM (cubic feet per minute).
It's a crucial metric to compare one unit with another - essentially how powerful it is. We need to know this.
2/

CADR pt2
Problem; it isn't always easy to find this figure. Some give you instead a 'suitable for room size' number. Not that useful because you need the CADR to work out how many air changes per hour (ACH) you're getting. Personally, I aim for 6 ACH as a covid mitigation.
3/

CADR pt3
Often, you can only find CADR for the machines max setting. Most machines have more than 1 setting. Loss of CADR is not always proportional & so speed 2 may not be 2/3 of speed 3. This makes things awkward. Assume CADR is lower than proportional to give you headroom.
4/

CADR pt4
A worked example. If you have a room sized 5m x 3m with a ceiling height 2.4m, your room has an air volume of 36m3 (multiply all dimensions to get volume 5m x 3m x 2.4m =36m3).
For 6 ACH, you need a CADR of 36m3 x 6 = 216m3/hr.
5/

CADR pt5
In my view, if you cant find CADR value, move on. Manufacturers should be open about this and if the info is hard to find, there's probably a good reason. Selling points are rarely hidden.
6/

2. Noise pt1
Nobody wants a machine that annoys them. They'll be tempted to turn it off or down & therefore it will not do the job intended.
Most machines give you a noise value for it's lowest setting. Stuff like 'whisper quiet'. You are looking for a number in decibels dB.
7/

Noise pt2
But what we're really looking at is what is the noise at max speed. Mainly because we only have CADR for max speed but we almost want best performance. Some machines have a noise of up to 70dB. This is intolerable for most. Ideally <50dB imo.
8/

Noise pt3
But what is acceptable for some is not for others. Need to think where it's being used and how close to it people are. So for a classroom for example <45dB may be more likely suitable. It's subjective and it can also depend on the pitch of the noise.
9/

Noise pt4
It can be that you need to have more than one device running to reach the right CADR at an acceptable noise level. Remember though that noise is additive (complex topic). Simply, 2 machines running together is a little louder than 1 -- 3 a little louder than 2 etc.
10/

3. Filter grade
For a commercial air filter to be regarded as HEPA, it must have a H13 or H14 grade filter. It get's more technical than that but for a buyer, this is what you need to know. Higher grade = more particles captured in 1 pass through the air filter.
11/

Filter grade pt2
Some air filters have lower grade filters. Take the IKEA versions for example. They have a grade below H13 called EPA12. This still works fine for removing virus particles. It just takes more than 1 pass to remove them all. We shouldn't exclude lower grades.
12/

Filter grade pt3
There is however a bottom end of grades that work for removing virus particles. You are very unlikely to come across one on a commercial air filter. Just ensure you look at which grade. So EPA11 & 12 (not HEPA) as well as H13 & H14 (HEPA) will all work.
13/

Filter grade pt4
For DIY versions, we often use lower grade filters because we want to achieve very high air flow and fast room clearance. To save confusion (as it's complex) I'm limiting this thread to commercial air filters and what to know and look of / be aware of.
14/

4. Cost
There is such a wide range of prices. Very cheap machines <£100 are unlikely to have good performance. So low CADR. Very expensive machines >£600 are likely to be overpriced versus equal or better performing machines. One way to look at this would be CADR per £.
15/

Cost pt2
CADR per £ is a good metric to allow comparisons but one must also consider the other factors in this thread. Especially noise. So if CADR per £ is good but noise is too high, the balance is wrong.
It is about finding the right balance of performance for the price.
16/

Cost Pt3
There are a lot of good machines that fall between £140 - £250 price bracket. I'd be looking at CADR of 300m3/hr to 500m3/hr over that price range with noise on max as low as possible based on where it will be used. It's a balancing act.
17/

Cost pt 4
One thing that can really bump up the cost are other tech's built in. You really only need good air filtration and I wouldn't pay extra for built in sensors. I definitely wouldn't pay extra for added extras like ionisers or UV bulbs or plasma tech.
18/

5. Hidden extras / tech
Simple rule; air filters remove particles from the air. Everything else has the potential to add stuff to the air and change the air chemistry. So ionisers blast out ions, some things blast out oxidants, others create ozone. Simple filtration is best.
19/

Hidden extras / tech Pt2
This market is badly regulated. It is ripe for marketing misinfo, bold claims and ineffective gadgets to make it seem like it's more powerful. So look out for what's onboard and look out for misleading terms. For example:

20/

Hidden extras / tech Pt3
So my view is that it's better to avoid anything that is added to a machine. It will be unlikely to be better than simple filtration, could be worse & could potentially be dangerous. It always adds to the cost and also the power usage. Swerve it.
21/

5. Replacement filter cost
It is important to understand that filters will need to be replaced at some point. Make sure you look at a) how much that replacement costs and b) its availability. Staying with a well known brand often assures availability is good.
22/

Replacement filter cost Pt 2
If your unit has a pre-filter, this may become blocked more quickly than the HEPA filter. It gathers dust & lint & hair. It's likely that you can extend the time between filter changes by regularly vacuuming the pre-filter:

23/

7. Auto mode
Some machines have built in sensors. They often have an auto setting which uses these sensors to decide how fast the machine needs to run.
When using as a Covid mitigation, NEVER use the machine on auto. It will not be as effective. It isn't reacting to virus.
24/

Auto mode Pt2
You should always be working out room air volume, ACH required, CADR needed to achieve that -- run the machines continually in manual on the speed setting that achieves the CADR required.
Auto overrides all your calculations & often runs on very low speed.
25/

8. Carbon filter
Some machines offer the option of a carbon filter. Some have a carbon filter as standard. It is not needed as a covid mitigation. It is able to remove gaseous contaminants from the air that the HEPA filter cannot. So it can be beneficial.
26/

Carbon filter Pt 2
However, if the primary use is as a covid mitigation, a carbon filter isn't needed. It has a drawback in that it can lower the CADR of the air filter if a carbon filter is added. Why?-it makes it harder work for the fan to pull or push air through this too.
27/

Final thoughts
This is quite a complex topic and I have simplified parts. There are other considerations like power usage, airflow direction, physical dimensions and so on. I have provided my opinion to help people understand the main aspects of what they are buying.
28/

Final thought Pt2
For filtration to work you need to ensure it is sized & used correctly and maintained. Needs be quiet enough to be tolerated.
I used ACH here as a rough metric but there are ways to calculate CADR needed, especially if occupancy is more than a few people.
29/

Final thoughts pt3
This market is in need of strict regulation.
Manufacturers should be clear about the machine's performance.
Added tech should be proven effective, published & peer reviewed.
Terminology & standards should be clear and consistent globally.
30/

11. Resources & useful tweets
I intend to keep adding to this thread as there are many good resources that would be useful for this topic in general. There are also many great contributors on here and so I will add good threads and tweets from others over time.
31/end of guide

Two places I recommend looking for air filters:
Clean Air Stars where they have compiled a list of options, built in calculator, recommendations & buying links.

Smart Air; an honest company with brilliant products. Great people.
cleanairstars.com/hepafilters/#H…
smartairfilters.com/uk/en/shop/

Thread regarding why I think there is a good case for air filtration in addition to whatever ventilation you have (always improve ventilation as much as poss). Filtration is supplementary, effective, reliable and complimentary:

Super thread here from @DavidElfstrom with details of the Levoit Core 400S air filter. CADR and noise level at each different speed -- note, for UK, we need to convert David's CFM figures to m3/hr (1 CFM = 1.699m3/hr):