HEPA vs. ULPA: Why Airflow Matters More Than 99.97% | Jaspr
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Secondary Keywords: HEPA vs ULPA, ULPA filter, is HEPA overrated, HEPA 99.97%, MPPS filter, most penetrating particle size, high airflow air purifier, air purifier airflow, CADR vs HEPA, air scrubber vs HEPA, air purifier filter seal
Why HEPA Is Overrated: If Efficiency Is Everything, Why Don't We All Use ULPA?
By Michael Feldstein, Founder of Jaspr
If HEPA is so good, why don't we all use ULPA?
Because maximum filter efficiency is not the same thing as maximum air-cleaning performance.
Most people have heard of HEPA.
Almost nobody has heard of ULPA.
ULPA stands for Ultra-Low Penetration Air filtration.
And ULPA filters can be substantially more efficient than HEPA filters.
Under the ISO classification system described by ASHRAE, HEPA Group H filters range from approximately 99.95% to 99.995% efficiency at their Most Penetrating Particle Size, while ULPA Group U filters range from approximately 99.999% all the way to 99.999995% at MPPS.
Read those numbers again.
HEPA:
99.97-ish%.
ULPA:
99.999%+
So if the best air cleaner is simply the machine with the highest filtration percentage, why aren't we all using ULPA filters in our bedrooms?
Why does everybody stop at HEPA?
Because the real engineering problem isn't:
How do I capture the highest possible percentage of particles in one pass?
It's:
How do I remove the greatest amount of pollution from the air people are actually breathing?
Those are not the same question.
And understanding the difference changes how you should shop for an air purifier.
What is a ULPA filter?
ULPA is an extremely high-efficiency particle filter typically used in applications where extraordinary levels of particle control are required.
Think:
Semiconductor manufacturing.
Pharmaceutical production.
Cleanrooms.
Nuclear applications.
Highly controlled medical and industrial environments.
ASHRAE describes both HEPA and ULPA filters as standard technologies for cleanrooms and other specialized environments requiring exceptionally low particle concentrations.
ULPA is not fake.
It is not inferior filtration.
It is actually more efficient filtration.
Which is exactly why ULPA is such a useful way to understand what's wrong with the consumer obsession with the word HEPA.
If the biggest percentage automatically meant the best air cleaner:
ULPA should have won.
It didn't.
Because filters exist inside systems.
Why isn't every home air purifier ULPA?
Because increasing filtration efficiency can increase resistance to airflow, and air cleaning depends on moving air as well as filtering it.
The fan has to pull air through the filter.
The more resistance the filtration system creates, the harder the fan must work to maintain airflow.
The U.S. Department of Energy notes that the pressure drop of ULPA filters is frequently greater than that of standard HEPA filters.
ASHRAE similarly warns that increasing filter efficiency can increase pressure drop and potentially reduce airflow if the system is not designed to compensate for it.
Can engineers solve that?
Of course.
You can use more filter surface area.
Better media.
Deeper pleats.
A stronger fan.
A larger machine.
More power.
ASHRAE specifically notes that increasing pleated-filter surface area can reduce pressure drop.
But now we've proven the point.
We're designing an entire air-cleaning system.
We're no longer shopping for the biggest percentage printed on a filter.
Is HEPA bullshit?
No. HEPA is excellent filtration technology. Treating “HEPA” as proof that an air purifier is excellent is what I think is bullshit.
The EPA defines HEPA as a mechanical filter capable of theoretically removing at least 99.97% of particles around 0.3 microns under the relevant conditions.
That's excellent.
But consumers often see:
99.97%
and interpret it as:
This machine removes 99.97% of the pollution in my bedroom.
That is not what the number means.
The efficiency describes what happens to particles that actually travel through the filter.
It doesn't tell you how much contaminated room air reaches that filter.
It doesn't tell you how quickly the room gets cleaner.
It doesn't tell you whether air leaks around the filter.
And it doesn't tell you whether you'll tolerate running the machine all night.
You're not buying a filter.
You're buying an air-cleaning system.
What does 0.3 microns actually mean?
This may be the most misunderstood number in the entire air-purifier industry.
People constantly say things like:
“HEPA filters particles down to 0.3 microns.”
That makes it sound as if 0.3 microns is the smallest particle HEPA can capture.
It isn't.
0.3 microns is approximately the hardest particle size to capture, not the smallest.
The EPA describes 0.3 microns as the “worst case” and explains that particles larger and smaller than that are captured with even higher efficiency.
The technical concept behind this is called:
MPPS.
What is MPPS?
MPPS means Most Penetrating Particle Size. It is the particle size that is hardest for a particular fibrous filter to capture.
For most HEPA and ULPA filters, ASHRAE says the MPPS falls roughly between 0.1 and 0.3 microns, depending on characteristics including the filter material and air velocity.
ISO 29463 actually specifies testing high-efficiency filters at their MPPS.
This produces a filtration curve.
Efficiency is high for larger particles.
It decreases as particle size approaches the MPPS.
Then something surprising happens.
As particles become even smaller, efficiency starts increasing again.
Why can filters capture particles smaller than 0.3 microns even better?
Because very tiny particles don't behave like little cannonballs flying straight through holes in a net.
They move erratically.
One of the dominant mechanisms is called diffusion.
Extremely small particles undergo Brownian motion, wandering irregularly through the air and increasing their chances of colliding with filter fibers.
ASHRAE describes diffusion as an important capture mechanism for very small particles and notes that the lowest efficiency generally occurs around the MPPS.
The EPA puts the conclusion even more simply:
Particles larger or smaller than 0.3 microns are trapped with even higher efficiency.
So:
0.3 microns is not a cliff.
A 0.29-micron particle does not suddenly sneak through.
And a 0.1-micron particle isn't automatically harder to capture than a 0.3-micron particle.
That's why marketing something as:
“Filters down to 0.3 microns”
can create completely the wrong mental model.
What matters more, 99.97% filtration or airflow?
Here's the easiest way I know how to explain my entire philosophy around air cleaning.
Would you rather have:
95% of $5
or:
99% of $1?
95% of $5 is:
$4.75
99% of $1 is:
$0.99
The percentage is lower.
The result is dramatically bigger.
Now replace dollars with air.
Imagine two hypothetical machines, ignoring bypass and other variables for a moment.
| Machine A | Machine B | |
|---|---|---|
| Filtration efficiency | 99.97% | 95% |
| Air processed | 100 CFM | 500 CFM |
| Simplified equivalent cleaned airflow | ~100 CFM | 475 CFM |
Which one would I rather have scrubbing a polluted room?
Machine B.
Not because 95% is magically better filtration.
Obviously it isn't.
But:
95% of a lot of air can clean far more air than 99.97% of very little air.
That's the concept people miss.
Your lungs don't breathe filtration percentages.
They breathe air.
Is 99.97% better than 95%?
If every other variable is identical, absolutely.
If two machines both process 500 CFM, have equally good seals, equal noise, equal power consumption, and one captures 99.97% while the other captures 95%:
Give me 99.97%.
That's obvious.
But air-cleaner design doesn't happen in a spreadsheet where everything else magically stays equal.
Filters create resistance.
Fans have performance curves.
Noise matters.
Energy matters.
Filter area matters.
Machine size matters.
And actual airflow matters.
The question isn't:
How high can we make one efficiency percentage?
It is:
What combination gives us the greatest useful clean-air delivery?
What is CADR and why does it matter more than a HEPA badge?
CADR, or Clean Air Delivery Rate, tells you how much useful filtered air the entire machine delivers.
That's much closer to the question I care about.
AHAM describes CADR as the volume of filtered air delivered by an air cleaner.
The EPA uses CADR as a primary metric when recommending how consumers size portable air cleaners for rooms.
Think about what CADR is trying to combine.
Efficiency × airflow.
That's the useful result.
You can have phenomenal single-pass efficiency but terrible clean-air delivery.
Or you can combine highly effective filtration with a huge amount of airflow and produce substantially more clean air.
That's why I would much rather know:
What's the smoke CADR?
than simply:
Does it say HEPA on the box?
Does airflow change filter efficiency?
Yes. Air velocity is one of the variables that can affect filtration efficiency and the MPPS.
ASHRAE explains that filtration efficiency depends on particle size and, to a lesser extent, airflow through the filter.
For a given filter, lower airflow rates can produce higher efficiency for certain smaller particles, which is one reason it is important to understand the airflow at which a filtration rating was measured.
This doesn't mean legitimate testing is fake.
Formal standards specify testing conditions.
But it does mean:
A filtration-efficiency percentage without the test conditions is incomplete information.
I want to know:
What was tested?
At what flow?
What was the filter face velocity?
What was the pressure drop?
And, most importantly:
What happens when that filter is installed inside the complete machine running at the speed I actually use?
Are HEPA tests performed at artificially low airflow?
This is where I want to be careful.
Legitimate HEPA testing standards do specify test conditions. It would be wrong to say that proper HEPA testing simply lets manufacturers choose any arbitrary airflow they want.
ISO 29463 provides standardized methods for evaluating high-efficiency filters at MPPS, and DOE specifications include airflow-resistance requirements for qualifying HEPA filters.
But there is still a huge distinction consumers need to understand:
Testing the filter is not the same thing as testing the complete air purifier.
A filter can produce an impressive laboratory efficiency result.
That doesn't automatically tell you the whole machine's:
CADR.
Airflow.
Noise.
Room-cleaning speed.
Or performance at a realistic operating setting.
And ASHRAE notes that the same filter's actual performance can change with factors including air velocity, loading, installation, and bypass.
So I don't want air-cleaner companies to stop publishing filter-efficiency testing.
I want them to publish more.
Tell me the filter efficiency.
Then tell me:
How much clean air does the finished machine deliver?
Why is whole-machine testing more useful?
Because I don't breathe air inside a filter-testing rig.
I breathe air inside a bedroom.
A complete air cleaner has:
A fan.
A filter.
A housing.
Seals.
Airflow pathways.
Controls.
Different operating speeds.
And leaks, if it isn't engineered properly.
Whole-machine performance incorporates those variables.
That's one reason CADR is useful.
It evaluates the particle-removal performance of the appliance as a system rather than merely telling you about a piece of filter media.
When I'm comparing machines, I want both types of information.
Component performance tells me whether the engineering pieces are good.
Whole-machine performance tells me whether those pieces work together.
What does the filter seal have to do with HEPA performance?
Almost everything.
If dirty air can travel around the filter instead of through it, the theoretical efficiency of the filter media doesn't matter for that bypass air.
This is another reason the HEPA badge can be misleading.
You can buy extraordinary filtration media.
Put it into a poorly sealed housing.
Allow dirty air to sneak around the perimeter.
And lose much of the benefit you were bragging about.
EPA's guidance for HEPA vacuums makes exactly this distinction: the machine must be designed so that the air drawn into it passes through the HEPA filter rather than leaking around it. EPA specifically warns that simply retrofitting a HEPA filter into equipment that was not designed and sealed for it may not provide equivalent performance.
ASHRAE makes the same point in HVAC systems.
A high nominal efficiency does not guarantee high real-world efficiency when gaps, bypass, installation, velocity, and other system factors are involved.
So when I hear:
99.97% filter
my next question is:
Great. How much of the machine's air is actually being forced through it?
Can a tiny filter gap matter?
Yes.
Especially when you're obsessing over decimals.
Think about how ridiculous this can become.
We're arguing over:
99%
versus:
99.9%
versus:
99.97%
while potentially letting some meaningful fraction of the airflow completely bypass the filter.
ASHRAE warns that even small leakage through filter racks or around seals can significantly reduce installed filtration performance.
So before I start worshipping the third decimal place of a filtration rating:
Show me the seal.
That's part of the machine too.
Why does noise matter as much as filtration?
Because an unplugged 99.97% filter has an effective CADR of:
zero.
I learned this firsthand during the 2016 Fort McMurray wildfire response.
We put an industrial air scrubber into a homeowner's house.
It was designed to move a lot of air.
The homeowner unplugged it.
It was too loud.
That experience stayed with me.
A machine can have extraordinary laboratory specifications.
But if someone cannot tolerate living beside it:
it stops cleaning air.
That's why I don't think the goal for a residential air cleaner should be maximum airflow at any cost either.
The goal is:
the greatest amount of useful clean-air delivery that someone can comfortably run continuously.
That's a more difficult design challenge.
And I think it's the right one.
Why we call Jaspr an air scrubber
I came from restoration.
Wildfires.
Floods.
Mold.
Water-damaged buildings.
In those environments, nobody cared about winning an acronym competition.
The job was:
Move contaminated air through effective filtration and keep doing it.
That's what I mean by air scrubbing.
I didn't want to put a screaming industrial restoration machine in people's bedrooms.
But I also didn't want to build a tiny consumer purifier where filtration terminology mattered more than the amount of room air actually being cleaned.
I wanted both.
Serious airflow.
Highly effective filtration.
A well-designed airflow path.
A sealed filtration system.
Enough carbon to meaningfully address another class of pollutants.
And a machine quiet enough that people leave it running.
That's Jaspr's design philosophy.
How does Jaspr think about filter efficiency?
We don't think efficiency doesn't matter.
It matters enormously.
We think system performance matters more than any single component specification.
Jaspr currently publishes whole-machine maximum CADR figures of:
| Particle | Jaspr CADR |
|---|---|
| Smoke | 340 CFM |
| Dust | 356 CFM |
| Pollen | 361 CFM |
Jaspr also publishes 114 CFM of clean-air delivery at fan speed 1, its quietest operating setting, along with 1 lb of activated carbon and onboard PM2.5 and VOC sensing.
Those are whole-machine specifications intended to tell you more about what the complete air scrubber is doing, rather than relying on one filter-efficiency claim.
What should I ask instead of “Does it have HEPA?”
If I'm buying an air cleaner, these are the questions I care about:
-
What's the whole-machine CADR?
-
How much air does it move?
-
What clean-air delivery do I get at the quiet setting?
-
What filtration efficiency does it achieve at the relevant airflow?
-
What's the pressure drop across the filtration system?
-
How is the filter sealed?
-
Can air bypass the filter?
-
How big is the room I'm asking it to clean?
-
How much activated carbon or other gas-removal media is actually inside?
-
Will I comfortably leave this machine running continuously?
Now we're evaluating an air cleaner.
Not shopping for a sticker.
So should I buy a HEPA air purifier?
Maybe.
There are excellent air cleaners that use HEPA filters.
There are mediocre air cleaners that use HEPA filters.
The term alone doesn't answer the question.
I would rather have someone buy a machine after understanding:
airflow + filtration + sealing + room volume + runtime
than simply walking into a store and saying:
“It says HEPA, so I'm good.”
That is the misconception I'm arguing against.
Not HEPA itself.
So what does ULPA teach us about HEPA?
This is why I love the ULPA example.
ULPA forces you to confront an obvious fact:
There is no magical point at 99.97% where filtration engineering ends.
We can go higher.
A lot higher.
We already know how.
ULPA does it.
So why don't we put ULPA in absolutely everything?
Because when engineers design an air-cleaning system, they don't optimize one isolated number.
They balance:
capture efficiency
with:
airflow
with:
pressure drop
with:
filter area
with:
fan performance
with:
energy
with:
noise
with:
size
with:
cost
with:
the actual application.
That's the lesson.
HEPA isn't some mystical finish line.
It's a filtration standard.
The goal is clean air.
Bottom Line: Is HEPA the most important thing in an air purifier?
No. The goal of an air cleaner is not to own the most efficient filter. The goal is to remove as much pollution as practical from the air people are breathing.
HEPA is great technology.
ULPA is even more efficient technology.
MPPS explains why 0.3 microns is not the particle-size cutoff most people think it is.
Airflow explains why a phenomenal filter inside an underpowered machine may clean a room slowly.
CADR gives you a better picture of the useful clean air the machine actually delivers.
And sealing determines whether your dirty air even passes through that impressive filter in the first place.
So stop asking only:
Is it HEPA?
Ask:
How much clean air does it actually deliver?
Because if I have to choose between:
99% of $1
and:
95% of $5
I'm taking the $4.75.
Every time.
Your lungs don't breathe filter-efficiency percentages.
They breathe air.
Frequently Asked Questions
What is ULPA?
ULPA stands for Ultra-Low Penetration Air filtration. Under ISO classifications described by ASHRAE, Group U ULPA filters range from approximately 99.999% to 99.999995% efficiency at their Most Penetrating Particle Size. They are commonly used in specialized cleanrooms, pharmaceutical, semiconductor, nuclear, and other highly controlled environments.
Is ULPA better than HEPA?
ULPA provides higher particle-capture efficiency than HEPA, but that does not automatically make a ULPA-equipped air cleaner better for every application. Airflow, pressure drop, fan design, filter area, noise, energy use, and overall clean-air delivery also matter. DOE notes that ULPA filter pressure drop is frequently greater than standard HEPA filter pressure drop.
Why don't home air purifiers use ULPA?
Some could. But extremely high-efficiency filtration may create more airflow resistance, requiring a system designed with sufficient filter surface area and fan capacity. Residential air cleaning is therefore an optimization problem rather than simply a competition for the highest single-pass efficiency.
What does MPPS mean?
MPPS means Most Penetrating Particle Size. It is the particle size that a particular filter captures least efficiently. For most HEPA and ULPA filters, it commonly falls around 0.1 to 0.3 microns, depending partly on filter characteristics and airflow.
Is 0.3 microns the smallest particle HEPA can capture?
No. EPA says particles larger and smaller than 0.3 microns are captured at even higher efficiency. The 0.3-micron figure represents approximately the difficult-to-capture region, not a minimum particle-size cutoff.
Is 99.97% filtration better than 95%?
At identical airflow and with all other variables equal, yes. But air-cleaner performance depends on both efficiency and the amount of air processed. A slightly lower single-pass efficiency combined with dramatically higher airflow can produce substantially more clean-air delivery.
Is airflow more important than HEPA?
Airflow and filtration efficiency work together. Once filtration is highly effective, the amount of contaminated air reaching the filter becomes extremely important. This is why whole-machine metrics such as CADR are useful when comparing portable air cleaners.
Can filter efficiency change at different airflow rates?
Yes. ASHRAE notes that filter efficiency depends partly on airflow and that it is important to know the airflow at which a filter is rated. The MPPS can also shift with filtration velocity.
Does the seal around an air purifier filter matter?
Yes. If air bypasses the filter, that air isn't receiving the advertised filtration. EPA specifically requires HEPA vacuums used in lead-renovation work to be designed so intake air passes through the HEPA filter without leaking around it.
What matters most when choosing an air purifier?
Look at the complete system: clean-air delivery, airflow, filtration efficiency, room size, filter sealing, performance at realistic fan speeds, noise, runtime, gas-removal media, and maintenance.
Sources
U.S. Environmental Protection Agency. What Is a HEPA Filter? HEPA efficiency, 0.3-micron particle size, and the fact that particles larger and smaller than the MPPS can be captured at higher efficiency.
ASHRAE Handbook, Clean Spaces. HEPA and ULPA classifications, MPPS, filtration mechanisms, efficiency ranges, filter design, and typical applications.
ASHRAE Handbook, Air Cleaners for Particulate Contaminants. HEPA/ULPA construction, airflow resistance, pressure drop, residential limitations, and system performance.
U.S. Department of Energy. Handbook for Use With DOE-STD-1269-2022. Airflow resistance and the observation that ULPA filter pressure drop is frequently greater than standard HEPA.
ISO 29463. International classification and standardized testing of high-efficiency filters at their Most Penetrating Particle Size.
ASHRAE. How Do Particle Filters Work? Filtration mechanisms, airflow effects, resistance, and the importance of the airflow at which a filter is rated.
ASHRAE RP-1649. Research comparing laboratory and installed filter performance, including effects from bypass, air velocity, loading, and installation.
U.S. Environmental Protection Agency. HEPA-vacuum guidance explaining why a HEPA filter alone is insufficient if the machine allows air to leak around the filter.
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