Dust Control: HEPA Extraction and CFM Matching
The number on the box is measured somewhere the dust never goes. Between that rating and the shroud sit a hose, a filter and every seal in the system, and each one is airflow you paid for and do not get.
A vacuum’s rated airflow is measured at its own inlet, with no hose on it. What reaches the shroud is less, and the largest single deduction is usually the hose, where diameter matters far more than length. It then falls further through the shift as the filter loads, which is why a pre-separator and a filter cleaning mechanism are not optional extras. Match the vacuum to the airflow the tool manufacturer specifies for that machine, check both CFM and water lift rather than CFM alone, and remember that HEPA describes a filter rather than the system it sits in.
The vacuum is an engineered control, not housekeeping
Dry grinding concrete generates respirable crystalline silica, and grinding vs acid etching sets out why that is a serious, regulated hazard rather than a nuisance. What matters here is the consequence for equipment: the control is attached to the machine and runs the entire time it is cutting. It is not a tidy up afterwards, and protective equipment is not a substitute for it.
That framing changes how the vacuum is chosen. A machine bought to clean the site at the end of the day is specified on price and capacity. A machine that is the control has to keep delivering a particular airflow for hours, through a real hose, with a filter that is filling up. Those are different purchases, and the rest of this page is about the gap between them.
The rating is measured where the dust never goes
The airflow figure on a vacuum is taken at its own inlet with nothing attached. That is a reasonable way to compare two machines and a poor way to predict what happens at a shroud, because on site there is always a hose, a shroud, and a filter in some state of loading between the rating and the work.
The hose is usually the largest single deduction, and the surprising part is which property of it matters. Length costs you something. Diameter costs you far more, because resistance to airflow climbs very steeply as the bore narrows, much faster than it rises with length. Adding another length of the correct hose is a smaller mistake than fitting a narrower one because it happened to be in the van, and it is the second of those two that people do without thinking about it.
| What it is | Why it costs you airflow | What to do about it |
|---|---|---|
| Hose diameter | Resistance climbs very steeply as the bore narrows, far faster than it does with length. | Use the diameter the machine was designed around. Never substitute a narrower hose because it was to hand. |
| Hose length | Every extra run adds resistance, though far less per foot than narrowing the bore does. | Move the vacuum with the machine rather than adding hose to reach it. |
| Filter loading | Concrete dust is fine and abundant, so resistance grows steadily through the shift. | A pre-separator and a filter cleaning mechanism, plus emptying on a schedule rather than on feel. |
| Leaks and seals | Air entering anywhere except the shroud does no work, and the machine cannot tell you. | Check cuffs, the canister gasket, the lid seat and the bag fit as part of setup. |
| Shroud contact | A worn brush or a shroud lifted over an uneven slab breaks the capture at the source. | Replace brushes as wear items. Capture that fails at the head cannot be recovered downstream. |
Reading across a row: the thing in the airflow path, the reason it reduces what reaches the shroud, and the response. No airflow figures are published here, because the number that matters is the one the tool manufacturer specifies for your machine, and a figure invented for a table would be worse than none.
CFM and water lift are two numbers, and you need both
This is the specification trap, and it costs money at the point of purchase rather than on site. The two headline figures on an extractor describe different abilities, and a machine can be strong in one and weak in the other.
CFM is volume: how much air the machine moves. Water lift, also called sealed suction or static pressure, is force: how hard it can pull against resistance. A vacuum with high CFM and low water lift moves a great deal of air with nothing in its way and collapses the moment it has to pull through a long hose and a partly loaded filter. One with high water lift and modest volume holds its suction but may not shift enough air to carry what a wide head produces.
Concrete extraction needs enough volume to capture the dust and enough force to keep delivering it as the system gets harder to pull through. Compare both figures on any two machines, and pay attention to how each is quoted rather than only to which is larger.
The suction fades all day, and nothing sounds different
Concrete dust is fine and there is a great deal of it, so a filter loads quickly, and a loading filter is a resistance that grows for as long as you keep cutting. The machine note does not change while the capture quietly drops, which puts this in the same family as the profile drift on a shot blaster: a piece of equipment delivering steadily less than it did at the start of the shift, with no signal that it is happening.
Two things exist to fight it, and a serious extractor has both. A pre-separator, usually a cyclone, drops the bulk of the material out before the filter sees it, so the filter is dealing with fines rather than with the whole load. A filter cleaning mechanism, a pulse or a shaker, knocks the accumulated layer off during the shift rather than at the end of it. Without a pre-separator, a vacuum can be adequate for the first hour and marginal by mid morning.
The habit that follows is to empty and clean on a schedule rather than when the machine feels tired. By the time it feels tired you have already been grinding at reduced capture, and there is no way to go back and collect what was missed.
HEPA is a filter rating, not a system rating
Dust escapes at the weakest point in a system rather than through its best component, so a HEPA cartridge guarantees very little on its own. In a housing that does not seal, behind a loosely fitted bag, or on a machine with a perished lid gasket, it is not delivering the performance its rating describes.
- The chain includes everything. Shroud contact with the floor, every hose joint and cuff, the canister seal, the lid seat, and the bag. One bad link is the whole result
- Emptying is part of the control. It is the moment captured dust has its best chance of getting back into the air, so treat a bag or cartridge change as a task with its own protection rather than as cleaning up
- Filters are wear items with a service life. A cartridge that has been knocked, washed when it should not have been, or simply run too long is not doing what the label says
- Consumables belong in the rate. Filters, pre-filters and bags scale with grinding hours, so they are a direct cost rather than an overhead. The grinding cost calculator has a line for exactly this
So what number do I actually match?
The airflow the tool manufacturer specifies for that machine. There is no universal figure that replaces it, and this site is not going to publish one, because it would be invented rather than derived.
The requirement genuinely is machine specific. It depends on how wide the shroud is, how well it seals to the floor, and how much material the head removes, so a figure that suits a small handheld grinder is nowhere near enough for a wide planetary machine. It is also where the regulation points rather than legislating a number: in the United States, the construction standard for respirable crystalline silica, OSHA 29 CFR 1926.1153, includes a route built around using a dust shroud with a vacuum that provides the airflow the tool manufacturer recommends, together with a high efficiency filter and a means of keeping it clean. Other countries have their own equivalents, and the standard itself is the authority on its own requirements rather than anything written here.
So the sequence is: take the figure from the machine’s documentation, choose an extractor that exceeds it rather than meets it exactly, then confirm it can still deliver through the hose you actually run and with a filter that is halfway through its day. That last step is the one that separates a specification from a working control.
Where to go next
For the hazard this equipment exists to control and why it is not optional, grinding vs acid etching sets out the silica case alongside the very different hazards of the chemical route. For the sequence the whole operation sits inside, how to prep concrete for epoxy runs the four gates in order.
If dust sensitivity is the deciding factor on a particular building, grinding vs shot blasting covers why a closed loop process can win a job on that alone, and shot blasting concrete covers what running one actually involves. For where extraction consumables belong in a price, grinding cost per square foot puts them in the rate rather than in overhead.
Frequently Asked Questions
What CFM do I need for my grinder?
The figure the tool manufacturer specifies for that machine, and there is no universal number that replaces it. We are not going to publish one, because it would be invented.
The airflow a shroud needs depends on how wide it is, how well it seals to the floor, and how much dust the head generates, all of which are properties of a specific machine. A number that suits a small handheld grinder is nowhere near enough for a wide planetary machine. This is also where the regulation points: the United States construction standard for respirable crystalline silica, OSHA 29 CFR 1926.1153, offers a route where you use a shroud with a vacuum providing the airflow the tool manufacturer recommends, rather than naming a figure of its own. Other countries have their own equivalents.
So start at the machine’s documentation, then check that the vacuum can still deliver that figure through the hose you actually use, which is the part most people skip.
Why does my vacuum feel weaker on site than it did in the shop?
Because the rating was measured at the vacuum’s own inlet with nothing attached, and on site there is a hose, a shroud and a partly loaded filter in the way. The rating is a ceiling, not a delivery.
The hose is usually the largest single deduction, and the surprise is which property of it matters. Length costs you something. Diameter costs you far more, because the resistance to airflow climbs very steeply as the bore narrows, much faster than it does as the hose gets longer. Reaching for a narrower hose because it was the one in the van is a bigger mistake than adding another length of the correct one.
The rest is leaks. A cracked cuff, a shroud brush that no longer touches the floor, a lid that is not seated, or a bag fitted loosely all bleed air into the system, and air that enters anywhere other than the shroud is doing no work at all.
What is the difference between CFM and water lift?
They are two different numbers describing two different abilities, and a vacuum needs both. Buying on one of them is the classic way to end up with a machine that underperforms on a real hose.
CFM is volume: how much air the vacuum can move. Water lift, also called sealed suction or static pressure, is force: how hard it can pull against resistance. A vacuum with high CFM and low water lift moves plenty of air when nothing is in its way and collapses as soon as it has to pull through thirty feet of hose and a loaded filter. A vacuum with high water lift and modest CFM can hold its suction but may not shift enough volume to carry the dust a wide head produces.
Dust extraction on concrete needs enough volume to capture and enough force to keep delivering it through a long hose and a filter that is filling up. When you compare two machines, compare both numbers, and look for how they are quoted rather than only how large they are.
Why does the suction fade as the day goes on?
Because concrete dust is fine and there is a great deal of it, so the filter loads quickly, and a loading filter is a resistance that grows all shift. The machine sounds the same while delivering steadily less, which is why this is worth knowing rather than discovering.
Two things exist to fight it, and a serious extractor has both. A pre-separator, often a cyclone, drops the bulk of the material out before it ever reaches the filter, so the filter deals with the fines rather than the load. A filter cleaning mechanism, usually a pulse or a shaker, knocks the accumulated layer off during the shift instead of at the end of it. Without a pre-separator a vacuum can be fine for the first hour and marginal by mid morning.
The habit that follows is simple: empty and clean on a schedule rather than when the machine feels tired, because by the time it feels tired you have already been grinding at reduced capture.
Is a HEPA filter on its own enough?
No, because HEPA describes the filter, not the system it is sitting in, and dust escapes at the weakest point rather than through the best one.
A HEPA cartridge in a housing that does not seal, behind a bag that is fitted loosely, or on a machine whose lid gasket is perished, is not delivering HEPA performance. Neither is one that has been knocked, washed when it should not have been, or run so long that it is damaged. The filter is one component in a chain that includes the shroud’s contact with the floor, every hose joint, the seal on the canister and the way the machine is emptied.
Emptying is the part that gets overlooked, because it is the moment the captured dust has the best chance of getting back into the air. Handle it as part of the control rather than as cleaning up afterwards, and treat any bag or cartridge change as a task with its own protection.
Do I still need extraction if I am wet grinding or shot blasting?
The hazard changes shape rather than going away, and each method has its own version of the problem.
Wet grinding suppresses dust at the point it is made, which is genuinely effective, but it converts the problem into slurry that has to be contained, collected and disposed of, and a slurry that dries out on a floor or in a bucket becomes airborne dust again. It is a different management task, not the absence of one.
Shot blasting recovers its abrasive and debris in a closed loop and puts very little into the air when it is working properly, which is one of its real advantages. The qualifier is the same one that applies to any extractor: that depends entirely on the collector being matched, emptied and maintained. A blaster with a clogged or undersized collector is not a clean process, and the edges are being ground with a machine and a vacuum anyway.