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Three tenths of a micron is where the optics give up, not where the aerosol stops. Below that threshold scattered light falls away so steeply that a single particle no longer produces a pulse an affordable detector can separate from its own noise, so the instrument simply never registers the event. The air continues to hold particles down to a few nanometres, and in most indoor situations those unseen particles outnumber everything the instrument does count, often by one or two orders of magnitude. They contribute almost nothing to the weight, which is why the mass reading stays calm while an entire population goes unrecorded.
Why 0.3 micron is where the optics stop, not where the particles stop
A laser counter sizes a particle by how brightly it scatters. When a particle is comparable to or larger than the wavelength of the light, scattering is efficient and the relationship between diameter and pulse height is usable. As the particle shrinks below the wavelength the behaviour changes character entirely, and scattered intensity begins to follow the sixth power of diameter. Drop from three tenths of a micron to fifteen hundredths and the signal does not halve, it falls to roughly one part in sixty four.
Detector noise does not fall with it. The photodiode, its amplifier and the stray light bouncing around the chamber all produce a background that stays where it is, so somewhere in that collapse the particle pulse sinks into the noise and the event becomes undetectable. Instrument designers place the lowest threshold just above that crossing point, which is why a figure near three tenths of a micron appears on so many different instruments from so many different makers. It is a property of the physics and of the cost of optics, not a coordinated choice.
Every instrument discussed here lands on the same lower limit from different directions. The WapoRich three channel laser counter states channels at 0.3, 2.5 and 10 micron with a range up to two million pieces per litre, a mass range for PM2.5 and PM10 of zero to one thousand micrograms per cubic metre, an accuracy of plus or minus twelve micrograms per cubic metre and a resolution of one. The six channel cleanroom counter states 0.3, 0.5, 1.0, 3.0, 5.0 and 10 micron. The JCKJEDAT JD 3003 handheld counter states 0.3, 0.5, 1.0, 2.5, 5.0 and 10 micron with a 2.8 inch display. Six channels against three, different middle thresholds, identical floor.
Ultrafine particles carry almost no mass and most of the number
Aerosol scientists describe indoor and urban air as a set of overlapping modes. The nucleation mode sits below about a tenth of a micron and is produced fresh by anything that burns or anything hot enough to vaporise and recondense: a gas flame, a toaster element, a candle wick, a soldering iron, a laser printer fuser. The accumulation mode, roughly a tenth to a couple of microns, is what those fresh particles grow into as they collide and coagulate over minutes to hours, and it is the mode that optical counters see best. The coarse mode above a couple of microns is mechanical in origin: abrasion, resuspension from floors, skin, fibres and soil.
The number concentration is overwhelmingly in the first of those modes and the mass is overwhelmingly in the third. Because weight scales with the cube of diameter, a single ten micron fragment weighs as much as a million particles of a tenth of a micron. That is the arithmetic that lets a gas hob fill a kitchen with a vast population of fresh particles while the microgram reading creeps up by a modest amount, and it is why a blank looking PM2.5 figure is not evidence that nothing happened.
What follows for the instrument is specific rather than general. Switch on a gas ring with a counter running and the 0.3 micron channel will climb, because some of the fresh population has already coagulated past the threshold by the time it reaches the inlet. The climb understates the event, the degree of understatement depends on how far the air travelled and how long it took, and no setting on the instrument can recover the missing fraction.
Channel boundaries are cumulative or differential, and the manuals rarely say
There are two conventions for reporting channels and they produce very different numbers from the same air. A cumulative instrument reports, for each threshold, every particle at or above it. A differential instrument reports, for each band, only the particles that fall between that threshold and the next one up. Contamination control standards are written in cumulative terms, so most instruments aimed at that work report cumulative counts, but a monitor aimed at indoor air may well report differential bands without saying so.
| Threshold | Differential count in this band | Cumulative count at or above this size | Ratio between the two presentations |
|---|---|---|---|
| 0.3 micron | 700,000 | 1,000,000 | 1.4 |
| 0.5 micron | 240,000 | 300,000 | 1.3 |
| 1.0 micron | 48,000 | 60,000 | 1.3 |
| 3.0 micron | 9,000 | 12,000 | 1.3 |
| 5.0 micron | 2,400 | 3,000 | 1.3 |
| 10 micron | 600 | 600 | 1.0 |
The figures above are an illustration of the arithmetic rather than a measurement of any room, and the point is the final column. The two presentations never differ by a huge factor in the fine channels of a typical distribution, which is exactly what makes the confusion dangerous: a mismatch of thirty or forty percent looks like a plausible instrument disagreement rather than an obvious unit error. Which convention each instrument uses is not stated in the listing bullets for any of the counters here, so the manual has to settle it before two sets of counts are compared.
What the three channel and six channel layouts here actually resolve
Channel count changes the resolution of the picture between the fixed end points, and nothing else. Setting the four listings side by side makes the pattern plain.
| Listing | Channels stated in the bullets | Smallest channel | What sits below the smallest channel |
|---|---|---|---|
| WapoRich three channel laser counter | 0.3, 2.5, 10 micron | 0.3 micron | Nucleation mode, unmeasured |
| Six channel cleanroom and HVAC counter | 0.3, 0.5, 1.0, 3.0, 5.0, 10 micron | 0.3 micron | Nucleation mode, unmeasured |
| Six channel air particle meter | 0.3, 0.5, 1.0, 3.0, 5.0, 10 micron | 0.3 micron | Nucleation mode, unmeasured |
| JCKJEDAT JD 3003 handheld counter | 0.3, 0.5, 1.0, 2.5, 5.0, 10 micron | 0.3 micron | Nucleation mode, unmeasured |
Note the detail in the third column of the two six channel layouts. One places a threshold at 2.5 micron and one at 3.0 micron, which looks trivial and is not, because the 2.5 micron boundary lines up with the definition of PM2.5 and the 3.0 micron one does not. An instrument whose thresholds straddle the definition has to interpolate to produce its mass figure, adding one more assumption to a conversion that already carries several. The three channel layout, meanwhile, is the one that reports at exactly the two boundaries the mass definitions use, which is a coherent design for a monitor and a coarse one for contamination work.
Reading a count with its lower cutoff written next to it
Treat the smallest channel as part of the reading, in the same way a temperature is meaningless without a scale. A count of one million particles per litre at or above 0.3 micron and a count of one million per litre at or above 0.5 micron describe very different air, and a note that says only one million per litre describes nothing. Write the threshold, the convention and the lower limit into the record every time, because the person reading it later will not be able to reconstruct any of the three.
Then be clear about what the number cannot support. It cannot support a statement about ultrafine exposure, since that population is below the floor. It cannot be compared with a reading from an instrument with a different lowest channel without an explicit correction nobody can make reliably. It can support a comparison of the same place against itself, a comparison between rooms measured with the same instrument on the same day, and a judgement about which mode a rise belongs to. Those are real and useful conclusions, and they are the honest boundary of the instrument. For how the different layouts read in practice see the WapoRich three channel writeup and the six channel counter writeup; the same discipline about recording the configuration alongside the figure applies to multi channel temperature logging, where a channel number without a probe location is equally unreadable later.
Frequently Asked Questions
Why do almost all handheld counters stop at 0.3 micron?
Because the light a particle scatters collapses as it gets smaller. Once the diameter falls well below the wavelength of the laser, scattered intensity falls roughly as the sixth power of diameter, so halving the size cuts the signal by a factor of about sixty four. Three tenths of a micron is close to the point where a pulse from a single particle still stands clearly above the electrical noise of an affordable photodetector. Going lower requires a longer optical path, a brighter source, a condensation stage or a cooled detector, none of which fit in a handheld body.
Are ultrafine particles harmless because they weigh so little?
Their negligible weight is the reason mass based instruments barely see them, not a statement about their effects. Particles well under a tenth of a micron deposit efficiently deep in the lung precisely because they are small enough to follow the airstream rather than impact on an airway wall. Health research treats them as a separate exposure metric for that reason. What is true is that no instrument in the handheld class described here reports them, so a low reading says nothing about that fraction either way.
Is a six channel instrument better than a three channel one?
It resolves the distribution more finely between the same two end points, which is useful when you are trying to work out what kind of material you are looking at rather than how much of it there is. A three channel layout gives you a coarse shape. A six channel layout can show that the rise after a job sat in the one to three micron band rather than in the sub micron band, which points at mechanical abrasion rather than combustion. Neither layout extends the lower limit, so neither sees any more of the air than the other.
How do I tell whether my channels are cumulative or differential?
Compare adjacent channels in dirty air. If every channel reads lower than the one below it, without exception and by a wide margin, the instrument is almost certainly reporting cumulative counts, since a cumulative series can only decrease as the threshold rises. If a middle channel ever reads higher than the channel below it, the series must be differential. The manual should say outright, and when it does not, that comparison is the quickest practical test.
Does the 0.3 micron channel count everything above 0.3 micron?
On a cumulative instrument, yes, that is exactly what it claims, so the figure includes every particle the optics can detect at any larger size. On a differential instrument the same channel means particles between 0.3 micron and the next threshold only. The two readings can differ by a large factor in the same air, which is why the cumulative or differential question has to be settled before any two counts are compared.