Yes, exactly - you mentioned the PlanTower PMS5003 so I managed to find a datasheet for that here:
Similarly, I found an article that did a teardown of the IKEA Vindstyrka AQ monitor here:
which mentions that the sensor used is a Sensirion SEN54, for which the datasheet is here:
Now looking at both of these datasheets, you can see that the minimum particle size that can be detected is 0.3um. Interestingly, as I mentioned before, the definition of the PMx categories is detecting the mass of particles of that size and below. And looking at the SEN54 datasheet for example, you can see that this is implemented as expected where every mode from PM1 to PM10 includes particles down to the minimum resolution of 0.3um, so I would expect it would work the same for the PMS5003 too.
So you can see that neither of these sensors can directly detect any UFPs, because they are below their minimum particle size.
Now the way that normal PMx ratings work is by detecting the mass of particles in that size category to give a ug/m3 reading, but as shown in this article, an additional problem is that because UFPs are so small, their mass is also small, so they would barely change the ug/m3 reading, even if the sensor’s minimum particle size was small enough.
So really in order to properly detect UFPs, a sensor would need to have a minimum particle size on the order of about 10nm or less (rather than the current 300nm for those 2 sensors), and it would need to have either a true particle counter, or if it was doing it by mass, then it would need some kind of special detection category to filter by only UFPs (such as PM0.1), as well as a high resolution on the ug/m3 reading in order to see the small mass increases caused by UFPs.
So we know that normal AQ sensors cannot detect UFPs directly, but what about indirectly?
Well according to this study that I linked previously (https://www.tandfonline.com/doi/full/10.1080/02786826.2017.1342029), the breakdown of particle emissions from a 3D printer as size vs mass shows that for a “long” print job (7 hours+), only 14% of the total mass of the particle emissions are 300nm or bigger. And for shorter print jobs, the ratio is even worse, with 12% or less being 300nm+ (i.e. detectable).
The paper says that the reason why shorter print jobs have a higher concentration of UFPs is because a lot of the particles are first formed as very small (<7nm), and then coagulate into large particles by clumping together. So for any print job, the amount of UFPs, and the percentage they make up of the total, is biggest at the start, but the longer the job goes on, the more the particles grow and shift into the >300nm range, before reaching steady state at around 7 hours+.
So essentially you could in theory indirectly sense UFPs, but firstly it should only show up after some time has passed (not right at the start of the print job) and the particles have had a chance to grow into the detectable range, and also you would need to multiply the sensor’s ug/m3 value by some ratio, to get the amount of the total particle emissions, instead of just the 300nm+ emissions. So for a 7h+ print job, you would be looking at a multiplier of 7.14 times what the sensor says for the true ug/m3 reading, and for shorter print jobs it would be an even higher multiplier. Also, this is just a mass reading, but we know from the papers I linked previously that UFPs have a higher toxicity than 300nm+ particles, so you could argue that you might multiply the ug/m3 reading even further to account for this skewed particle distribution if you wanted to compare the reading to some standard “safe” thresholds for air quality.
Now so far in this post I’ve been considering how effective a sensor would be if you were either placing it inside the printer, or if you put it outside the printer but you assume that the printer is not fully airtight and it can leak UFPs/particles. That last statement I’m still uncertain about because I couldn’t find any hard evidence, but I’ve seen some reports before of the enclosure on the P1S/X1C having some gaps between the panels and not being fully airtight.
In addition, there is the rather obvious large hole at the back of the printer where the chamber fan and activated carbon filter is. Since this filter is only good for getting rid of VOCs, you would assume that it would allow UFPs and other particles to pass through without issue, because the particles on the inside are not guaranteed to go through the bento box instead of just escaping through the carbon filter?
But if you ignore that issue or somehow find a way to make sure there is no exit path for the UFPs without going through a HEPA filter first, then yes I agree with what you said about how you could safely assume that any readings on the air quality meter are purely going to be about the less dangerous 0.3um size particles or bigger, since the UFPs will already have been removed by the HEPA filter before the air gets to the AQ monitor on the outside.
Personally though, I am a little concerned that just using the bento box and the stock P1S/X1C enclosure would not be enough to guarantee UFPs won’t escape filtration.
In which case, you would have to fall back on the low accuracy method of just multiplying the ug/m3 meter reading in order to account for those leaking UFPs, and hope that the resulting true particulate emissions after multiplication are low enough to be acceptable. But if they are not at an acceptable level, you could use an air purifier on the outside of the printer either as a replacement for the bento box, or in addition to the bento box and put the purifier on a low setting to mop up whatever escaped the bento box.
But on the other hand, I could just be being far too cautious about the safety side of things, and in reality as some people have mentioned, bento box + stock enclosure may be good enough to have the printer in the same room as you for long periods without any noticeable health effects.
