Good hint, I will probably do that. Still bugs me… 
Btw, I just finished a near 4 hours print with Silk PLA (I went pretty slow…). The readings, may they be too low, did not change at all during the entire print, even though I removed the top lid and opened the front door for the entire time. I had the meters placed about 1m away from the printer.
It’s probably generating some particles (PLA does not really generate many VOCs), but not enough to register on your device. It’s a much safer material and definitely low risk.
This is probably an oversimplification, but you can think of your house as having all the polutants of outdoor air, plus all your indoor polutants, but less circulation to get rid of the ones you generate inside. In the typical US home, the air inside a house is fully exchanged every 2-3 hours (through drywall, cracks, joints, doors, windows, etc). Outdoor pollutants can definitely end up in your room, even from cars or neighbors that use gas lawn mowers. So basically you’re starting from a baseline of 12 PM 2.5 (in your area).
Then you have all your indoor pollutants. Basically any combustible source (cooking, oven, water heater, gas furnace) will generate VOCs. Building materials, rugs, and paint can release particles when disturbed. Any piece of electronics can release small amounts of particles or VOCs when heated. They probably can collect on surfaces and be disturbed later during activities like vacuuming. Chemicals in building materials and products can produce VOCs and mold is another big contributor in homes. Most houses have a small amount of mold somewhere. If all your windows were open, your healthier outdoor air is going to be cleaning a lot of this stuff up. Even if you disturb something, with more circulation you’re less likely to breathe it in. With all your windows and door closed, your house will cycle every 2.5 hours and most likely be more polluted than outdoor air, which cycles much faster.
Obviously there’s exceptions to everything. Apartment dwellers with good building filtering systems in big cities may have much healthier indoor air. NYC PM 2.5 is 31 right now and I wouldn’t be surprised if many people’s indoor air was better. California during wildfire season is another obvious case. Probably anywhere in China might be healthier inside.
Anyone who opens their windows is going to normalize to outdoor levels pretty quickly, whether they be good or bad. Only way to get lower values than outside is to seal your house the best you can and run filters. The more polluted the outdoor air is, the easier it will be to have healthier indoor air. If the outdoor air is fairly healthy, you’re going to have to run a lot of HEPA filters to have healthier indoor vs. outdoor air. The filter industry probably wouldn’t exist if everyone could open their windows without worrying about freezing or frying.
Drywall, wood, treated floors, tile, carpet, computers, VOCs from furnaces and cooking in other rooms, small amounts of mold from bathrooms around the house. If you have central air conditioning all the rooms get mixed together.
Any particles smaller than 2.5 microns (PM 2.5) will end up in your lungs fairly easily, and your body has limited capacity to expel them naturally, which is why it’s important to stay below certain levels on average.
But to put it all in context for risk, I think outdoor PM 2.5 levels have contributed to 1 year life expectancy drop world wide and 2 years in more polluted countries. Obviously on an individual level it’s going to vary from person to person, with some people losing many more than two years, and maybe a one person gaining a year because they’re fueled by microplastics particles.
Obviously a hyperbole but you get the idea.
Flow rate is going to matter. I’m doing a print right now PLA polyterra @ 235, line size (.4 x .8 @ 80m/s) for a flow rate of around 25. This is both slightly higher than average temps and melting a lot of plastic at once, but it’s supposed to be PLA and safe. If you’re doing PLA @ 220 with a line size of (.2 x .4 @ 100m/s), your flow rate is around 8. If you’re melting 33% less plastic you’ll probably get a lot fewer particles. Here’s my quick test below with a reputable < $100 PM 2.5 meter. I calibrated the meter before hand and verified it was within 20% of my nearest station.
PM 2.5 readings - 33 ug/m3
PM 2.5 readings + 400 cfm hepa-ish/carbon filter next to printer - dropped to 12 within a few minutes.
(Note: All the recycled cardboard in the polyterra probably isn’t healthy).
Finally, I ran one more test with a filtered 10x10ft room keeping the sensor about 8ft way from the printer. The pm 2.5 readings started at 4 and I left the air filter off. Over a course of a 60 minute print it climbed to about 20.
Just wanted to add that during printing with eSun PLA+ I could not see any change in particle amount, I placed the meters around 1m away from the printer and I printed with the upper glass lid removed. The readings stayed at my insanely low values all the time. 
I got another “proof” that my meters are measuring reasonable values (imho): I desoldered some suspicious capacitors from a mains supply board of a faulty oscilloscope, after using the soldering iron for a just few minutes, the PM2.5 values went up to values around 20-25. Sure enough these values settled quickly after finishing my soldering work.
The flow rate surely matters, the high values recorded during ABS+ printing were most likely partly caused by printing at about 20mm³/s using a 0,6mm nozzle.
your sensor probably doesn´t register smaler than pm2,5 ultra fine particles ufp are even smaler. check out some youtube videos about 3d printing air quality. air exchange with the environment is the best
