TLDR; The lack of outdoor, ambient humidity in desert climates means we have higher airborne concentrations of the smallest, most dangerous ultrafine particles. Unfortunately, I don’t think the sensors in the Purple monitors are able to accurately report those. 
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We live in the Maricopa/Pinal county areas of Arizona. After our family became sick from a myriad of indoor air quality issues (mold/mycotoxins, VOCs from new construction materials/HVAC defects, etc.) - I assumed our outdoor air must be better than our indoor air.
At that point we began to open our windows as much as possible. I also created some DIY fresh air intakes. Fast forward several years and we were all suffering from a whole new set of health issues. Among mine were non-alcohol induced fatty liver disease, a failing gallbladder, hormonal imbalances, etc. My kids were struggling with all kinds of issues too - chronic stomach problems, behavioral/neurological symptoms, etc. Even our dog was battling bizarre health symptoms we couldn’t seem to resolve. To be clear, we are a very health conscious family, and hyper aware of hidden toxins (including EMFs), so this was all very confusing.
Out of desperation, we left the Maricopa/Pinal area and stayed in Flagstaff/Coconino County for 5 months. While there, we kept the windows open as well. Miraculously, and quite quickly, nearly all of our symptoms improved or resolved, including the dog’s. After returning to the Maricopa/Pinal area, and opening the windows again, our symptoms all returned within 2 weeks.
So, I began to suspect an issue with the outdoor air quality in the area and quickly went down the rabbit hole of research. I soon learned that both Maricopa and Pinal County have notoriously bad outdoor air quality. I purchased a Purple Air Zen monitor and installed it on our back patio. I thought we could simply close the windows when our monitor began registering unhealthy values and open them when it didn’t. However, that did not seem to work. The monitor rarely reported unhealthy values (according to the WHO standards), even when I looked at the raw PM2.5 data.
So, I dug deeper. There is a county monitor located about a block from us. I quickly noticed that the county monitor was not reporting PM2.5 or Ozone, only PM10. I contacted the Pinal County air quality dept. They confirmed that their air monitor network was over 30 years old and had never been updated despite the significant population increases.
The EPA requires that air monitor networks are reassessed every 5 years to ensure that they are in compliance. Pinal County just reassessed last year so I ran that report through Claude, along with the EPAs regulations. Claude confirmed that Pinal County is in violation of at least 4 different regulations, including missing at least one PM2.5 and Ozone monitor in our city. From what I can tell, the EPA does not have the staffing/resources/funding to enforce these regulations so it’s like an honor system at this point. 
(I did the same with Maricopa’s 5 year assessment and Claude did not find any violations.)
Initially, IQAir was also reporting a state-level monitor located at that same site with PM2.5 values. The values being reported from that monitor were higher than those being reported by our Purple Zen monitor and more inline with the clinical symptoms we are experiencing when opening the windows. However, it appears that may have been a mistake. The AZDEQ site only indicates the Pinal county-installed monitors and none of them are at that site. And the monitor has now disappeared from the IQAir app and it only shows my Purple monitor in the area.
At any rate, I have noticed that our Purple Zen does really well at reporting larger PM. For example, when someone in the neighborhood has an outdoor fire going. And when our neighbors were shooting off aerial fireworks for New Year’s, it went crazy. Outside of that, it’s almost always reporting healthy levels, even when our symptoms tell us otherwise.
Because we live in such a dry climate, the smallest, ultrafine particles are more likely to become airborne and stay airborne. So, I started investigating the accuracy of air sensors when measuring ultrafine particles. It seems the smaller the particles, the more difficult (and expensive) it becomes to accurately measure them. For most of the U.S., the outdoor humidity will regularly reach levels that naturally control the smallest particles. The outdoor humidity ‘weighs down’ (for lack of a better term), the ultrafine particles. For those areas, it’s more the larger, easier to measure particles that are an issue. But here, that’s not the case, especially with the climate changes we’ve been seeing.
As much as I wanted to be empowered to monitor my own air quality, I don’t think $300 will get it done in our climate. And I don’t have $30K to drop on my own commercial grade air monitor. Perhaps regulatory agencies like the EPA should prioritize enforcement in areas with the driest climates, since that seems to be where it’s most critical to measure ultrafine particles, requiring prohibitively expensive equipment.
I’m still very grateful these Purple devices and community exist. But I think folks should be aware that mileage may vary, depending on the climate circumstances. 