Hi,
I can get my home interior to US EPA Conversion 2.5 PM to AQI 12 or so with air purifiers, and I typically close the windows at outside AQI 28. The readings above are for my Zen, outside on the second floor balcony.
I am finding it difficult to get my head wrapped around this. The Zen cannot distinguish between particle source, only be adjusted with the Conversion tool. The closest official US EPA/CARB station - about 4 miles away - is showing AQI 43 - most typically the Converted US EPA is closer and higher.
So… I see AQI 28 as the actual top of the “Good” Range. 18 is well below that, we can say that 38 and 43 are close so the US EPA Conversion is working well enough, assuming conditions in the region are relatively homogenous… and they are somewhat higher.
None of is objectively very bad air quality, but I can’t understand my own guidelines given the nearly 100% difference between the two Conversions.