# Purple US EPA Index Seems Strange

**URL:** <https://community.purpleair.com/t/purple-us-epa-index-seems-strange/22898>\
**Category:** User Experience\
**Created:** [August 24, 2026, 3:50pm UTC](https://community.purpleair.com/t/purple-us-epa-index-seems-strange/22898 "2026-08-24T15:50:39Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![davepwhite](https://avatars.discourse-cdn.com/v4/letter/d/13edae/32.png) [@davepwhite](https://community.purpleair.com/u/davepwhite)\
**Post date:** [August 24, 2026, 3:50pm UTC](https://community.purpleair.com/t/purple-us-epa-index-seems-strange/22898/1 "2026-08-24T15:50:39Z")

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![PurpleIndex](https://us1.discourse-cdn.com/flex020/uploads/purpleair/original/2X/2/2ea89956d4b20f7e8d3e9edf2084cd3c3857761b.jpeg)

I have a Purple Flex installed outside and I monitor it from my phone’s home screen using the “widgets” offered on the Purple Map. I like to know the current readings so I use the “real time” overlay along with the US EPA conversion and showing the US EPA PM 2.5 data layer. I also want to know both PM2.5 and AQI values at any given time. So I actually use a widget for each and just combine them into a single html page that gets shown on my phone’s home screen.

The attached screen show shows this at 2 adjacent points in time: 8:11am and 8:16am this morning. As you can see, the PM2.5 level is exactly the same at 2.

What I cannot understand is how/why at exactly the same PM2.5 level the computed/displayed AQI should be different: 9 and 11? From what I understand, the PM2.5 value is simply put through a fixed formula to produce the index. So how can the same input produce two different outputs?

Can anyone help me understand this? Thanks

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<div class="post-metadata">

**Author:** ![davepwhite](https://avatars.discourse-cdn.com/v4/letter/d/13edae/32.png) [@davepwhite](https://community.purpleair.com/u/davepwhite)\
**Post date:** [August 24, 2026, 5:05pm UTC](https://community.purpleair.com/t/purple-us-epa-index-seems-strange/22898/2 "2026-08-24T17:05:52Z")

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Someone over on Reddit said: **For US EPA AQI, 0-50 = 0-9ug/m3. So this is LIKELY a product of PA only showing the whole concentration value (I.e., 2.1 vs. 2.2 ug/m3).**

So I decided to start running pm2.5 values between 1.5 and 2.5 thru the EPA formula I have (got elsewhere here). I found that at pm2.5 = 1.6 I get AQI = 9. At 2.0 I get 11, at 2.2 I get 12 and so on. So I guess the Reddit answer is correct - the display of the pm2.5 value is being rounded while the computation for AQI is using the raw value.

A bit confusing but…