The specifics of how confidence is calculated are found here: How the Confidence Score is Calculated
The Confidence Score
The Confidence Score is a numerical value displayed on the PurpleAir map ranging from 0 to 100 percent. Every sensor that contains two channels—the PurpleAir Classic, Classic SD, Flex, and Zen—has an assigned Confidence Score. PurpleAir Models that contain one channel—the Indoor and Touch—are assigned a confidence score of 30 by default.
The Confidence Score is a measure of how confident PurpleAir is in the readings a sensor is reporting. That is, it reflects PurpleAir’s belief about the reliability and credibility of a sensor’s data. A higher confidence score indicates a greater level of confidence in the accuracy of the sensor’s data, while a lower score suggests a lower degree of confidence.
PM2.5 data is omitted from affecting the confidence score if it results in a US EPA PM2.5 AQI above 2,000
Sensor Opacity
It is possible that while viewing the map, you might see a sensor that has a lowered opacity. In other words, it’s a little bit see-through. This is correlated with the sensor’s confidence score. After confidence drops past a certain point, that sensor’s dot on the map will have a lowered opacity. Indoor sensors—those with only one laser counter— will always have a lowered opacity on the map because we can never be 100% confident in their readings. See image below:
Channel Indicators
You may also see on the map that your sensor has a downgraded channel. There are two ways that this can happen. First, the system detected that one of the channels in your device was outputting erroneous readings, and it automatically downgraded the channel. Second, a PurpleAir staff member determined that your sensor was outputting erroneous readings and manually downgraded a channel.
Downgrades directly affect a sensor’s confidence score. A sensor with one downgraded channel will appear see-through, as seen above. A sensor that has had both channels downgraded will appear as though it is offline––it will be grayed out on the map.
Channel Output Deviation
PurpleAir sensors, and the laser counters within them, are highly useful tools, but they are not perfect. Each device contains two internal laser counter channels, and while these channels often track closely with one another, some variation is expected.
A large deviation between channels may indicate an issue with the sensor or its readings, but this is not always the case. Environmental factors, sensor aging, and dust accumulation can also contribute to inconsistent readings.
To help interpret these differences, it’s important to consider the Maximum Consistency Error as defined by the laser counter manufacturer, Plantower. For both the PMS5003 and PMS6003 laser counters used in PurpleAir sensors, the accepted tolerance range is:
- ±10μg/m³ @ 0~100 μg/m³
- ±10% @ 100~500 μg/m³
Please note, these ranges are for raw PM2.5 readings, and not AQI levels.
Understanding this margin can help determine whether a variation is within expected behavior or may require attention (e.g., cleaning the sensor or further diagnostics).
Learn More
PurpleAir Map Guide
View Your Sensor on the Map
Why Should I Put my Indoor Sensor on the Map