I live in Kamloops, BC. On days that looks smoky I go to Purple Air as well as the Canadian Government’s AHQI website to check the air quality readings.
Lately, today included, it’s smoky outside. The air is tinted orange and it smells like a campfire everywhere outside.
When I look at Purple Air today it’s been reading (so far) at 1-3, low risk. What I’m seeing and smelling outside doesn’t seem to match.
On Purple Air’s live map when I change it to another set of data like the US EPA, or the Australian PM 2.5 it’s reading as much higher and more dangerous levels.
What’s going on?
Thanks for your time
This is because of the conversion rate of the Canadian AQHI.
- The Air Quality Health Index or (AQHI) is a scale designed in Canada to help understand the impact of air quality on health.
PurpleAir sensors count the particulate matter that is in the air and report it to our systems. The PurpleAir Map takes the values reported and lists them for users to access. Users can select the filters and data layers on the map to convert our reporting of particulate matter into the different scaling systems that are created by different national and scientific groups.
The example you encountered is also affected by the fundamental differences between these conversions and their factors.
- The AQI is a tool used to communicate to the public how polluted the air is.
- The AQHI takes into account the health effects and environmental concerns associated with varying concentrations of pollution.
Sourced from Canada.ca the AQHI was created to report on the health risk posed by a specific level of air quality.
I hope this assists when selecting the conversion factors we have available. Linked here is a list of the index layers that the PurpleAir map employs.
Rach. I had the same question living in Regina, SK. There are a couple of reasons the PA map calculation of the Cdn AQHI does not seem to match up with what our official agency Environment Cda reports. The AQHI is calculated based on the combined effect of 3 components (PM2.5, Ground Level Ozone, Nitrogen Dioxide). See Air Quality Health Index (Canada) - Wikipedia. The PA sensors only measure PM and that is all the PA map is using to calculated the AQHI. The second difference is that many provinces including BC/SK also use a variation of the AQHI they call AQHI-Plus during the summer months when forest fire smoke is the dominate AQ concern. That formula is much simpler. Wildfire Season AQHI-Plus = ceiling value (the closest integer greater than or equal to a given number) of the 1-hour PM2.5 concentration divided by 10. I believe BC like SK use the AQHI+ when the PM2.5 reaches 51 ug/m3. Therefore PM2.5=51 will be reported as AQHI 6…61 would be 7 and so on. See https://www.tandfonline.com/doi/full/10.1080/10962247.2020.1797927 for studies from your province.
I prefer to switch the map to show me Raw PM2.5 concentration and convert using the AQHI+ formula. I also use the ALT CF=3 conversion factor as the PA devices mostly over estimate PM and I have found that setting more closely matches official sources. Play around with the different settings. Lots of good data to play with.
One other good way to compare the PA devices to official sensors in Canada is to use this AQmap (EN). Originates in your province as well. It shows PM2.5 only but a good resources to see how PA stacks up against official sensors.
Hope this helps.
This is excellent information!