Tom Butt
  E-Mail Forum – 2020  
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  Air Quality Confusion
August 25, 2020

There is a plethora of air quality test data available on the Internet that report PM 2.5 , the fine particulate matter that makes smoke from wildfires unhealthy. To add to confusion, the concentration pf PM 2.5 is reported in two ways, as micrograms per cubic meter (μg/m3) and as Air Quality Index (AQI). The UN World Health Organization (WHO) uses μg/m3, and US EPA uses AQI.

The WHO air quality guidelines for particulate matter is as follows:
Air pollutant WHO Exposure Guideline

  • PM2.5 10 µg/m³ annual mean
  • 25 µg/m³ 24-hour mean
  • PM10 20 µg/m³ annual mean
  • 50 µg/m³ 24-hour mean

The annual mean exposure guideline of 10 µg/m³ guideline is broadly referred to as the authoritative global guideline for PM2.5 exposure to be limited to, in order to minimize the risk of negative health effects.
By comparison, the US EPA’s Air Quality Index represents PM2.5 levels below 12 µg/m³ as within the "Good" category, which is very close to WHO standards. Thus, the two different standards are relatively comparable, as shown in the chart below.

The system with the most sensors in Richmond is the one set up pursuant to AB 617 Air Monitoring in Richmond. The data is available on an hourly basis through Ramboll Shair at the website There are dozens of sensors reporting in Richmond. The only downside is that the data is reported hourly, so conditions can change quickly before the data catches up. That explains why you might see an all blue map on Ramboll-Shair and then look out the window to see thick smoke.

Figure 1 - Image from Ramboll-Shair

Another frequently accessed site is Purple Air, a volunteer driven system of air sensors that use WiFi to in real time to the PurpleAir map. Here are only half a dozen Purple Air sensors reporting in Richmond. It is not clear, but there may be a one to two hours lag time.

Figure 2 - Purple Air Map (

Here are some additional sources for current and forecasted air quality conditions and for wildfire smoke and health information.

  • U.S. EPA also has data and helpful information on its Airnow website:
    • Air quality and health information:
    • Interactive map with fire, smoke, and air quality data: This map shows latest AQI from regulatory monitors and low-cost PurpleAir sensors, as well as locations of fires and smoke plumes. (Note that data from low-cost sensors have greater uncertainty than regulatory monitors, but can help add to the overall picture of air quality across the area.)

At the end of the day, probably the most reliable test is the smell test. If it smells like smoke and looks like smoke, it probably is smoke. If you go out, wear your N95 mask.