Making Sense of Air Quality



With wildfires moving across California, many of us are concerned about air quality. Before the fires of 2017, few of us knew and understood phrases like SF AQI PM2.5: 154. Now we are all instant experts in air quality. The 2020 fire season started early with a freak dry lightning storm moving across Northern California with many strikes causing the dry tinder to burn all across our part of the state. As of this writing, over 3 million acres have burned.

Experts measure air quality on a scale from 1 to 500 called the AQI index. The index has six distinct bands.

Daily AQI ColorLevels of ConcernValues of IndexDescription of Air Quality
GreenGood0 to 50Air quality is satisfactory, and air pollution poses little or no risk.
YellowModerate51 to 100Air quality is acceptable. However, there may be a risk for some people, particularly those who are unusually sensitive to air pollution.
OrangeUnhealthy for Sensitive Groups (USG)101 to 150Members of sensitive groups may experience health effects. The general public is less likely to be affected.
RedUnhealthy151 to 200Some members of the general public may experience health effects; members of sensitive groups may experience more serious health effects.
PurpleVery Unhealthy201 to 300Health alert: The risk of health effects is increased for everyone.
MaroonHazardous301 +Health warning of emergency conditions: everyone is more likely to be affected.

EPA Air Quality

The EPA operates many high-quality air quality sensors throughout the United States. These sensors detect four critical components in air quality: ground-level ozone, particle pollution, carbon monoxide, and sulfur dioxide. If you are near an air quality sensor, just go to and input your location. Alternatively, iPhone users can ask Siri, “what’s the air quality?” or “what’s the air quality in San Francisco?”.

In regular times, these sensors generally give useful data points. Still, the fires highlight key nuances and how air quality is measured and communicated.

Purple Air

The folks over at PurpleAir have developed a low-cost air quality sensor. A PurpleAir sensor only costs about $250. Because of its low-cost price point, significantly more PurpleAir sensors cover the landscape. However, PurpleAir sensors only measure one component of air quality: particulates. Particulate matter, specifically PM 2.5 is often linked to heart and lung issues. Particulates thus are important to watch.

PurpleAir’s coverage highlights that local air quality can be very nuanced. Shifting winds can bring particulates from far-off fires quickly to a new area. Also, areas with geographic features such as mountains can trap air and cause significant differences in air quality.

In seasons of rapidly changing air quality, the PurpleAir sensors will differ sometimes significantly from the EPA counterparts. This occurs for two reasons. First, the PurpleAir sensors deliver real-time readings of air quality. The EPA sensors transmit an average reading over the past 1 to 3 hours. Also, PurpleAir sensors are known to over-report the amount of particulate matter. There are two conversion factors available in the web interface provided by the Lane Regional Air Pollution Agency in Oregon (LRAPA) and the University of Utah (AQandU).

In the screenshot above, check out the legend in the lower left. Uncheck the box for inside sensors. These sensors measure the air quality indoors, which can significantly differ from quality based on the efficiency of the home owner’s HVAC system. Indoor purple air sensors have a thick black line around the value on the map. The conversion factor noted above can be set down in the upper right of the legend. Toggling this option will more closely bring purple air sensor value with the EPA value.

While PurpleAir does not have a dedicated mobile app, iOS users can download Local Haze which aggregates data from PurpleAir sensors. The map view and the ability to favorite a sensor are handy features!

Tracking Smoke

Software engineers at the EPA then combined all of the data from PurpleAir, their own sensors, active fires, and smoke plumes in a new portal at This map view gives a significantly better view of the activity in one model. Currently, the west is experiencing massive amounts of fires. As winds tend to blow west to east in North America, the Great Plains are starting to get some of the smoke from Western fires.

It’s important to note that smoke can sit in many different layers of the atmosphere. We can see the smoke plume over Kansas, but air quality remains good. It is likely that the smoke plume is higher in the atmosphere and not affecting ground-level air quality. On Wednesday, September 9, 2020, the San Francisco Bay Area had intensely orange skies from the smoke. Since the smoke was pushed further up in the atmosphere from a strong Pacific Marine layer, air quality remained good even though the sky had an eerie, smoky look to it.

Monitoring Changing Conditions

The question that rests on everyone’s mind is, “When will this get better?”. The difficult answer is we don’t know. Rain is still likely a couple months out. CalFire regularly posts updates on the containment efforts at Once fires are contained, fires will then burn out, and then the smoke will cease.

During this time of the year, the West Coast experiences “fire weather.” Meteorologists note extreme fire weather with the term Red Flag Warning. Red flag warnings dictate weather with warm (not always hot) temperatures, extremely low humidity, and steady wind. These conditions allow a fire to rapidly move.

The wind has the power to move both fire and smoke. is a wind portal that tracks a number of meteorological factors. In the context of this blog, we are looking at air quality. On the right-hand side of the screen, click the car to pull out the air pollution menu. Then, click the PM 2.5 icon to see air pollution most closely associated with smoke. This will change the screen to highlight particulate air pollution data.

Next, move the slider at the bottom left of the screen to see the forecast over the next four days. models the change in winds and the resultant change and air pollution to wildfire smoke. Here in the San Francisco Bay Area, Western winds from the Pacific Ocean usually mean good air quality. As of this writing, northern winds bring smoke from the LNU complex and southern winds from the CZU complex fires.

Ultimately, the best answer is continued forest management over the long term and rain to help us in the short term. The hard answer though that forest management needs to be a continued priority in California. There is no easy answer to get us out of the current situation.

A colleague of mine sent me a great article about forest management over the last number of decades in California. If you’re interested in learning more, I encourage all my readers to read this article: They Know How To Prevent Mega Fires. Why Won’t Anyone Listen?

If you have any tips, I’d love to hear in the comments!


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2 responses to “Making Sense of Air Quality”

  1. Mom Avatar

    Great blog… helps you understand what’s really happening

  2. Nina Belott Avatar
    Nina Belott

    Wow very informative article. Thanks Dan

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