Hixon Center for Climate and the Environment

Air quality resources and tips

Last updated Jan 9, 2025

During major wildfire events there is often greater interest and awareness of air quality. We hope this document provides clear and actionable information for the community. Please feel free to share this information and document with anyone it can help.

TLDR - For the best information on air quality during a wildfire, focus on PM2.5 and use the EPA Fire and Smoke Map or the Purple Air website. This applies to the smoke of active fires. Following a fire, there may be debris in the air that is not well captured by PM sensors, including toxic chemicals, lofted into the air from the burned remains of structures.

Air quality primer: There are many types of air pollutants, so “air quality” is a sometimes unhelpful generalization. The air quality index (AQI) is a shortcut, but can be ambiguous and incorrect for your personal location. Instead, look for levels of the relevant pollutant (in the case of wildfire that would be fine particulate matter).

Common air pollutants of concern in our area, Los Angeles, include gas phase species (ozone and nitrogen oxides) and fine particulate matter (PM2.5). There are high quality, regulatory instruments for these pollutants sparsely distributed in the LA region, and if you live or work near one of these, the measurements can be very useful. However, most of the time, there is not a station near you, or it is not measuring all of the various pollutants.

In the case of wildfire, it’s safe to assume you want to know about ground level fine particulate matter, or PM2.5. Looking at the wrong pollutant can be very misleading.

What can we do: The EPA’s Fire and Smoke map (Figure 1) is a great choice for information during wildfire events, because it gathers observations from multiple sources and provides the spatial data helpful to folks who might be deciding to stay home, go to work, or check on friends and family.

Here we can see the air quality in Claremont on Jan 9 at 6:30 am as “moderate” (selecting a marker will tell you that the source of this data is in fact the low-cost Purple Air sensor network, which measures only fine particulate matter). Therefore, this map is using the most relevant pollutant.

Figure 1 - The EPA Fire and Smoke Map for 6:30 am on Jan 9, 2025. As of 10 am Jan 9, 2025 the air quality is rapidly decreasing from moderate to poor.

You can also directly use the Purple Air website, (Figure 2) to see the live data from all available sensors. These low-cost sensors are accurate enough for PM2.5 to give a sense of the air quality across a neighborhood or region, especially if multiple sensors in the same area are in agreement. And, they measure the relevant pollutant for wildfire smoke. Even though they aren’t as accurate as the regulatory stations, their higher spatial density and the fact that they are measuring PM2.5 makes them a great choice for assessing your exposure during a wildfire.

Figure 2 - Purple Air web interface for PM2.5 AQI for 6:30 am on Jan 9, 2025. As of 10 am Jan 9, 2025, the local sensors are indicating red/poor air quality.

Better still, we have both rooftop (Jacobs building) and interior Purple Air sensors at Harvey Mudd’s campus (Figure 3), visible for real time observations on the map linked above. The indoor sensors are indicated with a black outline on the circles.

Figure 3 - Map of HMC Purple Air sensors and current reading at 10 am on Jan 9, 2025.

The above sources are much more appropriate than what is likely being pulled into your weather app, such as AirNow’s Nowcast. As an example, here is the morning “nowcast” for Claremont on Jan 9 at 6 am, indicating the air quality is GOOD, referencing ozone. But it’s not good as we can see in the PM data in Figure 2. Why is ozone being used? We are experiencing major wildfires, and it’s winter, so particulate matter, and not ozone, is the most relevant pollutant to air quality right now. Ozone is highest in the peak of summer, when high temperature and elevated photochemical activity can generate high concentrations. Information about ozone is not helpful for wildfire impacted air quality in winter.

Figure 4 - Nowcast for AQI for Claremont, 6 am Jan 9, 2025

In fact, if you use the map feature (Figure 5) , you can see that ozone IS quite low (AQI values for ozone are in the green circles) and also, that there are major hazards for PM2.5 (shown in the colored contours).

Figure 5 - Overlay of the AQI for ozone (in circled numbers) with rough contours of the AQI for PM2.5. Ozone is very low, while PM2.5 is heavily impacting some areas due to the fires for 6:30 am on Jan 9, 2025.

The reason Claremont’s AQI is “good” in the NowCast is because our nearest monitoring station, indicated as Pomona/Walnut Valley, does NOT measure PM2.5! This makes the information in Figure 4 irrelevant for assessing air quality today, but that’s not obvious to most users.

In summary, during wildfire events you should be focused on determining the PM2.5 levels near you, and you can use the EPA Fire and Smoke map, or the Purple Air network, but you should be wary of the Nowcast AQI (often pulled into weather apps, unfortunately).

Depending on your personal health and level of risk tolerance, the PM levels will indicate different actions for different people (e.g. staying indoors with filters running vs going for a walk). We won’t comment on that, as it’s so personal. But generally when the readings are colored red you should consider limiting your exposure.

For more information, feel free to reach out to me, Lelia Hawkins, lhawkins@g.hmc.edu.