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Understanding Our Air Quality in a Wildfire Prone World
Thursday, September 16, 2021



Do you often find yourself checking the Air Quality Index on your smartphone or computer to see when is the best time to go outside? You’re not alone.

With all the active wildfires in the West spewing smoke, drought conditions kicking up dust and a pandemic pushing people to opt for outdoor activities, it’s no wonder many feel the need to monitor the invisible microscopic pollutants floating around that could pose health risks when they embed into lungs.

The Environmental Protection Agency has set up a color-coded system to reflect the amount of major pollutants present in ambient air, alert the public of possible concerns, and make recommendations on the level of exposure some might tolerate. AQI ratings start at green: good in the 0 to 50 range; then go to yellow: moderate 51-100; orange: unhealthy for sensitive groups 101-150; red: unhealthy 151-200; purple: very unhealthy 201-300; and top off at maroon: hazardous 301 and higher.

EPA’s website,, features maps that display this information for locations all over the country. The agency and U.S. Forest Service recently updated their smoke and fire map to help pinpoint where wildfires and smoke drift are impacting communities. A dot marks an air monitor and a square marks an air sensor, which is notable because both measure quality differently. Monitors are regulated by the EPA, whereas the sensors belong to a crowd-sourced network.

In Ketchum, for example, a dot shows where the Idaho Department of Environmental Quality installed and maintains a permanent regulatory air quality monitor outside Ernest Hemingway STEAM School. The unit costs thousands of dollars, is about the size of a refrigerator, and wrapped in colorful vinyl depicting student artwork that won a contest a while back.

As Idaho DEQ Monitoring Analyst Roger Sauer explains: The monitor has a roll of filter tape. Air is pulled through the filter. The retained fine particle pollutants are known as PM2.5 because they are two and a half microns or less in width. This includes dust, ash, and soot. They are weighed to establish an AQI report every hour. That information is posted online on and at where additional AQI numbers are calculated by temporary monitors placed strategically during wildfire season.

Ketchum’s air is also measured by what are called “low-cost sensors” that people or “citizen scientists” have bought from PurpleAir for $249 to $279 and set up outside. The 3.5 inch by 3.5 inch, 5-inch-tall sensor needs a power source and WIFI so that its two lasers can count particles and then relay the average every 10 seconds. Owners are invited to register their sensors online to share AQI data on the map. Currently, the map indicates owners at Heidelberg, Bigwood, Sun Pointe and West Ketchum. The map also shows one sensor reporting AQI in Gimlet and two in Hailey.

The black circles on the PurpleAir map mean those sensors are set up indoors. On the fire and smoke map, only outdoor PurpleAir sensors are marked--they appear as squares.

PurpleAir’s Ethan Breinholt claims the company has sold more than 20,000  sensors worldwide since 2015.

All of the sensors are maintained by individual users, and they would have to contact PurpleAir for repairs and troubleshooting, he said. But if a sensor is way off, the company will “automatically disable” it, he said.

Ben Seely, Air Quality Monitoring Supervisor with Idaho DEQ, says PurpleAir sensors could read two to three times higher than what regulatory air quality monitors do, so the EPA uses a “correction equation” to address this bias when reporting the AQI on AirNow maps.

The EPA also uses an algorithm to estimate current AQI readings based on many hours of past data and ongoing wind conditions. New this year is PM2.5 trend information, which EPA’s Enesta Jones says describes how conditions have changed recently—"usually over the last 30 minutes.”

It’s no wonder then, as the saying goes, the numbers are all over the map, because readings are constantly changing when the wind switches speed and direction, a smoke plume arises, a construction site emits irritants, traffic patterns or temperatures shift.

As for the accuracy of all the apps that are available on smartphones, Seely comments that they may be grabbing raw data and transferring it in some way.

According to its website, BreezoMeter, which provides data on iPhones’ weather app, collects information from many sources including satellites, and applies more than 30 algorithms and machine learning to come up with its own AQI.


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