Where low cost air quality sensors are — and aren’t — in the Bay Area


Last summer, as wildfires raged across the state and smoke cloaked the Bay Area, East Palo Alto resident Mark Dinan did what many Bay Area residents were doing — he pulled up a map made by the company PurpleAir showing real-time air quality measurements. The numbers came from a network of low-cost sensors sold by the company that people install at their homes.

As the region’s air quality and the effects of wildfire smoke continued to rise as a pressing issue in the Bay Area, the number of private, low-cost air quality sensors have skyrocketed in the past year— PurpleAir sensors are among the most popular brands of monitors.

These sensors, which users can install in their homes to collect and access real-time data, offer a hyperlocal perspective compared to government-run air monitoring stations, which tend to show data for the broader region.

But Dinan, who works as a tech recruiter, noticed something strange last August: East Palo Alto had no sensors — a stark contrast from across the freeway, where the traditionally wealthier Palo Alto had dozens. He posted a screenshot showing the discrepancy in the PurpleAir Users Facebook group.

“It was like, what’s going on here?” he said. “It was like somebody redlined East Palo Alto.”

Such occurrences aren’t limited to Dinan’s small part of the Bay Area. The relatively “low-cost” sensors, which cost about $200 per device, appear to be concentrated in the more affluent areas of the region, according to a Chronicle analysis of the PurpleAir sensor network.

“Those discrepancies are quite stark,” said Michael Flagg, principal air quality specialist at the Bay Area Air Quality Management District. “That speaks to the kind of inequity of the access to information.”

Data provided by these at-home monitoring networks help inform a much deeper and localized understanding of air quality in an area, and what to do when the air quality is bad or hazardous, he said. In lower-income areas where there are fewer sensors or even none, the data is limited or non-existent, and residents there are left without access to the same rich information that could affect their health and well-being.

Counties with higher median household income tend to have a higher concentration of PurpleAir sensors per person. Of the nine counties in the Bay Area, Marin County had the highest concentration, with about 31 sensors per 10,000 people, compared with about 8 in San Francisco and 4 in Solano. Marin County’s median household income was about $111,000 in 2019 compared with Solano’s $87,000, according to the latest data from the U.S. census.

The PurpleAir sensor network has exploded in size in the past year, data shows, attracting thousands of new users as record-breaking fires spread smoke and ashes all over the region, sounding alarm over toxic air. The number of unique PurpleAir sensors in the Bay Area’s nine counties multiplied rapidly, especially after August 2020.

Adrian Dybwad, the founder…

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