“I felt betrayed as a citizen,” he said. “They’re polluting at our doorsteps and we know it and we can see it and we can smell it, and now we can measure it.”

He has been building the network out this fall and hopes to use the data to push the government to crack down. Poor and minority communities are disproportionately affected by air pollution, and a revived environmental justice movement is rising.

One home that Mr. Dixon outfitted with a PurpleAir this summer is across the river from Edgar Thomson Steelworks and near a coke refinery that emits a rotted sulfur smell. That’s where Robin Kornides, 63, lives with her husband and makes her living selling sewn and knitted goods.

One recent November afternoon, she stood on her front porch in a sweatshirt and gloves, taking a break from caring for a troupe of pet pigeons to talk about the gadget.

But other days, she is not so lucky.

“A third of the time it’s bad enough that I don’t want to go outside, but it mostly depends how the wind is blowing,” Ms. Kornides said. On the worst days, she added, “It’s like trying to breath Jell-O.”

That day there was a cool crisp east wind — she thinks of it as the good wind.

“It makes me feel justified to complain at least, and I’m a visual person, so I like seeing the color on the app, ” Ms. Kornides said. “But I can always smell it first, that coke smell.”