When wildfire smoke drifts into the Methow Valley, it tends to stay, settling in the folds of the Cascade foothills like a choking fog. Recent summers have brought weeks-long binges of unhealthy air to one of Washington state’s poorest counties, rivaling some of the most polluted cities in the world. Countering this intensifying threat are small nonprofit organizations such as the Methow Valley Citizens Council, which has been distributing air purifiers, maintaining a network of air quality monitors, and spreading the message about how to keep safe when the smoke rolls in. Much of that work was funded by a...