Where there's wildfire, there's smoke and Arizona health officials say the soot in the air is "off the carts".
That's bad for both residents and firefighters.
Calmer winds helped firefighters gain some ground, but also concentrated the sooty air, keeping it stagnant and raising pollution to levels officials hadn't seen yet.
Mark Shaffer of the Arizona Department of Environmental Quality says the smoke creates a "serious potential health problem."
Officials plan to bring in additional air quality monitoring equipment, but they say people should just stay away.
The second largest wildfire in state history has claimed more than 30 homes and forced nearly 10,000 people to flee.
It's poised to cross into New Mexico and still threatens towns and major electrical transmission lines.