As soon as an evacuation order was lifted in her neighborhood, Arlynn Page raced back to her charred street in Altadena, Calif., to see what was left of her hillside home. The two houses next door were rubble, but hers was unscathed.

Then she went inside. A stinging haze hung in the living room. Her mattresses, rugs and couches reeked like a chemical campfire. Ms. Page, 55, flung open the windows and doors to let in the breezy afternoon air, but she was still choking.

“I have such a headache,” she said through a mask. “There’s so much smoke.”

This was the vexing reality that thousands of displaced people across Los Angeles faced, as they were allowed back home this weekend for the first time since fleeing the firestorms. Their homes had escaped the annihilation that burned 12,000 other structures, but were nonetheless filled with ash and smoke damage.

As waves of residents return to their homes in the coming weeks, many more people are likely to encounter similar surprises. Wildfires not only burn down structures, but emit smoke, ash and heat that suburban homes are rarely built to withstand.

“It smelled worse inside our house than outside,” said Marcos Barron, 53, who snapped on a respirator and face shield, as he headed back into his mountainside home.