Air quality alerts become the new normal for millions

As smoke and smog spread, cities are learning to live with warnings that were once rare.

The alerts, once seasonal, now arrive with unsettling frequency.

Schools, workplaces and hospitals are adapting their routines.

Breathing lessons

The long-term health toll is only beginning to be understood.

"We are asking children to check the air like they check the weather."

Analysts point to a handful of forces driving the story forward:

• More frequent alert days
• New indoor-air standards
• Uncertain long-term effects

Clean air, once assumed, is becoming something to plan around.