As US wildfires pollute the skies, a loophole is obscuring the impact. Can it be fixed?

Experts agree it's time to change the Clean Air Act's exceptional events rule - but offer different solutions.

• Revealed: how a little-known pollution rule keeps the air dirty for millions of Americans
• What you need to know about the loophole hiding the extent of wildfire pollution
• In Detroit, a magic wand' makes dirty air look clean - and lets polluters off the hook

During wildfire season in the western US, soot-clogged skies have long triggered public alerts with advice like: Shut the windows and stay indoors. For those who can afford it: Use an air filter. As Canadian wildfire smoke curled down to Kentucky this year, officials began to do the same thing.

On alert days, smoke's there when you wake up in the morning, it's there when you're going to bed at night," said Michelle King, the assistant director of the Louisville metro air pollution control district.