Revealed: how a little-known pollution rule keeps the air dirty for millions of Americans

Major investigation shows local governments are increasingly exploiting a loophole in the Clean Air Act, leaving more than 21 million Americans with air that's dirtier than they realize

• What you need to know about loophole hiding extent of US wildfire pollution

A legal loophole has allowed the US Environmental Protection Agency to strike pollution from clean air tallies in more than 70 counties, enabling local regulators to claim the air was cleaner than it really was for more than 21 million Americans.

Regulators have exploited a little-known provision in the Clean Air Act called the exceptional events rule" to forgive pollution caused by natural" or uncontrollable" events - including wildfires - on records used by the EPA for regulatory decisions, a new investigation from the California Newsroom, MuckRock and the Guardian reveals.