Poor air quality returns to US north-east from Canada wildfires

New York City, parts of Pennsylvania and Baltimore all issued warnings as 421 wildfires continue to burn up northPoor air quality returned to the north-east US on Sunday, although it was nowhere near as bad as the heavy haze that recently shrouded the region and triggered global headlines as wind-borne smoke from raging Canadian wildfires caused orange skies, thick smog and record-setting pollutant levels.On Sunday morning, a smoke plume moved across New York City, leaving the air quality index