US states near the Canadian border including Minnesota, Wisconsin, Michigan and Illinois were particularly choked, while the Northeast including New York was also experiencing deteriorating air quality.On Thursday evening, tracker IQAir said Chicago and Detroit were the top polluted cities in the world, with New York not far behind.The most populous US city was shrouded by the smoke that state authorities warned contained fine particulate matter deemed unhealthy for everyone across the New York metro area and Long Island.Even worse air enveloped the central and western regions of the state.Authorities encouraged New Yorkers to spend as little time outside as possible, with Mayor Zohran Mamdani warning of the "serious threat" of heavy heat and unsafe air.Libraries and train stations were offering free masks, while hundreds of cooling centers were open across the city for those without access to air conditioning."This could become the most significant smoke event in New York City since 2023, and conditions will be closely monitored for any deterioration," the city's Department of Emergency Management said.That year, the city's skies turned an eerie orange, with the air quality index reaching a dangerous 465.'Our chests feel tight'