Dangerous wildfire smoke originating from Canada is spreading across the United States, impacting a population of 100 million. New U.S. cities are now experiencing hazardous air quality as the smoke drifts south. NBC News correspondent Tom Costello is reporting on the latest developments. The smoke poses significant health risks, particularly for vulnerable populations. Residents in affected areas are being advised to take precautions, including staying indoors and wearing masks. This event highlights the increasing impact of climate change and the potential for transboundary air pollution.