Canada's worst wildfire season has caused toxic smoke to spread across the US, affecting air quality in 20 states. The smoke contains fine particulate matter, which can cause short-term health effects such as irritation of the eyes, nose, and throat, as well as long-term effects like asthma and heart disease. Major cities including Chicago have experienced poor air quality, prompting warnings for vulnerable groups to stay indoors. Red flag warnings have also been issued in parts of Washington, Oregon, and Colorado due to dry conditions that increase the risk of wildfires.