A noticeable change filled the air last summer—an actual change in air quality. Wildfires in Canada produced smoke-filled skies and wood-burnt, chemically scented air. National media showed images of orange-tinged skies in New York City and other urban areas. Unhealthy air quality plagued much of the United States during our hottest months, and at various times several U.S. cities had the worst air quality in the world.