Beginning in June of last year, measurement stations across the U.S. began to regularly measure maximum particulate levels in the "Unhealthy for Sensitive Groups" range. From July to September, stations in all three regions reported "Unhealthy" air pollution levels, particularly in California and the Pacific Northwest. In November, during California's "Campfire" wildfire, some stations in Southern California reported particulate levels high enough to warrrant a "Hazardous" rating from the EPA, advising anyone in the impacted area to stay inside.
This chart shows the maximum AQI measured by region throughout the fire season of 2018. The red line represents California, the blue line shows the Pacific Northwest (Oregon and Washington), and the black line shows the rest of the United States.
Chart courtesy of Puraka Children & Adult Pollution Masks.