18 of the 30 highest PM2.5 levels since 1999 were caused by two groups of wildfires. Air monitoring station locations are shown in parentheses next to each date. Note that these numbers represent the highest 24-hour PM2.5 readings recorded at a single station on each date (the average reading for that entire day). This is therefore a list of the 30 days with the highest readings, NOT the 30 highest readings ever recorded.