I did notice that during the 90s the average hazardous and dangerous days were a lot more common. This was easiest to see using the pie chart. I also noticed that we have pretty much ended the output of SO2 since 1980 in which it was the most common main pollutant. This decrease started in the 2000's and a shift began to happen towards NO2 and the days Ozone. We can also see that there. I have attached pie charts from 1980 vs 2018 respectively of the pie charts for main pollutants and air quality.
Although these charts show both extremes, the general trend is that the shift happened gradually from a dependancy on SO2 gasses which was harming the air quality to PM10 and Days ozone which made the air quality better in turn.