This is to do with the alert levels set in the government’s Air Quality Index. They relate to short term (daily) predictions and measurements of air quality. These are important for people with respiratory health problems but not strongly related to the long – term health effects on the population from poor air quality that are represented by the annual mean measurements. So pollution levels in Bristol as measured by the air quality index can be low throughout the year, but we still breach the annual mean objective for nitrogen dioxide.