There are two principal ways in which air quality is assessed: air quality standards and air pollution index and banding. Air pollution is assessed against air quality standards. The air pollution index has been developed to provide a simple number scale of how low or high air pollution is each day; 1 being low, 10 being very high.
Air quality is assessed by comparing measured levels collected from monitoring stations with air quality standards; find out more at the London Air website. The standards are based on the knowledge we have about the effects that each pollutant has on health and on the environment at a certain concentration over a certain time.
AQ standards also keep track of whether air pollution is getting better or worse. As pollutants are measured in different ways it is difficult to compare them directly, so often it is the exceedence of the standards that are compared. An exceedence of a standard is the period of time (usually expressed in days) for which the concentration of a pollutant is higher than its standard.
Each AQ standard has an air quality objective. The objective is a target date by which time exceedences of a standard must not rise above a specified number. Learn more about this by visiting the DEFRA website.