The following charts show how the annual average concentration of NO2 has changed since 2018 at sites being monitored long-term using difusion tubes in St Albans and District.  The tables between the charts indicate the location of the sites shown by (the local authority's) code numbers.  This information derives from the local authority's Air Quality Annual Status Reports - which include maps identifying the sites. The data shown includes estimates where the local authority data is either missing or likely to be erroneous.  The data shown is "raw" - it has not been "adjusted for bias". Assuming no bias, annual average concentrations prior to the end of 2019 considerably exceed those since 2020.  As at the end of 2022, the trend was upwards at some sites, down at others but generally flat.  For calendar year 2022, most of those concentrations were between two and three times greater than the World Health Organisation ("WHO") guideline.   Those concentrations exceeded the UK legal limit at three sites and were less than double the WHO guideline at two sites.