The acidity critical loads values are based on a 1km grid for the UK. Consequently, for sites that are larger than 1 km2 will have many critical load grid squares covering the site. For this reason we give a maximum and minimum set of critical loads. For initial assessments, the minimum set of critical levels should be used as this follows the precautionary approach.
The critical load function tool provides a % of critical load for process contribution, background and predicted environmental concentration (eg process contribution plus background).