The long-term critical level of ozone for crops is considered to protect also other plants such as trees and natural vegetation . In collaboration with the World Health Organization ’ s Regional Office for Europe ( WHO/EURO ) , a critical level expressed as an AOT60 ( accumulated exposure over a threshold of 60 ppb , i.e . 120 μg/m3 of zero ppb.hours ) , calculated over one year , was adopted as a surrogate for the WHO Air Quality Guideline for the purpose of integrated assessment modelling .