A critical level of ozone for human health is represented by the WHO Air Quality Guideline level for ozone of 120 ug/m3 as an 8-hour average . 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 ug/m3 , calculated over one year , was adopted as a surrogate for the WHO Air Quality Guideline for the purpose of integrated assessment modelling .