<= 10 (WHO
[2]
, reference level for the annual mean)
> 10 <= 20 (EU
[1]
, limit value for human health, annual mean – to be met by 1st Jan 2020)
> 20 <= 25 (EU
[1]
, target value for human health annual mean – to be met by 1st Jan 2015)
> 25 <= 30 (nonattainment)
> 30 (nonattainment)