These values are set by the Ambient Air Quality Directive (2008/50/EC) and the Air Quality Standards (Wales) Regulations 2010.
Mae’r Rheoliadau hyn yn diwygio Rheoliadau Diogelu Iechyd (Coronafeirws, Teithio Rhyngwladol) (Cymru) 2020 a Rheoliadau Diogelu Iechyd (Hysbysu) (Cymru) 2010.