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Substance                   Averaging Period               Objective Levels                            Date by which                                                                                                                                               objective to be met

Benzene                      running annual mean         16.25 µg/m3 ( 5 ppb)                      31 December 2003

                                    annual mean                       5 µg/m3 (1.54 ppb)                        31 December 2010

1,3-Butadiene              running annual mean          2.25 µg/m3 (1 ppb)                        31 December 2003

Carbon monoxide        running 8-hour mean          10 mg/m3 (8.6 ppm)                       31 December 2003
(CO)                                              
                          
Lead                            annual mean                       0.5 µg/m3                                       31 December 2004

                                    annual mean                       0.25 µg/m3                                     31 December 2008

Nitrogen dioxide          1-hour mean                       200 µg/m3 (105 ppb), not be          31 December 2005
(NO2)                                                                     exceeded > 18 times a year 
                   
                                   Annual mean                      40 µg/m3 (21 ppb)                           31 December 2005

Particulate matter        24-hour mean                    50 µg/m3, not be exceeded             31 December 2004
less than 10 microns                                             > 35 times a year
in diameter (PM10),      
Gravimetric*                annual mean                      40 µg/m3                                          31 December 2004
                             
Sulphur dioxide           15-minute mean                266 µg/m3 (100 ppb), not to be        31 December 2005
(SO2)                                                                    exceeded > 35 times a year       
 
                                    1-hour mean                    350 µg/m3(132 ppb), not to be          31 December 2004
                                                                            exceeded > 24 times a year   
           
                                    24-hour mean                  125 µg/m3 (47 ppb), not to be           31 December 2004
                                                                             exceeded > 3 times a year                


Provisional national objectives for air quality, for the protection of human health

                        
Benzene                       running annual mean      3.25 µg/m3 (1 ppb)                               31 December 2010                            
Carbon                         running 8-hour mean      10 mg/m3 (8.6 ppm)                               1 January 2005
monoxide (CO)             

Polycyclic                     annual mean                 0.25 ng/m3                                              31 December 2010
aromatic
hydrocarbons
(PAH)                          

Particulate                    24-hour mean                50 µg/m3not to be exceeded                  31 December 2010
matter less than                                                > 10-14 times a year,
10 microns in                                                    for London
diameter                       annual mean                 23-25 µg/m3, for London                        31 December 2010
(PM10),
gravimetric*                  

*Gravimetric refers to the method of measuring PM10