Table 1
Typical United States urban concentration of chemical carcinogens (ng m
−3
)
Pollutant
Year
1960
1970
1980
1990
PAH (as B(a)P)
20
4
1
0.5
Cr (all valence states)
40
10
6
4
Ni
100
60
20
10
As
25
12
5
3