Chemistry Reference
In-Depth Information
O
Cytochrome
P-
450
Scheme 3.1
PAH emissions fell from 6463 to 1745 t, the PCB
emissions fell from 6.8 to 3.2 t and the PCDD/PCDF
emissions fell from 1083 to 325 g TEQ [4].
Cl
Cl
Cl
Cl
Suspended particles.
Airborne particles are diverse
in size and composition. Owing to coagulation at the
lower end and rapid deposition at the higher, the
majority by mass are in the range 0.1-10 mm. Tran-
sient nuclei from condensation of vapour, e.g. from
motor vehicle exhausts and H
2
SO
4
from SO
2
oxida-
tion, peak at around 0.02-0.03 mm. Coagulation of
smaller particles produces a second peak at around
0.2-0.3 mm and mechanically generated particles,
e.g. volcanic and wind blown dusts, peak at around
10 mm. Traditionally, atmospheric particulate mea-
surement has been either as black smoke or as gravi-
metrically determined particulate matter (PM), i.e.
PM
10
(in practice this equates to particles of less than
10 mm in diameter). Black smoke principally comes
from coal burning and diesel exhausts, of which the
latter has come to dominate with 95% of the total.
The PM
10
is dominated by road traffic emissions
and secondary pollutants (ammonium sulfate and
ammonium nitrate particles), with contributions
from dried sea spray and wind blow dusts.
Particulate air pollution was one of the first types
of pollution demonstrated to have serious health
effects and there is evidence linking ambient par-
ticulate levels, irrespective of their chemical compo-
sition, with lung disfunction, respiratory disease and
mortality [7].
Black smoke emissions in the UK declined from
1.07 Mt in 1970 to 0.31 Mt in 1997, due largely to
the switch from coal to smokeless fuels and natural
gas. This has been offset partially by the increase in
diesel-engine road vehicles. The PM
10
emissions in
the same period declined from 0.51 to 0.18 Mt [4].
Cl
Cl
3,3
¢
,4,4
¢
,5,5
¢
-hexachlorobiphenyl
Scheme 3.2
thought to be less than other species and therefore
extrapolation of laboratory data is difficult.
Polychlorinated dibenzo-
p
-dioxins (PCDD) and
polychlorinated dibenzofurans (PCDF), with chlo-
rine atoms substituted at some or all of the num-
bered positions, originate in combustion processes.
They are formed in incinerators or runaway exother-
mic chemical reactions, as happened at the ICMESA
chemical plant in Seveso, Italy, in 1976 (see Scheme
3.3). They exhibit a range of toxicities, depending
upon the chlorine atom substitution. Only those
chlorinated at the 2, 3, 7 and 8 positions are
considered significant. Of these, 2,3,7,8-
tetrachlorodibenzo-
p
-dioxin is the most toxic and
addition of further chlorine atoms reduces the toxi-
city. The furans generally are less toxic than
the equivalent dioxin. In summing the toxicity of
PCDDs and PCDFs, each is weighted by its toxicity
relative to 2,3,7,8-tetrachlorodibenzo-
p
-dioxin and
expressed as grammes of toxic equivalent (TEQ).
The only undisputed effect in humans is the
choracne (a disfiguring skin ailment) resulting from
high exposure at Seveso. The PCDDs and PCDFs also
were present as contaminants in Agent Orange, a
defoliant used during the Vietnam War to which vet-
erans were exposed.
The UK emissions of persistent organic pollutants
decreased significantly between 1990 and 1997. The
Heavy and toxic metals.
Heavy metal pollution of
the troposphere mainly concerns lead, although local
discharges (e.g. arsenic from copper smelting and
