Chemistry Reference
In-Depth Information
60
50
40
Birds of prey
All samples
below 2 ppm
All samples
above 40 ppm
20
0
2
5
10
20
40
60
Non-predatory birds
40
All samples
below 0.5 ppm
All samples
above 20 ppm
20
0
0.5
1
2
5
10
20
Mercury Content in Parts per Million
fIgure 8.4
Mercury residues in the livers of Swedish birds (from Walker 1975).
to CH
3
-Hg X and do tend to undergo biomagnification, but they are generally more
easily biodegradable to inorganic mercury and tend to bioaccumulate less strongly.
At one time, a major source of organomercury pollution in Western countries was
fungicidal seed dressings used on cereals and other agricultural products (see IAEA
Technical Report 137 1972). Another important source was organomercury antifun-
gal agents used in the wood pulp and paper industry. Most of these uses were discon-
tinued by the 1970s, but certain practices have continued into the 1990s, including
the use of phenylmercury fungicides as seed dressings in Britain and some other
countries. In the 1950s and early 1960s, Sweden and other Scandinavian countries
had serious pollution problems due to the use of methyl mercury compounds as seed
dressings. Deaths of seed-eating birds and raptors preying upon them were attributed
to methyl mercury poisoning (Borg et al. 1969). Thus, as with dieldrin, bioaccumula-
tion led to secondary poisoning. Interestingly, seed-eating rodents contained lower
mercury levels than seed-eating birds. Some data for total mercury levels found in
Swedish birds during the mid-1960s are shown in Figure 8.4. Most of the mercury
was in the methyl form. Findings such as these led to the banning of methyl mercury
seed dressings in Scandinavia. Other forms of organomercury seed dressing (e.g.,
phenyl mercury) were not implicated in poisoning incidents and continued to be mar-
keted in many Western countries after methyl mercury compounds were banned.
8.2.4 T
of x i c i T y
of f
o
r g a n o m e r c u r y
c
o m p o u n d s
The toxicity of organomercury, like that of certain other types of organometals, has
been related to their strong affinity for functional -SH groups of proteins (Crosby
1998). Exposure of animals to organic mercury leads to a reduction in the num-
ber of free -SH groups in their tissues. Both mercury and methyl mercury bind
strongly to these groups. This is consistent with the wide range of physiological and
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