You can read the full paper @ http://www.ehponline.org/members/2005/8009/8009.pdf

The analysis in the study below was done on Nigerian cocoa beans and
products but it's true about other cocoa producing countries (all are
Developing Countries) where leaded gasoline is used.  Some Latin and
South American countries are in the process of phasing out leaded gas
but it's hard to know at what stage they are wrt lead pollution.
Needless to say that Africa is the main or perhaps the only source of
cocoa-liquor for European cocoa end-products that due to a very long
tradition, are still considered the best.
Cocoa beans are processed in the growing countries up to the stage of
cocoa-liquor, which is then exported to the producers of cocoa end-
products.

Arbor

Environ Health Perspect. 2005 Oct;113(10):1344-8.  Links
Comment in:
Environ Health Perspect. 2006 May;114(5):A274-5; author reply A275.
Lead contamination in cocoa and cocoa products: isotopic evidence of
global contamination.Rankin CW, Nriagu JO, Aggarwal JK, Arowolo TA,
Adebayo K, Flegal AR.
Environmental Toxicology, WIGS,University of California, Santa Cruz,
California, USA. rankin@etox.ucsc.edu

In this article we present lead concentrations and isotopic
compositions from analyses of cocoa beans, their shells, and soils
from six Nigerian cocoa farms, and analyses of manufactured cocoa and
chocolate products. The average lead concentration of cocoa beans was
</= 0.5 ng/g, which is one of the lowest reported values for a natural
food. In contrast, lead concentrations of manufactured cocoa and
chocolate products were as high as 230 and 70 ng/g, respectively,
which are consistent with market-basket surveys that have repeatedly
listed lead concentrations in chocolate products among the highest
reported for all foods. One source of contamination of the finished
products is tentatively attributed to atmospheric emissions of leaded
gasoline, which is still being used in Nigeria. Because of the high
capacity of cocoa bean shells to adsorb lead, contamination from
leaded gasoline emissions may occur during the fermentation and sun-
drying of unshelled beans at cocoa farms. This mechanism is supported
by similarities in lead isotopic compositions of cocoa bean shells
from the different farms (206Pb/207Pb = 1.1548-1.1581; 208Pb/207Pb =
2.4344-2.4394) with those of finished cocoa products (206Pb/207Pb =
1.1475-1.1977; 208Pb/207Pb = 2.4234-2.4673). However, the much higher
lead concentrations and larger variability in lead isotopic
composition of finished cocoa products, which falls within the global
range of industrial lead aerosols, indicate that most contamination
occurs during shipping and/or processing of the cocoa beans and the
manufacture of cocoa and chocolate products.

PMID: 16203244 [PubMed - indexed for MEDLINE]