L and M Eriksson. 2003. Is the decline of the increasing
incidence of non-Hodgkin's lymphoma in Sweden and other countries
a result of cancer preventive measures? Environmental
Health Perspectives. online 2 July 2003
in most western countries, non-Hodgkin's lymphoma (NHL) increased
in Sweden during much of the second half of the 20th century.
the final decade, however, trends had reversed, with NHL falling
NHL incidence in Sweden; adapted from Hardell and Eriksson
in the SEER registry for men in the US are consistent with the
Swedish trend, with steady increases in the incidence rates from
1973-1990 (+ 3.6% per year), a slower increase from 1990-1995 (+
1.6% per year) and a decline from 1995-1999 (0.9% per year).
this paper Hardell and Eriksson propose that the recent
decline may be the result of reduced exposures to several pesticides
and persistent organic pollutants that studies have associated
with elevated risk of NHL.
argument is based on several inter-related observations.
the delay between when NHL begins and when it is diagnosed is
usually years if not decades, if the observed decline in NHL cases
is due to changes in environmental conditions, those changes would
have begun several decades ago.
risk is firmly linked to impaired immune function.
types of chemical exposures have been consistently linked to elevation
in NHL risk. The chemicals involved suppress immune function.
A striking example
of this was published in 1997, showing an interaction between
PCBs and Epstein-Barr virus in elevating risk to NHL.
exposures have declined over the past few decades because of government
policies restricting or banning certain compounds.
do they find? Hardell and Eriksson pull together two decades
of their own and others' research on NHL that has consistently revealed
associations between NHL risk and exposure to a series of contaminants.
They briefly review scientific findings that link NHL to 4 types
of exposures affecting the immune system: chlorophenols
the pesticides 2,4-D and 2,4,5-T), organic solvents, persistent
organic pollutants (PCBs and dioxins) and HIV virus. Exposures to
the chemicals have decreased since the 1970s; in Sweden, HIV exposure
is low and stable.
then ask the following: if these associations are causal, how many
cases could have been avoided by elimination of the exposures? They
do this by focusing on the percentage of NHL cases that are plausibly
caused by one chemical exposure or another, called "the attributable
fraction." These fractions typically are low, running from
3% (chlorophenols) to 25% (persistent organochlorines). But even
at the lower range, they indicate, for example, that 30 cases of
NHL out of 1000 could have been avoided by avoiding exposures to
chlorophenols. For the persistent organic pollutants, that number
is 250 out of 1,000 cases that could have been avoided, according
to their calculations.
to Swedish and other data, the highest human exposures to persistent
organic pollutants occurred during the 1970s, just before bans began
to be implemented. Exposures have decreased since, with the pace
of decline varying depending upon when and where restrictions and
bans were implemented.
Hardell and Ericksson's estimation, known decreases in the body
burdens of several of the chemicals implicated in causing NHL have
decreased sufficiently so that, if the chemicals actually have caused
NHL, then we should be witnessing a reversal in the decades-long
rise in NHL incidence. The graph above
shows that it is.
does it mean? If this is true, it's good news, because
it means that for at least one cancer, we are beginning to win the
war... not via the focus on cures pursued by the health establishment
for decades with at best mixed success, but instead through prevention.
Decades of hard work to design and implement better chemical regulations
will have bourn fruit. And new public health measures, like the
2001 Stockholm Convention on Persistent
Organic Pollutants, should add momentum to efforts at cancer
data and reasoning presented by Hardell and Ericksson fall far short
of proving their theory. They make a plausible case, however, that
opens up a broad new front in arguments about the costs and benefits
of reducing exposures.