Our methodology also presented some limitations. The
validity or appropriateness of the dispersion models have
not been assessed (and had to be assumed over time in
this study of long-term effects), because we lacked the necessary
household and soil measurements in the contaminated
areas. Residence location as a surrogate of exposure
cannot distinguish contributions from the direct and the
indirect exposure pathways (e.g., from air to soil and
home-grown produce). Moreover pollutant-specific deposition
models (dioxin, metals, and dusts) are interrelated
and thus pollutant effects are difficult to parse. However,
the estimate of dioxin emissions should not be considered
as a tracer for the whole mixture, since experts specifically
assessed dioxin emissions. Finally, it was impossible to
determine whether cases developed NHL in the same
block group where they were diagnosed. Considering the
long exposure-to-effect interval, in- and out migrations
may have occurred, inducing a potential non-differential
misclassification.
This is an ecologic study, that does not deal with individual
subjects or individual-level traits or exposures, but
rather with the characteristics of block groups. Although
some relevant confounding factors were considered at the
block group level (socioeconomic status, population density,
etc.), we lacked individual information pertaining to
residence history, occupational history, food consumption,
etc., that could potentially confound the relationship
between dioxin exposure from the municipal solid
waste incinerator and NHL. Thus, we cannot firmly
exclude the possibility that residual confounding affected
the reported regression coefficients.
We found a significant association among females but not
among males. It has been shown that the male-to-female
incidence rate ratios are greater than 2 for high grade or
peripheral T-cell NHL (constituting, however, only about
17% of all NHL) [24]. The risk for NHL among men versus
women may also vary by site. A predominance of NHL
regional to the respiratory tract among men, and the retroperitoneum
among women, has been reported in a
Swedish study [25]. Risk factors (e.g. environmental or
occupational hazards) are therefore probably different for
men and might blur an association with environmental
dioxins [25]. Future epidemiological studies, at the indi-