analyses for each study separately, and then pooled the
studies using models in which a binary variable for study
was added to the covariates. To address the question of
whether construction workers have an excess risk of
lung cancer, subjects who held at least one occupation
in the construction industry (as previously defined) were
compared to those who never held an occupation in the
sector. First we included all study subjects in an analysis,
and second we restricted the study base to blue-collar
workers in the study sample. These analyses were further
stratified by large subdivision of the construction industry
and by duration of employment in construction.
To examine which agents commonly found in the construction
industry environment carry an excess risk of
lung cancer, we restricted the study base to subjects who
had ever worked in the construction industry. That is,
for each agent, the reference category consisted of those
construction workers not exposed to that agent.
We first established a list of agents to consider. Among
the 294 agents in the database of agents coded by our exposure
experts, we selected those that satisfied the following
criteria: the lifetime prevalence of exposure to the
agent among construction workers was more than twice
that among non-construction workers, at least ten cases or
ten controls were exposed among workers who had the
Construction industry as the industry code, and the lifetime
prevalence of exposure to the agent among controls
was greater than 5 %. Twenty agents satisfied these criteria.
For each agent we derived an exposure index, based
on the reliability of exposure, the frequency of exposure,
the total duration of exposure and the relative concentration
of exposure. The index has three categories: i)
non-exposure; ii) non-substantial exposure; and iii) substantial
exposure. Non-exposed is self-explanatory. Substantial
exposure comprised subjects who had been
exposed with a likely or definite reliability to medium or
high concentrations for more than 5 % of their workweek
and for at least 5 years . Non-substantial exposure
comprised those with less than substantial exposure.
Differences in ORs between the two studies were tested
by introducing an interaction term between the binary indicator
for study and the exposure variable of interest