There are several strengths to this study. It is nationwide,
with more than half of the farmers in the country involved. It
is therefore a fairly large study group with good statistical
power. It is also a fairly homogeneous group with most of the
farmers having similar farms and using similar farming practices.
The study uses questionnaires that have been
standardized in the usual way.12,13 All the study subjects,
both farmers and non-farmers, have the same access to
a nationalized health care system.
This study also has a number of weaknesses. The
response rate is lower than in previous studies on European
animal farmers that have up to 60e80% response rate.3,4 A
relatively low response rate, albeit in a study population
including the whole population of farmers in Iceland, raises
the possibility of under-representation of individuals with
respiratory disease in the study group or over-representation
in the comparison group. Low smoking rates among
farmers may result in lower prevalence of respiratory
disease and a healthy worker effect may also be a factor in
this study. However, a healthy worker effect would likely
have been present in other studies and low smoking rates
are common in farming populations.5 Sending the questionnaire
to participants in the wintertime and early spring
might cause a potential seasonal effect. However, the
questions ask about the last 12-month period, suggesting
that this is less likely to be a confounding factor.
The high prevalence of respiratory disorders commonly
present among the generally healthy low smoking population
of agricultural workers found in numerous other
studies1e4 was not present in our study of all Icelandic
farmers. Although the potential for bias certainly exists,
this improvement may quite possibly be due to widespread
changes in farming practices that have taken place over the
past two to three decades. The public health implication of
this finding, if supported by further studies, is considerable
as the high volume of respiratory ailments historically
linked with agricultural work may thus be avoided with
modernization of farming practices.