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Email “ divyang.shah[at]
HWE is a phenomenon observed initially in
studies of occupational diseases: Workers usually
exhibit lower overall death rates than the general
population, because the severely ill and chronically
disabled are ordinarily excluded from employment
“ Last, 1995.
Another definition by McMichael (1987) who first
give its name is: HWE refers to the consistent
tendency for the actively employed people to have
a more favourable mortality experience than the
population at large.
However, other occupational epidemiologists
describe HWE as the reduction of mortality or
morbidity of occupational cohorts when compared
with the general population.
Letâ„¢s try to understand it by an example.
Doll and co-workers studied on gas workers
exposed to carbonised coal. They measured
The Healthy Worker Effect (HWE) phenomenon is
under debate and a lot of discussion during the
last years. Some epidemiologists regard HWE as
an ordinary method problem, while other
considered it as a field of science by itself.
In this article I shall explain different definitions of
HWE and the historical back ground. What is
important to understand this phenomena and what
is the core of HWE, by that I mean the crux of the
problem. I will also address about the different
factors affecting the phenomena and how to
minimize it.
measured Standardised mortality rate (SMR
i.e. mortality rates after eliminating possible
effect of age differences in workers and
general population) for groups of gas workers
with different exposure. Following were their
Mortality (SMR-, all causes) of gas workers
compared with national experience
Heavy Exposure Intermediate No Exposure
(Doll et al 1965).
The SMR is less than 100 in unexposed
Historical Background:
The phenomena was observed in 1885 when
William Ogle found that mortality rates is
depending on the difficulty of the work where
some occupations may repeal, while others
may attract the workers. On other words, the
more vigorous occupations had relatively
lower mortality rate as compared with the
death-rates in occupations of an easier
character or the unemployed. Almost One
hundred years later McMichael coined the
term HWE to describe this phenomena in
1974. A year later, Goldsmith (1975) pointed
out that most industrially- employed cohorts
should be expected to have better life
expectancy than unemployed persons. SMR
close to unity (100) is used as an indication of
absence or a low degree of the HWE.
Importance of HWE:
Any occupational study looking for the worker
health could potentially face this problem. It is
a type of bias. The question is - Is it a serious
bias? Actually, most of the study indicated thatPage 43

Journal of HSE & Fire Engineering
Issue 2 March 2009
Page 34
HWE will reduce the association between
exposure and outcome by an average of 20-30
percent. Letâ„¢s study previous example of the
British gas workers
SMR in non exposed worker is less than 100. So
this reduction in SMR could be leading us to
conclude that the condition among the workers
is good and no harmful effect was seen. And of
course it is not true. It may partially or
completely mask excess mortality or morbidity
that is caused by harmful exposure.
Trying to understand HWE was not easy since
there was a lot of explanation and debate about
its nature. Some of the scientists considered
HWE as a source of selection bias; others
considered it as a confounding. A third group
considered it as a mixture of both. While others
considered it as a comparison problem.
Selection Bias: Error due to systematic
differences in characteristics between those who
are selected for study and those who are not.
The selection bias occurred from the initial
choosing of workers (mainly healthy workers)
and the factors that influence the continuity of
work such as leaving the work because of
sickness. To put it simple, HWE refers to the
initial hiring into the workforce and the
subsequent factors which influence continuing
Confounding: A situation in which a measure
of the effect of an exposure on risk is distorted
because of the association of exposure with
other factor(s) that influence the outcome under
The confounding factor is the (unmeasured)
health status of the group of employees.
Going back to our example GHS is associated
with exposure. (Employment in industry and
associated with outcome death).
There is a third school of thought. The third
opinion considered HWE as a confounder and a
selection bias since itâ„¢s very difficult to
differentiate between them.
The last opinion was a comparison problem.
According to Olli Miettinen; the best reference for
a population under study (as an example A) in a
specific time is with the same population at the
same time without the exposure. And that is
Components of HWE:
Healthy hire effect:
Employers have the right to reject certain persons
for employment because of physical disabilities,
or because of poor general health. Employer will
exclude those obviously at high risk. Person
selection may also be influenced by person habits
and physical conditions such as weight, smoking,
or alcoholism. This effect will vary according to
the labour situation, i.e. during period of labour
shortages less fit workers could be included into
the labour force whereas during periods of labour
surpluses employers can be much selective.
Healthy worker survivor effect:
Workers who do not have strong motivation to
work because of health problems do not present
themselves for employment (self-selection). They
mainly change their work frequently or retired
early. They change their job for different reasons
including health. The effect will be reduced after
15 years of entry to the industry.Page 44

Journal of HSE & Fire Engineering
Issue 2 March 2009
Page 35
The length of time the population has been
HWE is a characteristic of actively employed
workers. Incomplete follow-up of the out-
migrating section of the cohort, which could
result in failure to track every individual to
determine his vital status. Reduction in health
status could occur without any relation to
Monson (1986) divided the follow-up into two
phases; a dynamic phase and a stable plateau.
The dynamic phase characterized by increase
Relative Risk (RR) with years of follow-up and
then RR becomes constant after some years of
follow-up (plateau).
Beneficial effect of work:
Improved access to health care, routine disease
screening and Physical exercise is the beneficial
effect of work.
While there is a wide agreement on the first
three factors, there is debate about the extent on
the beneficial effect. Doll considered the low
mortality is the result of true benefit of work on
Factors affecting HWE:
HWE is not constant. Rather it varies depending
on choice of comparison population. These
factors that are affecting HWE are also varied
between studies.
Time related factors:
Age at Hire:
Workers with high age will be highly influenced
by selective processes. Because, the proportion
of persons attaining the required level is likely to
be smaller in the old age group.
RR for all causes of mortality
Age at Hire
Age at risk:
Means the age at any point in follow up that
shows the outcome (death). Increasing age at
risk will increase the period of follow-up and thus
reduce the HWE.
RR for all causes of mortality by age at risk
Age at Hire
55-64 65-74 >=75
Fox & Collier
McMichael et al
Delzell and
Monson (1981)
Duration of employment:
Increasing the duration of employment will
increase the effect since many sick people will
leave or change the work into another safer work.
Socioeconomic status:
The HWE is stronger in more qualified jobs.
Professional workers demonstrate a stronger
overall HWE. Based on job classification, highest
socioeconomic status of work (white collar) has
high healthy worker effects since it requires high
qualification more than unqualified works.
Gender: Usually the effect is stronger for females
than males.
Avoiding HWE:
Many efforts are tried to minimize the HWE. The
most straightforward way of avoiding or
minimizing the HWE is not to use the general
population as a reference group. Instead use
active workers from another industry who do not
have the same exposure. Another way to
minimize HWE is comparing rates of the health
outcomes of interest between individuals with
high exposure and those with low or no
exposures. This is useful where the external
reference group is not ideal. However, it is not
likely that all occupational hazards pose gradient
effects on human health (many industries show
uniform exposure). One can also reduce HWE by
starting the study after a latency of time e.g. one
year, five years etc. where the HWE is high
during the period. Another way to minimize HWE
high during this period. Another way to minimise
HWE is to include experience of every person
who ever worked in a particular factory or
industry.Page 45

Journal of HSE & Fire Engineering
Issue 2 March 2009
Page 36
Summary and Conclusion:
HWE is caused by inadequate reference group
(i.e. comparison problem). If we find an ideal
reference group then the HWE will not exist. HWE
is a complex and problem creating bias; it
comprises several factors, and may be modified by
a number of factors. It is not possible to make
generalisations in a particular case of HWE.
Doll R, Fish REW, Gammon EJ, Gunn W,
Hughes GO, Tyrer FH, Wilson W (1965)
Mortality of gasworkers with special reference
to cancers of the lung and bladder, chronic
bronchitis, and pneumoconiosis. Br J Ind Med
22: 1“12.
McMichael, AJ. Standardized mortality ratios
and the "healthy worker effect": Scratching
beneath the surface. J Occup Med. 1976
Fox, AJ; Collier, PF. Low mortality rates in
industrial cohort studies due to selection for
work and survival in the industry. Br J Prev
Soc Med. 1976 Dec;30(4):225“230.
Delzell, E; Monson, RR. Mortality among
rubber workers. III. Cause-specific mortality,
Delzell, E; Monson, RR. Mortality among
rubber workers: X. Reclaim workers. Am J Ind
Med. 1985;7(4):307“313.
Hernberg, S. Evaluation of epidemiologic
studies in assessing the long-term effects of
occupational noxious agents. Scand J Work
Environ Health. 1980 Sep;6(3):163“169.
7. Gilbert, ES. Some confounding factors in
study of mortality and occupational
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