The review of occupational asthma by Nicholson et al. [1] is
comprehensive. It is an important report that is likely to be widely
read: the evidence review of low back pain has, for example, been one of
the most commonly downloaded articles [2]. The appearance in the
principal recommendations of the authors’ unsubstantiated opinions is,
therefore, concerning.
The review of occupational asthma by Nicholson et al. [1] is
comprehensive. It is an important report that is likely to be widely
read: the evidence review of low back pain has, for example, been one of
the most commonly downloaded articles [2]. The appearance in the
principal recommendations of the authors’ unsubstantiated opinions is,
therefore, concerning.
The authors also include issues in the principal recommendations that
are not supported by evidence. By including these with others the
appearance is given that evidence exists. They recommend that
"surveillance should include … immunological tests where appropriate" despite an absence of evidence that these tests are of benefit in surveillance. They also recommend that surveillance should be provided
"at least annually". No evidence is provided that surveillance should be
delivered on this basis. I am not aware of any studies comparing
surveillance intervals.
These issues have important implications for the practice of health
surveillance. Applied without thought they could lead to pressure on
employers to provide surveillance that is unnecessarily costly and
frequent. More importantly they could lead to pressure on employees to
accept invasive procedures of questionable or no value in surveillance
that could then have important implications for their employment. I have
noted in this journal on a previous occasion the tendency to interpret
data in a way that reinforces existing opinion to the potential
disadvantage of workers at risk of occupational asthma and their
employers [3].
This additional "expert opinions" complement the evidence review.
However, it would have been better to either distinguish them clearly from
the evidence or make it clearer that there is an absence of evidence to
underpin these opinions.
References
1. Nicholson PJ, Cullinan P, Newman Taylor AJ, et al. Evidence based
guidelines for the prevention, identification, and management of
occupational asthma. Occup Environ Med 2005;62:290–9.
2. Top ten downloads from Occupational Medicine. Occup Med (Lond)
2005;55:74.
3. RM Preece. Lung function decline in laboratory animal workers
[electronic response to Portengen et al. Lung function decline in
laboratory animal workers: the role of sensitisation and exposure]
occenvmed.com 2003 http://oem.bmjjournals.com/cgi/eletters/60/11/870#90.
The paper by Nicholson et al. provides useful evidence-based
guidelines for prevention, identification, and management of occupational
asthma (OA) based on a comprehensive review of the literature.[1]
Evidence statements 6 and 7 list the workers most commonly reported to OA
surveillance schemes or reported from population studies to be at
increased risk of developing asthma. Hairdressers are n...
The paper by Nicholson et al. provides useful evidence-based
guidelines for prevention, identification, and management of occupational
asthma (OA) based on a comprehensive review of the literature.[1]
Evidence statements 6 and 7 list the workers most commonly reported to OA
surveillance schemes or reported from population studies to be at
increased risk of developing asthma. Hairdressers are not included in
these lists, but we believe that they should be.
Data from the French national OA surveillance programme (ONAP) show that
hairdressing represents the fourth most frequent occupation (both sexes)
and the second most frequent occupation in women, among subjects with OA.[2]
According to this programme, the annual incidence rate was 308 per
million in hairdressers, whereas the mean annual incidence of OA in France
was estimated to be 24 cases per million workers. In Sweden, the self-
reported rate of OA in female hairdressers was 129 per million, whereas
the annual crude reporting rate for women was 70 per million.[3]
Several population-based studies have also shown an increased risk of OA
in hairdressers. In Finland, the risk of OA was significantly increased in
hairdressers, both in men (RR: 2.09; 95%CI: 1.12-3.89) and in women (RR:
1.61; 95%CI: 1.45-1.79), using a reference group with little exposure to
dusts, fumes, vapours, or other workplace air-pollution.[4] In Spain, an
increased risk of asthma, defined by the presence of wheezing or whistling
in the chest during the previous 12 months, was observed in hairdressers,
but was not statistically significant (OR: 1.94; 95%CI: 0.86-4.39).[5]
These data are also consistent with the results of a Swedish nationwide
retrospective study showing a moderately increased asthma risk among non-
smoking hairdressers [6], and with the results of a Finnish retrospective
cohort study using a self-administered questionnaire showing a relative
risk for developing asthma of 1.7 (95%CI: 1.1-2.5) among hairdressers,
compared with referents.[7]
References
1. Nicholson PJ, Cullinan P, Taylor AJ, Burge PS, Boyle C.
Evidence based guidelines for the prevention, identification, and
management of occupational asthma. Occup Environ Med 2005;62:290-9.
2. Ameille J, Pauli G, Calastreng-Crinquand A, Vervloet D,
Iwatsubo Y, Popin E et al. Reported incidence of occupational asthma in
France, 1996-99: the ONAP programme. Occup Environ Med. 2003;60:136-41.
3. Toren K. Self reported rate of occupational asthma in Sweden
1990-2. Occup Environ Med 1996;53:757-61.
4. Karjalainen A, Kurppa K, Martikainen R, Karjalainen J, Klaukka
T. Exploration of asthma risk by occupation: extended analysis of an
incidence study of the Finnish population. Scand J Work Environ Health
2002;28:49-57.
5. Kogevinas M, Anto JM, Soriano JB, Tobias A, Burney P. The risk
of asthma attributable to occupational exposures. A population-based study
in Spain. Spanish Group of the European Asthma Study. Am J Respir Crit
Care Med 1996;154:137-43.
6. Albin M, Rylander L, Mikoczy Z, Lillienberg L, Dahlman HA,
Brisman J et al. Incidence of asthma in female Swedish hairdressers. Occup
Environ Med 2002;59:119-23.
7. Leino T, Tammilehto L, Luukkonen R, Nordman H. Self reported
respiratory symptoms and diseases among hairdressers. Occup Environ Med
1997;54:452-5.
Firstly, latent period always refers to the period between the point
of the time when disease occurs and point of the time when the disease is
detected, while tumour induction time refers to the period between the
point of the time when the component cause (can be an exposure) is
satisfied and the point of the time when the disease is occurred.[1]
Thus only under the extreme condition that one
secon...
Firstly, latent period always refers to the period between the point
of the time when disease occurs and point of the time when the disease is
detected, while tumour induction time refers to the period between the
point of the time when the component cause (can be an exposure) is
satisfied and the point of the time when the disease is occurred.[1]
Thus only under the extreme condition that one
second of mobile use may cause brain tumours, while once the tumour is
‘born’, it can be detected, it would be possible to defined the latency
period as the period between ‘first use of a cellular or cordless
telephone until tumour diagnosis’.
Secondly, in the research exposure to ‘microwaves’ is classified as a
chronic exposure (such as smoking) rather than a point exposure (such as
exposure to toxic gas for one minute). While the exposure is measured by
the total hours use of different types mobile, and they compared the
effect of different exposure dose (lower or above 85 hours) under
different length of ‘latency period’. However the effect of certain dose
of exposure will only appear after the subjects already exposed to that
dose of exposure. For example, the effect of exposure to 85 hours of digital
mobile use on the incident of brain tumours, might only possibly appear
after the subjects has already been exposed to 85 hours of digital mobile use.
Similarly, it is unrealistic to measure the effect of exposure to 10 pack-
year smoking on risk of lung cancer, since the first day when the subjects
pick up their first cigarettes.
Furthermore, there may be subjects who use more than one kind of the
mobile that have been listed in the article, effect of multiple exposures
have not been measured in the research.
In addition people with ‘Zero’ exposure may not be a good comparison.
People who never use mobile may be rare in today’s society, as to say even
control for gender, age, socio-economic status, they may still have
special characters different from people who use mobile such as smoking,
alcohol use, diet habit, physical activity etc. Above all, exposure is always extremely hard to measure validly, and sometimes it may even hard to define, however these are always essential
for the accuracy of a study.
Reference
1. Rothman, J. & Greenland, S. Modern Epidemiology, 1998, Lippincott-Raven.
Sorahan et al. [1] recently published the results of a cohort
mortality and morbidity study of workers purportedly exposed to benzene in
the UK. Despite inherent problems with their data analyses, the authors
nevertheless concluded that “the study does not support claims that
exposure to benzene affects risks for lymphohaematopoietic malignancies
other than ANLL.” In my opinion, the discrepancies and...
Sorahan et al. [1] recently published the results of a cohort
mortality and morbidity study of workers purportedly exposed to benzene in
the UK. Despite inherent problems with their data analyses, the authors
nevertheless concluded that “the study does not support claims that
exposure to benzene affects risks for lymphohaematopoietic malignancies
other than ANLL.” In my opinion, the discrepancies and flawed analyses
preclude this study from being used to make any health determination for
workers exposed to benzene.
Lack of information on cancer deaths among cohort members:
The authors admit to evidence of “under-ascertainment,” or non-
identification, of cancer registrations. This is an understatement.
Indeed, with the exception of cancer of the lip (2 observed vs. 0.2
expected), the highest risk of cancer achieving statistical significance
was “cancers of uncertain origin,” SMR = 140, based on 68 cancer deaths, p
<0.001. Their morbidity analysis also suffers from “under-ascertainment” for specific types of cancer experienced by these workers
as the authors state that an estimated 60 cancer registrations remained
untraced.
Closer scrutiny of their data also reveals an even greater problem
with “under-ascertainment” of cancer deaths during the1968-74 follow-up
period--the period of death for those cohort members who likely
experienced the highest benzene exposure levels. For the 1968-74 follow-
up period, I estimate that 51% (46 of 91) of the cancer deaths had no
information provided on their specific type of cancer. My analysis is as
follows:
The data in Table 2 of the Sorahan et al. report [1] indicate
that 761 cancer deaths occurred during the entire follow-up period of 1968
-2002. In the text, the report states that 670 cancer deaths occurred
during the period 1975-2000. Therefore, 91 (761 - 670) cancer deaths had
to have occurred during the 1968-74 and 2001-2002 follow-up periods.
Of
these 91 cancer deaths, the number with undetermined type can be
calculated as follows: For the 1975-2000 period, “there were 102 deaths
for which no cancer registration had been received.” Of these, the Office
of National Statistics (ONS) produced cancer registrations for 80, leaving
22 cancer deaths for the period 1975-2000 as unspecified. Since 68 cancer
deaths could not be specified for the entire follow-up period of 1968-
2002, 46 (68 - 22) of the cancer deaths that had no information on type of
cancer had to have occurred between 1968-74 and 2001-2002. (Sorahan et
al. [1] state that they made no attempt to search for cancer registrations
for the cancer deaths that are known to have occurred during these follow-
up periods, but do not explain the rationale for this decision.)
If one
assumes that cancer registrations have improved over time, most of the
under-ascertainment in cancer registration is likely to have occurred in
the earlier period of follow-up, e.g., 1968-74. Because there was an
under-ascertainment of cancer registrations of up to 51% (46/91) for the
1968-74 period, the study cannot accurately calculate any reliable cancer
risk from exposure to benzene during this period. Given the rarity of
hematopoietic neoplasms, the identification of only a few among the
estimated 46 unspecified cancer deaths for this period could produce
highly significant results.
Lack of information on benzene exposure:
Information on benzene exposure was provided for only 130 of the 233
(56%) establishments that contributed cohort members to the study, and
this information was limited to the “average” benzene exposure at the
facilities in 1966-67. Given that the cohort included workers exposed to
benzene in the 1940s and that it was followed to 2002 for mortality, and
to 2001 for morbidity, the average exposure level for a “facility” based
on the 1966-67 period has little meaning in terms of benzene exposure to
the cohort members.
Further, the authors state “others might only be
exposed for a few hours each week; such information was not available to
the study.” Therefore, the exposure recreation is dubious at best given
(a) the paucity of exposure information and (b) the lengthy intervening
time period between initial exposure to benzene and the attempted exposure
determination. The inclusion of individuals in the benzene cohort who
were not exposed to benzene compromises the identification of the cohort
and dilutes estimates of relative risk of diseases related to benzene
exposure.
Lack of analysis by latency period:
The authors present data by period of death, but they do not present
information by latency. Such an analysis may provide useful information
(with more complete ascertainment of cancer deaths) as acute leukemia
appears to have a relatively shorter average latency period than B-cell
cancers such as chronic lymphatic leukemia, multiple myeloma and some non-
Hodgkin’s lymphomas (NHL), all of which have been associated with benzene
exposure, or benzene containing solvents [2-6]. For example, data from
the Hayes et al. study [6] indicate that the risk of ANLL/MDS was more
significantly associated with recent benzene exposure, whereas NHL was
associated with more distant exposure prior to diagnosis. While the
numbers are small in the Sorahan et al. study [1], the earliest period of
follow-up for which data are presented suggests that the risk for “ANLL”
is higher in the earlier 1968-75 period (SMR = 265) based on three deaths,
than from the more distant follow-up period of 1976-2002 (SMR = 169 )
based on 11 deaths. Unfortunately, this issue cannot be more fully
explored because of the limitations in ascertainment of specific cancer
deaths.
Lack of statistically significant excess of mortality from ANLL:
The authors erroneously state that their data demonstrate a
statistically significant excess of acute non-lymphocytic leukemia (ANLL).
In fact, an elevated risk for ANLL only achieves statistical significance
when two deaths from acute unspecified leukemia (AUL) are included as
deaths from ANLL. The authors inclusion of the AUL’s along with the true
ANLL deaths in the category of “acute leukemia deaths” would result in 14
observed vs 8.48 expected, SMR = 165, which also is not statistically
significant. The results of their morbidity analysis as well do not
demonstrate a significant excess of “ANLL” among the benzene cohort
members.
In summary, there are inherent data limitations in the Sorahan et al.
study [1]: (i) “under-ascertainment” of cancer deaths; (ii) unverifiable
benzene exposure for individual cohort members; (iii) inadequate attention
to analysis by latency; and (iv) improper categorization of ANLL. As a
result, the study provides little information upon which to evaluate
health risks from occupational exposure to benzene.
Peter F. Infante, D.D.S., Dr.P.H.
Adjunct Professor of Environmental and Occupational Health
School of Public Health and Health Services
The George Washington University
Washington, D.C. 20037
1. Sorahan T, Kinlen LJ, Doll R. Cancer risks in a historical UK
cohort of benzene exposed workers. Occup Environ Med 2005;62:231-236.
2. Glass DC, Gray CN, Jolley DJ, et al. Leukemia risk associated with
low-level benzene exposure. Epidemiology 2001;14:569-577.
3. McMichael AJ, Spirtas R, Kupper LL, et al. Solvent exposure and
leukemia among rubber workers: An Epidemiologic study. J Occup Med
1975;17:234-239.
4. Wolf PH, Andjelkovich D, Smith A, et al. A case-control study of
leukemia in the U.S. rubber industry. J Occup Med 1981;23:103-108.
5. Rinsky RA, Smith AB, Hornung R, et al. Benzene and leukemia: An
epidemiologic risk assessment. New Eng J Med 1987;316:1044-1050.
6. Hayes RB, Yin S-N, Dosemeci M, et al. Benzene and the dose-related
incidence of hematologic neoplasms in China. J Natl Cancer Inst
1997;89:1065-1071.
Kyle Steenland raises some interesting points in his commentary on silica [1] both on our papers reporting exposure assessment and mortality in the UK silica sand industry [2,3] and on the adverse effects of silica in general.
With the exception of one quarry, where other exposures such as polycyclic aromatic hydrocarbons could have occurred, no relationship was found with cumulative silica ex...
Kyle Steenland raises some interesting points in his commentary on silica [1] both on our papers reporting exposure assessment and mortality in the UK silica sand industry [2,3] and on the adverse effects of silica in general.
With the exception of one quarry, where other exposures such as polycyclic aromatic hydrocarbons could have occurred, no relationship was found with cumulative silica exposure in the UK silica sand study. Steenland points out that the estimated exposure levels were relatively low with only a few workers having a cumulative exposure over 1
mg/m3.years (in fact only 8% of the study population overall and 5 lung cancer cases had a cumulative exposure over 2 mg/m3.years). We agree that this may be why only 2 silicosis deaths were observed, although, as we point out in the paper, fibroses, including silicosis, are poorly recorded
on death certificates and therefore could not be accurately assessed.
Steenland draws attention to the current controversy over the relationships between silica, silicosis and lung cancer. The epidemiological literature is indeed inconsistent. OEM readers might be interested to know about a two day workshop organised by the European Association of Industrial Silica Producers (EUROSIL) that was held in New
York in August 2004, which brought together leading scientists from Europe, the USA, Canada, China, South Africa and Australia involved with the major studies in the industrial sand, diatomaceous earth, mining, heavy clay, granite, stone, pottery and brick industries [4] (Kyle
Steenland was invited but was unfortunately unable to attend). The aim of the workshop was to gain a clear understanding of the epidemiological work to date, and to prioritise future research needs. Following brief presentations summarising the results from these studies, break out groups evaluated the variations between studies in the design, definition and derivation of health outcomes, assessment of exposure, collection of confounding data and statistical methodology. The groups identified the knowledge gaps and discussed the feasibility and desirability of filling these.
The overwhelming conclusion from the workshop was that heterogeneity occurs across all aspects of both the actual nature of the industries in which silica exposure occurs and the design, conduct, analysis and interpretation of the studies that have been carried out.
Heterogeneity in one or more of the following might contribute towards the differing results:
The physico-chemical features of the silica, including geological species, chemical composition, percentage crystalline silica, particle size, freshness of the fracture.
Methods of measuring silica exposure, including sampler design, analytical technique, sampling strategy.
Exposure assessment methodology, including taking account of changes in technology, use of protective equipment such as respirators, retrospective extrapolation.
Derivation and definition of health outcomes, including accuracy and completeness of death, cancer and other registers, diagnostic procedures such as X rays, CT scans, use of pathological samples.
Methodology for collection of data on confounding variables such as smoking, which, in many studies, is limited.
Statistical methodology, including study size, follow-up time, adjustment for confounders and other relevant exposures, appropriate models.
Many of the above are, of course, inherent difficulties in occupational epidemiology in general. However, in the area of silica some of these differences may be so fundamental that it may be necessary to rethink the concept of the silica industry being a uniform entity. As the
IARC Working Group pointed out, given the wide range of populations and exposure circumstances some non-uniformity of results would be expected [5] and even within the same industry the results from different studies may vary. Some of these are highlighted below.
At the workshop the group discussing studies in the diatomaceous earth industry highlighted uncertainties in exposure assessment methodologies including the conversion factors used to convert total dust counts to respirable silica, extrapolation methods used for exposures prior to 1950 and lack of adjustment for calcining for exposure before 1930. They were also concerned about co-exposures to asbestos, lack of smoking data, and the difficulty of separating small round opacities from small irregular opacities when reading chest x-rays. Conversion factors
and extrapolation in exposure assessment were also of concern regarding the studies in the sand industry as was the origin and composition of the sand. For example, in the North American studies some sands were almost pure quartz, whilst others elsewhere were dune sand or feldspar sands.
There were also differences in the aluminium content. Workforces in mining were considered to have the advantages of large and stable workforces, good exposure measurement data and special health programmes and surveillance. Co-exposures, for example, to radon, asbestos and PAHs were
highlighted as particular problems, however, as was the variation in the quartz content of dusts from mines extracting tin, gold, or coal.
The group discussing the mining studies also drew attention to the need to investigate exposure metrics such as intensity and peaks of exposure as an alternative to cumulative exposure. Many of the above considerations were also identified as potential problems in the pottery, brick and granite industries. The researchers involved in the Vermont granite studies expressed interest in collaborating to explore the reasons why their studies had produced different results. This group drew attention to the issues of survivor populations and susceptibility. For example, in the studies in China, there is a high incidence of non-malignant respiratory disease at an early age that could impact on the results for lung cancer occurring at older ages.
One of the aims of the workshop was to try and target any future research so that key issues concerning adverse health effects of silica and uncertainties around current knowledge can be addressed. These include:
In what industrial settings, if any, does silica exposure at
current compliance levels, essentially at established ‘Western (US, EU)’ limits, cause cancer?
What role does silicosis (fibrosis) play?
Does the lung cancer risk increase for radiographic severity of silicosis?
Are there pathological differences between fibrosis from silica and asbestos that influence lung cancer?
How does non-malignant respiratory disease affect lung cancer risks?
From the various presentations and discussions, eight priorities were
identified:
1. Effectively control workers’ dust exposure and implement proper
evaluation and prevention measures.
2. Harmonise sampling and analytical methods for future collection of
dust measurements and develop a standardised job/task industry wide Job
Exposure Matrix (JEM).
3. In parallel, collect information on the type and use of personal
protective equipment and develop the methodology for incorporating this
into the JEM for future exposure assessment.
4. Investigate the toxicological potency of different types of silica
using industry samples
5. Focus on industries with similar exposures and review the
differences that may have given rise to different estimates of risk.
6. Consider whether pooling of the data might be useful and
investigate what this might entail, e.g. development of a harmonised JEM
and exposure assessment methodology, bearing in mind that indiscriminate
pooling might give misleading and imprecise results.
7. Consider whether current cohorts might be able to re-analyse their
data to address the priority areas of concern and/or whether they can
collect supplementary data to assist with this.
8. Alternatively carry out a new study(ies) but ensure that there is
an agreed protocol and a design that ensures knowledge gaps will be
filled.
Some of the above are already being developed. For example, the
Industrial Minerals Association in Europe is developing a harmonised dust
monitoring strategy and encouraging all its members to implement this.
IARC have classified silica as a Group 1 carcinogen, but rather than
focusing on hazard identification, workshop participants felt that the
focus should be on reducing exposure to a level that would protect against
silicosis, and thus probably lung cancer, and be technically achievable
across the whole industry. The need for enforcement of current national
standards is illustrated by findings of high percentages of samples taken
by the US Occupational Safety and Health Administration (OSHA) still
exceeding the OSHA Permissible Exposure Limit (PEL) of 0.1 mg/m3 [6]. For
example in 1999, 47% and 38% of samples in construction and manufacturing
groups, respectively, exceeded the OSHA PEL. The fact that silicosis (and
other potentially related diseases) still occurs is likely to be due to
the continuation of overexposure that is perhaps not reflected in some of
the industries investigated in much of the current epidemiological
literature, which tends to focus on producers rather than manufacturers or
users.
References
1. Steenland K Silica: déjà vu all over again? Occup Env Med 2005; 62: 430-432
2. Brown TP, Rushton L Mortality in the UK Industrial Silica Sand
Industry: 1. Assessment of exposure to respirable crystalline silica.
Occup Env Med 2005; 62: 442-445
3. Brown TP, Rushton L Mortality in the UK Industrial Silica Sand
Industry: 2 a retrospective cohort study. Occup Env Med 2005; 62: 446-452
4. Eurosil Proceedings of expert workshop: Epidemiological
perspectives on silica and health. Brussels, Belgium, European Association
of Industrial Silica Producers, 2005
5. IARC Monographs on the evaluation of carcinogenic risk to human,
vol. 68: silica, some silicates, coal dust and para-aramid fibrils. Lyon,
International Agency for Research on Cancer, 1997
6. NIOSH Work-related lung disease surveillance report 2002 (DHHS
(NIOSH) Number 2003-111) Cincinnati, Ohio, National Institute for
Occupational Safety and Health, 2003
The report on the occupational health risks of ethylene glycol
ethers is convincing while using the time-honoured indicators of female
reproductive health.[1]
The effects often coincide with or depend on nervous system toxicity of
e.g. solvents. The toxic effect of the ethylene glycol ethers seems to
stem from their end metabolites, the corresponding alkoxyacetic acids.
They seem to be inhibit...
The report on the occupational health risks of ethylene glycol
ethers is convincing while using the time-honoured indicators of female
reproductive health.[1]
The effects often coincide with or depend on nervous system toxicity of
e.g. solvents. The toxic effect of the ethylene glycol ethers seems to
stem from their end metabolites, the corresponding alkoxyacetic acids.
They seem to be inhibitors of succinate dehydrogenase [2,3] which is
critically associated with mitochondrial respiratory chain and links it
with the tricarboxylic acid circle. Thus, one would expect negative
consequences of exposure in highly oxygen-dependent organs, like brain and
kidneys. Chronic energy failure would also be harmful to the developing
foetus.
Kojo et al. [1] report their results on breast cancer risk among
airline cabin attendants in a nested case-control study. Increased
incidence of breast cancer has been repeatedly found among Finnish and
other airline cabin attendants and that is the motive of the study. The
results do not support the hypothesis that cosmic radiation exposure as
measured in the study is strongly linked to the inductio...
Kojo et al. [1] report their results on breast cancer risk among
airline cabin attendants in a nested case-control study. Increased
incidence of breast cancer has been repeatedly found among Finnish and
other airline cabin attendants and that is the motive of the study. The
results do not support the hypothesis that cosmic radiation exposure as
measured in the study is strongly linked to the induction of breast cancer
among the cabin attendants. I would like to comment on some aspects of
this study as well as how the results are interpreted. A cohort of Finnish
cabin attendants serves as a study base, cases of breast cancer were found
in the Finnish Cancer Registry and controls were chosen with matching on
year of birth from non-cases of the cohort. This setting seems to be ideal
and it should be possible to identify all cases of breast cancer that
occur in the study base. However, exposure information in the study was
collected after the cases of breast cancer have been diagnosed. This
procedure will influence validity because it opens up the possibility that
breast cancer may influence the access and the quality of the information
on exposure. This is exactly what happens in the study and I will come to
it later.
In the paper Kojo et al. state the following policy implications:
“There is no need to take occupational factors into account in breast
cancer prevention among cabin attendants.”[1] This is a surprisingly
determined generalisation in the light of the small material of the study.
The study has not convincingly demonstrated the absence of effect of an
occupational exposure on breast cancer risk among cabin attendants. There
is a well known definition of a negative study but the study of Kojo et al.
[1] does not fit into that definition. A true negative study must be large
and sensitive, and it must have accurate exposure data.[2] It seems to me
that the study of Kojo et al. is lacking in all three aspects.[1] I would
like to point out the following: The authors themselves discuss the
smallness of the material consisting of 27 cases. It also seems a rather
crude method to estimate cumulated radiation dose on basis of, among other
parameters, five questions on number of round-trip flights per month
divided in two or three decades. Further there was only 52% participation
rate in the questionnaire survey on exposure. The limitation of collecting
exposure information retrospectively has been addressed in a recent study
on airline cabin attendants.[3] That study did not suffer from low
participation.
Kojo et al. [1] have a problem with possible selection bias because of
poor participation when estimating the exposure to cosmic radiation. In an
attempt to evaluate this problem they calculate the odds ratio for breast
cancer for all subjects in the cohort (44 cases and 921 non-cases, all
subjects with known start and end of cabin work according to information
obtained from Finnair and the Finnish Cabin Crew Union). Based on this
information they calculate active work year and combine this with the
estimated mean annual cosmic radiation dose by calendar period [4] to
obtain a crude estimate of cosmic radiation dose for every person
explained in the Methods.[1] Not surprisingly they get a similar odds
ratio in the matched case-control study as in the analysis when they
calculate the odds ratio using the crude cosmic radiation exposure data in
the Results.[1] Both exposure estimates involve information from the
questionnaire survey with the poor participation rate. However, this fact
does not keep the authors from concluding on the similarity of the two
odds ratio in the Discussion.[1] Here we seem to be facing a phenomenon
called arguing in a circle. Arguing in a circle occurs when two or more
unproved propositions are used to establish each other. In this aspect I
would like to point out the interesting difference in the mode of
expression concerning these odds ratio in the Result section.[1] In the
main study with the smaller number of subjects in the univariate analysis
it is: “..cumulative radiation dose …..showed no effect on breast cancer”,
and in the multivariate analysis it is: "..cumulative radiation
dose...showed negligible effects on breast cancer", whereas the odds ratio
based on calculation on crude work years gets: “..using crude cosmic
radiation exposure data, the occupational radiation dose was not
associated with breast cancer…”.
The authors mention the limitation of their study due to
retrospective collection of the information on exposure. The case
ascertainment was retrospective and therefore eight cases (18%) were
deceased and lost from the study and authors assume the loss to be greater
among breast cancer cases due to excess mortality from breast cancer
compared to non-cases. This can indeed be calculated from the data given
in the paper to be 24 deaths or 2% of the non-cases. Thus proportionally
more cases than non-cases were lost because they were deceased and thus it
was not possible to collect information from them in regard to exposure.
In the study of Kojo et al. [1], breast cancer influences the access to
information on exposure and this has possibly introduced bias.
Kojo et al. [1] do not mention the possibility that the disease,
breast cancer, can influence the subjects answers, which may introduce
errors.[2, 5] This may particularly occur if cabin attendants who get
breast cancer or the non-cases suspect that there is an association
between occupation and cancer and such suspicion can easily arise based on
information via mass media if not from other sources. No information is
given in the paper on measures taken to avoid possible influence by the
authors knowledge of the aim of the study. There is no comment on whether
investigation on the exposure conditions were conducted blind as to the
case-control status of the cabin attendants. For example it is not very
clear who selected and how the representative routes were chosen. These
routes were later used to calculate radiation dose.[4] One can only
speculate whether these have introduced bias. However, the authors
excluded from the study those who had worked for less than two years as
airline cabin attendants and it appears not to have involved cases. This
exclusion based on exposure variable introduces bias towards the null
hypothesis.
In the conclusion of the Abstract and in the last sentence of the
Main messages it says that three occupational factors are studied and it
is stated that there is no clear evidence that they affected the breast
cancer risk. It is not a simple task to find where in the paper the
authors give a clear account of all three occupational factors, however,
it is easy to identify the cumulative radiation dose in mSv as an
occupational factor. The other two occupational factors are the disruption
of the sleep rhythm and the disruption of the menstrual cycle.
Inexperienced reader may be confused whether these factors belong to
outcome or exposure and the same may be valid for cases and non-cases. Are
these disturbances, identified with the other exposure data
retrospectively, so clearly related to the occupation as to serve as a
surrogate of exposures? Is it possible to escape sleep disturbances as a
consequence of long haul flight? Is it possible that breast cancer cases
get sleep disturbances of causes other than occupational? Is it possible
that these disturbances are not suitable exposure indicators?
Kojo et al. [1] divided the material into two parts in an attempt to
evaluate the possible longer recall period concerning flight activity,
disturbances of menstrual cycle and sleeping among those over 50 years of
age as compared to younger women. The next step was to calculate odds
ratio for breast cancer in the two groups (50 years of age and younger,
and over 50 years of age) associated with each of the three factors
separately i.e. cumulative radiation dose, sleep disturbances, and
menstrual disturbances. These calculations, with less than 27 cases in
each group, (the number in each group is not available in the paper) yield
six different odds ratio and wide 95% confidence intervals, which all
include unity. However, the authors conclude that the estimates were
comparable suggesting that the lower participation among those older than
50 years did not bias the results. The longer recall for the older women
is not mentioned in this respect, which was the goal in the outset. Here
the authors conclude firmly based on small material arguing for the
validity of their study. In this discussion, on what Kojo et al. [1] call
modifying effect of age, we are shown the range of exposure in the groups,
0-103.5 mSv for women 50 years of age and younger, and 0-136.8 mSv for
women over 50 years of age. It is rather confusing to see the range go
down to zero, given that airline cabin attendants with less than two years
career were excluded from the study.
In the Discussion Kojo et al. [1] inform us on the fact that the
excess risk in the incidence of breast cancer among Finnish cabin
attendants has persisted based on updated follow up. They suggest that
this risk is related to well known risk factors of breast cancer such as
family history of breast cancer and possibly to moderate or heavy alcohol
consumption. And the authors do not compare their findings with
information from other studies on breast cancer among cabin attendants
ignoring the benevolent recommendation given long ago.[5]
References
1. Kojo K, Pukkala E, Auvinen A. Breast cancer risk among Finnish
cabin attendants: a nested case-control study. Occup Environ Med,
2005:62;488-493.
2. Hernberg S. Introduction to Occupational Epidemiology. Chelesea:
Lewis Publisher, 1992.
3. Grajewski B, Atkins DJ, Whelan EA. Self-reported flight hours vs.
company records for epidemiologic studies of flight attendants. Aviat
Space Environ Med, 2004;75:806-810.
4. Kojo K, Aspholm R, Auvinen A. Occupational radiation dose
estimation for Finnish aircraft cabin attendants. Scand J Work Environ
Health, 2004;30:157-163.
5. Breslow NE, Day NE. Statistical methods in cancer research, Vol. I.
The analysis of case-control studies. Lyon: International Agency for
Research on Cancer, 1980.
We read with great interest the article by Mannes et al., which
related the adverse effects of ambient air pollution on birth weight.[1]
That article well described the effects of pollutant exposure on the risk
of low birth weight using a marker of small for gestational age (SGA).
However, that study presents some shortcomings.
First, gestational week at birth is obstetrically and sociall...
We read with great interest the article by Mannes et al., which
related the adverse effects of ambient air pollution on birth weight.[1]
That article well described the effects of pollutant exposure on the risk
of low birth weight using a marker of small for gestational age (SGA).
However, that study presents some shortcomings.
First, gestational week at birth is obstetrically and socially a more
important marker for infancy and childhood than birth weight.[2] In recent
studies such as Mannes�f, the gestational week at birth or both the
gestational week at birth and birth weight are used rather than birth
weight.[3] We are convinced that the gestational week should be
incorporated into their methods as an appropriate marker. Secondly, almost
all infants in multiple gestations are SGA even if the pregnancy course is
uneventful.[2] Accordingly, Mannes et al. were compelled to exclude
multiple gestations from the study materials. Finally, the blood-placental
barrier prevents various materials from passing through to the fetus in a
similar manner to that of the blood-brain barrier. Accordingly, it is
inferred that those materials do not easily reach the fetus even if they
can reach to the mother. Mannes et al.�fs study would have been better
researched and more useful if the above problems had been addressed in
their discussion section.
References
1) Mannes T, Jalaludin B, Morgan G et al. Impact of ambient air
pollution on birth weight in Sydney, Australia. Occup Environ Med
2005;62:524-30.
2) Cunningham FG, Leveno KJ, Bloom SL et al. Williams Obstetrics (22nd
edn) TX, McGraw-Hill 2005
3) Wiles NJ, Peters TJ, Leon DA et al. Birth weight and psychological
distress at age 45-51 years: results from the Aberdeen Children of the
1950s cohort study. Br J Psychiatry 2005;187:21-8.
We thank Dr. Rafnsson[1] for valuable comments on our paper.[2]
Rafnsson finds our policy implications surprising. In the light of present
evidence, we do not find further measures justified for reducing radiation
exposure among cabin crew. The justification for this view is the fact
that exposure limits common for all radiation workers, also apply for the
cabin crew. Dose monitoring indicates that the...
We thank Dr. Rafnsson[1] for valuable comments on our paper.[2]
Rafnsson finds our policy implications surprising. In the light of present
evidence, we do not find further measures justified for reducing radiation
exposure among cabin crew. The justification for this view is the fact
that exposure limits common for all radiation workers, also apply for the
cabin crew. Dose monitoring indicates that the cosmic radiation doses are
within the exposure limits. We see no reason to depart from the general
radiation protection principles.
Cohort studies have shown an excess risk of breast cancer in cabin
crew, in particular among those with long employment, with 1.5 – 3.4 fold
incidence compared with the general population.[e.g. 3] Nevertheless, the
radiation doses received are low and the expected effect based on previous
literature is very small, with RR well below 1.1.[4] Neither previous
studies nor our study have been able to identify the cause for the excess
incidence of breast cancer. Lack of association in our study does not
exclude the contribution of cosmic radiation in the development of breast
cancer, but it implies that other risk factors are likely to have a
greater role.
Rafnsson[1] finds our approach to occupational radiation dose
estimation crude. For cabin attendants, the only available source of
information on the number of flights during their career is the cabin
attendants themselves and thus, the questionnaire approach in exposure
assessment was appropriate. We used self-reported numbers of flights by
route and calendar period. We feel this is an improvement compared to
previous studies[e.g. 5-7], none of which have had any estimates of the
individual cosmic radiation dose. They have been based on surrogate
indicators such as length of employment or flight route type assignment.
Rafnsson[1] claims that we did not consider the possibility that
breast cancer can influence the subjects’ answers. Recall bias is
intrinsic in all case-control studies with subjects as source of
information and the issue was discussed in our paper.[2]
We excluded cabin attendants who worked for less than two years
because they had negligible exposure (due to very short period of cabin
work). In addition, several studies have shown that short-term employees
differ in terms of mortality and cancer risk from those with more stable
employment. Therefore, they are commonly excluded from occupational cohort
studies to avoid bias.
Our study has shortcomings inherent to retrospective case-control
study and to sparse data. Therefore, it cannot provide conclusive evidence
but does nevertheless supply new information. A large prospective follow-
up study with a large data set would be valuable. Currently, a
retrospective study, combining all the Nordic cabin crew cohorts with
comprehensive cancer incidence registration systems and improved dose
estimation algorithm is ongoing, and may be able to provide further
insight to the issue.
References
1. Rafnsson V. Retrospective assessment of exposure. Occup Environ
Med, electronic letter 25 Jul 2005.
2. Kojo K, Pukkala E, Auvinen A. Breast cancer risk among Finnish
cabin attendants: a nested case control study. Occup Environ Med
2005;62:488-493.
3. Pukkala E, Auvinen A, Wahlberg G. Incidence of cancer among
Finnish airline cabin attendants. BMJ 1995;311:649-652.
4. Boice JD Jr, Blettner M, Auvinen A. Epidemiologic studies of
pilots and aircrew. Health Phys 2000;79:576-684.
5. Haldorsen T, Reitan JB, Tveten U. Cancer incidence among Norwegian
airline cabin attendants. Int J Epidemiol 2001;30:825-830.
6. Rafnsson V, Sulem P, Tulinius H, et al. Breast cancer risk in
airline cabin attendants: a nested case-control study in Iceland. Occup
Environ Med 2003;60:807-809.
7. Reynolds P, Cone J, Layefsky M, et al. Cancer incidence in
California flight attendants (United States). Cancer Causes Control
2002;13:317-324.
In an interesting study published in the September 2005 issue of
Occupational and Environmental Medicine, Simoni and collegues reported the
relation between mould and/or dampness exposure and respiratory disorders
in children and adolescents in Italy [1]. The authors concluded that
wheeze and asthma can often be explained by exposure to home mould and
dampness, particularly in early life.
In an interesting study published in the September 2005 issue of
Occupational and Environmental Medicine, Simoni and collegues reported the
relation between mould and/or dampness exposure and respiratory disorders
in children and adolescents in Italy [1]. The authors concluded that
wheeze and asthma can often be explained by exposure to home mould and
dampness, particularly in early life.
Although the authors acknowledged the use of questionnaire data alone
to assess mould and dampness exposure will have limited their study, they
state that the validity of using questionnaires has been established.
In our own study, we investigated indoor exposure to dampness in 200
asthmatic and non-asthmatic children aged 4-17 [2]. We found that self-
reported dampness (by the parent/guardian) was significantly associated
with an asthmatic household, but no such association was found for
dampness observed by the field investigator or objective measures (using
an industrial dampmeter). Additionally, we have previously demonstrated
that the concordance between self-reported dampness and objective measures
is very poor [3]. In fact, there was almost complete disagreement between
self-reported dampness, visual inspection by a trained investigator and
measurement using an industrial dampmeter.
A study of the validity and determinants of reported home dampness
and moulds conducted by Dales et al reported evidence of systematic
reporting bias and recommended that objective measures rather than
questionnaires be used to clarify the health effects of indoor fungi [4].
Bearing in mind the evidence from these past studies, we feel that
the positive findings of Simoni et al should be interpreted with caution
and that all research involving home dampness should have some objective
data to back it up.
References
1) Simoni M, Lombardi E, Berti G et al. Mould/dampness exposure at
home is associated with respiratory disorders in Italian children and
adolescents: the SIDRIA-2 study. Occup Environ Med 2005; 62: 616-622.
2) Tavernier GO, Fletcher GD, Francis HC, Oldham LA, Fletcher AM,
Blacklock G, Stewart L, Gee I, Watson A, Frank TL, Frank P, Pickering CA,
Niven RM. Endotoxin exposure in asthmatic children and matched healthy
controls: results of IPEADAM study. Indoor Air. 2005;15 Suppl 10:25-32.
3) Frank TI, Pickering CAC, Fletcher G, Francis HC, Oldham LA, Kay
S, Frank P, Niven RMcL. (1999). Relationship between self reporting,
visible inspection and objective measurement of damp for determining damp
or mould contamination in houses. Proceedings of the 8th Internationional
Conference on Indoor Air Quality and Climate-Indoor Air '99, Vol. 2, pp564
-566.
4) Dales RE, Miller D and McMullen ED. Indoor air quality and
health: validity and determinants of reported home dampness and moulds.
International Journal of Epidemiology 1997; 26: 120-125.
Dear Editor,
The review of occupational asthma by Nicholson et al. [1] is comprehensive. It is an important report that is likely to be widely read: the evidence review of low back pain has, for example, been one of the most commonly downloaded articles [2]. The appearance in the principal recommendations of the authors’ unsubstantiated opinions is, therefore, concerning.
The authors also include issues...
Dear Editor,
The paper by Nicholson et al. provides useful evidence-based guidelines for prevention, identification, and management of occupational asthma (OA) based on a comprehensive review of the literature.[1]
Evidence statements 6 and 7 list the workers most commonly reported to OA surveillance schemes or reported from population studies to be at increased risk of developing asthma. Hairdressers are n...
Dear Editor,
Firstly, latent period always refers to the period between the point of the time when disease occurs and point of the time when the disease is detected, while tumour induction time refers to the period between the point of the time when the component cause (can be an exposure) is satisfied and the point of the time when the disease is occurred.[1] Thus only under the extreme condition that one secon...
Dear Editor,
Sorahan et al. [1] recently published the results of a cohort mortality and morbidity study of workers purportedly exposed to benzene in the UK. Despite inherent problems with their data analyses, the authors nevertheless concluded that “the study does not support claims that exposure to benzene affects risks for lymphohaematopoietic malignancies other than ANLL.” In my opinion, the discrepancies and...
Dear Editor,
Kyle Steenland raises some interesting points in his commentary on silica [1] both on our papers reporting exposure assessment and mortality in the UK silica sand industry [2,3] and on the adverse effects of silica in general.
With the exception of one quarry, where other exposures such as polycyclic aromatic hydrocarbons could have occurred, no relationship was found with cumulative silica ex...
Dear Editor,
The report on the occupational health risks of ethylene glycol ethers is convincing while using the time-honoured indicators of female reproductive health.[1]
The effects often coincide with or depend on nervous system toxicity of e.g. solvents. The toxic effect of the ethylene glycol ethers seems to stem from their end metabolites, the corresponding alkoxyacetic acids. They seem to be inhibit...
Dear Editor,
Kojo et al. [1] report their results on breast cancer risk among airline cabin attendants in a nested case-control study. Increased incidence of breast cancer has been repeatedly found among Finnish and other airline cabin attendants and that is the motive of the study. The results do not support the hypothesis that cosmic radiation exposure as measured in the study is strongly linked to the inductio...
Dear Editor,
We read with great interest the article by Mannes et al., which related the adverse effects of ambient air pollution on birth weight.[1] That article well described the effects of pollutant exposure on the risk of low birth weight using a marker of small for gestational age (SGA). However, that study presents some shortcomings.
First, gestational week at birth is obstetrically and sociall...
Dear Editor,
We thank Dr. Rafnsson[1] for valuable comments on our paper.[2] Rafnsson finds our policy implications surprising. In the light of present evidence, we do not find further measures justified for reducing radiation exposure among cabin crew. The justification for this view is the fact that exposure limits common for all radiation workers, also apply for the cabin crew. Dose monitoring indicates that the...
Dear Editor
In an interesting study published in the September 2005 issue of Occupational and Environmental Medicine, Simoni and collegues reported the relation between mould and/or dampness exposure and respiratory disorders in children and adolescents in Italy [1]. The authors concluded that wheeze and asthma can often be explained by exposure to home mould and dampness, particularly in early life.
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