Occup. Med. Vol. 49, No. 2, pp. 71-77, 1999
Copyright O 1999 Upplncott Williams & Wllklns for SOM
Printed In Great Britain. All rights reserved
0962-7480/99
Lymphopoietic cancer and other
major causes of death among
petrochemical researchers: an
update
R. J. Lewis,* C. M. Yarborough,* M. J. Nicolich* and
B. R. Friedlander*
*Exxon Biomedical Sciences, Inc., East Millstone, NJ, 08875-2350,
USA; +Caterpillar, Inc., Peoria, IL 61629-1410, USA
This study updates lymphopoietic cancer (LHC) mortality statistics and other major
causes of death through 1992 for 13,188 petrochemical researchers employed
between 1964 and 1986. Significant deficits of deaths were observed for all causes,
all cancers, ischaemic heart disease and all external causes. The subcategory of 'all
other LHC was elevated among males in an exposure class containing scientists and
engineers. This finding was statistically significant based on national but not state
comparison rates. Poisson regression analyses showed that increasing exposure
classes were not associated with LHC, but a relationship was noted for total years
worked. A non-significant increase in breast cancer among females was also
observed but was concentrated among the lowest exposure class. This study and
other similar investigations suggest various subcategories of LHC deaths are
marginally elevated among chemical researchers and engineers. Evidence for a workrelated LHC hazard for this population, however, has not been identified.
Key words: Laboratory workers; lymphopoietic cancer; mortality; occupational
epidemiology; petrochemical researchers.
Occup. Med. Vol. 49, 71-77, 1999
Received 1998; accepted in final form 4 August 1998
INTRODUCTION
Epidemiologic studies of chemical researchers and
engineers have reported on excess deaths due to various
lymphopoietic-related cancers. l~7 The mortality experience of 13,188 New Jersey (USA)-based employees of a
large petrochemical research and development (R&D)
company has been examined in two previous investigations.8'9 These studies indicate a favourable overall employee mortality profile compared with the general New
Jersey population. However, elevated mortality from all
lymphopoietic cancers (LHC) combined was observed
among scientists and engineers as compared with managers and support staff.9 Mortality due to all LHC combined was also elevated among research technicians as
compared with managers and support staff. A casecontrol study of LHC nested within this cohort did not
find any relationship between LHC-related causes and
Correspondence and reprint requests to: R. J. Lewis, Staff Epidemiologist,
Exxon Biomedical Sciences, Inc., Mettlers Road, CN 2350, Ea3t Millstone,
NJ 06875-2350, USA. Tel: (+1) 732 873-6284; Fax: (+1) 732 873-6009;
email: rjlewis©fpe.erenj.com
qualitative exposure scores for benzene, other solvents
and ionizing radiation.10
As the aetiology of increased LHC deaths among certain subgroups of workers remains unknown for this
petrochemical R&D worker cohort, we updated mortality through 1992 to continue monitoring LHC mortality
trends and other major causes of death. In addition, we
repeated the Poisson regression analyses of LHC deaths
using exposure measures developed in the previous
update,9 but with a larger number of cases and a minimum latency requirement. Finally, we analysed pre-1964
retirees separately to evaluate more historical exposures
separately, and to reduce the potential bias associated
with including only longer surviving retirees.
MATERIALS AND METHODS
Identification and definition of the study cohort is
described elsewhere.9 Briefly, the cohort consisted of all
New Jersey-based employees of a large petrochemical
R&D company who worked one day or more during the
period 1 January 1964 to 31 December 1986, as well as
72 Occup. Mod. Vbl. 49, 1999
retired employees alive on 1 January 1964. Employees
hired after 1986 were not included in this investigation
because the latency period for these individuals is limited, and the focus of this investigation was on LHC and
other chronic diseases. After selection based upon these
criteria, 13,188 workers were available for analysis.
The previous update9 determined vital status through
31 December 1986. Vital status for the update period
(1/1/87 to 31/12/92) was determined using the National
Death Index (NDI). Death certificates for deceased
employees identified in the update period were coded to
the Ninth Revision of the International Classification of
Diseases (ICD 9) by a trained nosologist. Since the 7th,
8th and 9th revision ICD codes do not alone identify all
mesothelioma deaths, death certificates were reviewed by
the nosologist for mention of this tumour.
The job exposure class categories used in the previous
update9 were used in this investigation. The job exposure
class category is a measure of the average per cent of
working time that workers would have potentially been
exposed to chemical or physical agents {e.g., organic and
halogenated solvents and radiation). A total of four job
exposure classes were created from these rankings: class
1 (e.g., research technician — exposed 60% to 100% of
the time — the highest exposure category); class 2 (e.g.,
mechanics — exposed 20% to 59% of the time); class 3
(e.g., scientists, engineers — exposed 5% to 19% of the
time) and class 4 (e.g., managers and support staff —
the lowest exposed group — exposed less than 5% of the
time). As in the previous update,9 an employee's job title
for which there was the highest potential for exposure
was used to assign a single job exposure class to each
worker.
The mortality experience of the cohort was compared
to the general United States and New Jersey (NJ) population using a modified life-table analysis provided by
the Occupational Cohort Mortality Analysis Program
(OCMAP).11 Standardized mortality ratios (SMR) and
95 per cent confidence intervals (CI) were calculated
only when the observed number of deaths was five or
more. The CIs for the SMRs were computed under the
assumption that the observed number of deaths followed
a Poisson distribution.
SMRs were calculated separately for active workers
(i.e., employed 01/01 /64-31/12/86) and retired employees
(i.e., retired and alive as of 01/01/64). As the SMRs
based on national and state rates were similar, only SMRs
based on state comparison rates are presented, unless
otherwise noted. In addition, we repeated the Poisson
regression analyses performed in the previous update9 to
examine LHC deaths in relation to job exposure class.
Job exposure class 4 (e.g., managers and support staff)
served as the referent group, and the potential confounders of age, years employed, gender and smoking were
included in all models. Because the purpose of this
analysis was to conduct a more rigorous examination
of potential aetiologic factors associated with LHC, we
used a 10-year latency period in all analyses (i.e., the first
10 years of time-at-risk beginning after the date of hire
were not considered). Analyses were conducted for all
LHC deaths combined, leukaemia and the category of
'all other LHC.
RESULTS
Description of the cohort
Demographic and employment characteristics of the
cohort are presented in Table 1. The majority of the
cohort comprised active male employees (i.e., employed
1/1/64-31/12/86). Average duration of employment
ranged from 6.5 years among active females to 23.2
years among retired employees. Smoking data were unavailable for retirees and for approximately 35% of the
active workers.
SMRs for active employees
Statistically significant deficits of deaths were observed
for all causes, all malignant neoplasms and several other
major causes of death among active males (see Table 2).
There was a slight deficit of deaths due to all LHC combined, while leukaemia and mortality from 'all other
L H C was only slightly increased. There were five deaths
due to mesothelioma among active males (data not
shown). An approximation of the SMR for mesothelioma using incidence rates from the Surveillance,
Table 1. Demographic, employment and other characteristics of active and retired employees, 1964-92
Characteristics
Job exposure class
Class 1 (e.g., research technicians)
Class 2 (e.g., mechanics)
Class 3 (e.g., scientists and engineers)
Class 4 (e.g., managers and support staff)
Race
White
Non-White
Mean length of employment (years)
Number of deaths
Unknown smoking status
•" Smoking data unavailable.
-Active males
Active females
Retired employees
No. (% of total)
No. (% of total)
No. (% of total)
1,927
1,451
4,127
1,490
(21.4)
(16.1)
(45.9)
(16.6)
462 (11.9)
146 (3.8)
285 (7.4)
2,977 (76.9)
57
80
80
106
8,333 (92.6)
662 (7.4)
12.4
806 (9.0)
2,862 (31.8)
3,465 (89.5)
405 (10.5)
6.5
94 (2.4)
1,363 (35.2)
320 (99.1)
3 (0.9)
23.2
295 (91.3)
(17.6)
(24.8)
(24.8)
(32.8)
R. J. Lewis ef a!.: Petrochemical researcher mortality update 73
Table 2. Observed and expected" deaths, standardized mortality ratios (SMR), and 95% confidence intervals (CO for active employees (all
races) by gender, based on state comparison rates, 1964-92
Males
Cause of death
Obs/Exp'
All causes
All malignant neoplasms
Digestive organs and peritoneum
Bronchus, trachea, lung
Breast
Prostate
Central nervous system
All lymphopoietic cancer
Lymphosarcoma, reticulosarcoma
Hodgkin's disease
Leukaemia, aleukaemia
All other lymphopoietic
Ischaemic heart disease
All external causes of death
Suicides
806/1,465.7
225/371.3
54/100.7
60/123.2
0/0.6
22/25.2
8/10.4
33/36.2
2/4.6
1/3.5
14/13.5
16/14.6
244/454.6
59/121.5
21/27.1
SMR
55b
61 b
54 b
49"
88
77
91
103
110
54 b
49 b
78
Females
Cl
Obs/Exp*
SMR
Cl
51-59
53-69
40-70
37-63
55-132
33-151
63-128
57-174
63-178
47-61
37-63
48-119
94/145.7
33/51.0
3/9.2
1/8.8
16/13.5
65 b
65 b
119
52-79
45-91
68-193
1/1.5
2/4.8
0/0.5
0/0.7
0/1.8
2/1.7
7/21.5
13/14.1
5/2.7
33 b
92
184
13-67
49-157
60-429
• Obs/Exp = observed/expected deaths, where expected deaths are based on New Jersey mortality rates, males and females, all races combined.
b
9 5 % Cl does not Include 100.
Epidemiology and End Results (SEER) program12 to
estimate expected deaths (invasive mesothelioma rates,
1973-1991) showed a twofold, non-significant elevation
(five observed, SMR = 228; Cl = 74-532).
Among active females, there was a statistically significant deficit of deaths due to all causes and all malignant
neoplasms (see Table 2). While small numbers limited
the assessment of most specific causes of death, a nonsignificant increase in breast cancer was observed. A
detailed analysis of breast cancer deaths showed most
decedents were employed in the lowest exposed job
exposure class [class 4, (n = 11); class 1, (n = 2); class
2, (n = 1); class 3, (« = 2)]. Further, breast cancer
mortality rates were similar among women with 15 or
more years employment (six observed; SMR = 115; Cl
= 42-250) vs. women with less than 15 years employment (10 observed; SMR = 121; Cl = 58-223). Finally,
deaths from suicides were non-significantly elevated (see
Table 2), although the SMR based on US rates for this
cause of death indicated a deficit (SMR = 96; Cl =
31-223). No deaths due to mesothelioma were observed
among the women studied.
categories except suicides, where a non-significant excess
was observed (see Table 4); suicides were not elevated
as compared with national rates (SMR = 91; Cl =
42-173). Workers in job exposure class 2 also had deficits
of deaths for all causes examined, with the exception of
prostate cancer, which was consistent with expected
rates (see Table 4).
Deficits were observed for most causes of death
among workers in job exposure class 3, although deaths
due to all LHC combined were non-significantly elevated
due primarily to increased deaths from 'all other L H C
(See Table 4). The category of 'all other LHC includes
certain lymphomas (largely non-Hodgkin's lymphoma)
and multiple myeloma. The SMR for the 'all other L H C
disease rubric was statistically significantly elevated
based on national (SMR = 204; Cl = 102-364) but not
Table 3. Observed and expected" deaths, standardized mortality
ratios (SMR), and 95% confidence intervals (Cl) for retired
employees (all races and gender), based on state comparison
rates, 1964-92
Obs/Expa
SMR
Cl
All causes
295/346.7
48/65.7
All malignant neoplasms
Digestive organs and peritoneum
16/21.6
Bronchus, trachea, lung
10/16.0
Breast
4/1.1
Prostate
10/9.0
0/0.7
Central nervous system
All lymphopoietic cancer
5/4.9
Lymphosarcoma, reticulosarcoma
0/0.8
Hodgkin's disease
1/0.2
Leukaemia, aleukaemia
3/2.3
All other lymphopoietic
1/1.7
Ischaemic heart disease
135/145.6
All external causes of death
8/7.4
Suicides
1/1.2
85 b
73 b
74
63
112
_
102
93
109
-
76-95
54-97
42-121
30-115
54-205
33-239
78-110
47-214
-
Cause of death
Retirees
Retirees had a statistically significant deficit of deaths
from all causes and all malignant neoplasms (see Table
3). Prostate cancer and external causes of death were
slightly, non-significantly elevated while mortality rates
from all LHCs combined were consistent with those
expected. There was one death due to mesothelioma
among the retired employees.
SMRs by job exposure class
Analyses by job exposure class were performed only for
active males given the limited number of retirees and
active females in each job exposure class. Employees in
job exposure class 1 had deficits of deaths for all disease
• Obs/Exp = observed/expected deaths, where expected deaths are based
on New Jersey mortality rates, all races and genders combined.
b
95% Cl does not include 100.
74 Occup. Med. Vol. 49, 1999
Table 4. Observed and expected" deaths, standardized mortality ratios (SMR), and 95% confidence intervals (CO for active males (all races) by
job exposure class, based on state comparison rates, 1964-92
Cause of death
Job exposure class 1
Job exposure class 2
Job exposure class 3
Job exposure class 4
fe.g, research technicians)
(e.g, mechanics)
(e.g., scientists, engineers)
fe.g, managers, support staff)
Obs/Exp*
264/389.1
All causes
All malignant neoplasms
67/99.1
15/27.4
Digestive organs and peritoneum
22/32.9
Bronchus, trachea, lung
Prostate
4/7.5
2/2.5
Central nervous system
7/9.1
All lymphopoietic cancer
Lymphosarcoma, reticulosarcoma
0/1.2
0/0.8
Hodgkln's disease
Leukaemia, aleukaemla
4/3.5
All other lymphopoietic
3/3.7
Ischaemic heart disease
98/126.2
17/26.7
All external causes of death
Suicides
9/5.9
SMR
b
68
68b
55b
67
77
78b
64
154
CI
Obs/Exp° SMR
60-77 206/333.0 62b
52-86
54/84.6 64 b
31-90
16/23.7 68
42-101
20/27.9 72
7/6.9 101
1/2.0
31-158
4/7.5 0/1.0
0/0.6
4/2.8 0/3.1
63-95
65/111.1 59"
37-102
13/19.1 68
70-291
3/4.2
-
C/
Obs/Exp* SMfl
54-71 233/559.5 42 b
48-83
73/141.8 52 b
39-110
18/37.4 48 b
44-111
12/475 25b
41-209
6/7.9 76
4/4.5
20/14.8 135
2/1.8
1/1.6
6/5.5 110
11/5.9 185
45-75
55/163.0 34b
36-117
19/56.7 34 b
6/12.8 47
C/
Obs/Exp'
SMfl
37-47
40-65
29-76
13-44
28-166
103/184.1
31/45.8
5/125
6/15.1
5/2.8
1/1.4
2/4.8
0/0.6
0/0.5
0/1.8
2/1.9
26/54.3
10/18.9
3/4.2
56»
68 b
-
82-208
-
40-239
93-332
25-44
20-52
17-102
41 b
40"
177
48b
53b
-
CI
46-68
46-96
13-96
15-87
57-412
31-70
25-97
-
* Obs/Exp » observed/expected deaths, where expected deaths are based on New Jersey mortality rates, males, all races combined.
b
95% CI does not include 100.
state comparison rates (see Table 4). Analyses by duration of employment for the 'all other LHC' category
indicated higher SMRs among those with less than 15
years employment in job exposure class 3 (six observed;
SMR = 236; CI = 87-515) compared with persons
having worked 15 years or more in this job exposure
class (five observed; SMR = 148; CI = 48-344). Finally,
workers in job exposure class 4 experienced fewer deaths
than expected for all major causes except prostate cancer,
which was non-significandy elevated.
SMRs by time period
Because the previous update study9 covered the time
period 1964-86, a separate analysis of LHC mortality
(the cause of death of a priori interest) during only the
update period 1987-92 was performed.
Table 5 presents LHC-related mortality during die
original and updated study periods for all active males
combined and for only active males employed in job
exposure class 3. For all active males combined, the
most notable rinding was a non-significant increase in
leukaemia mortality during the update period. Analyses
of only active males employed in job exposure class 3
were generally unremarkable, with die exception of an
apparent decrease in mortality for 'all odier LHCs' in die
update period to expected levels; however, these results
were based on small numbers.
Poisson regression analysis
Table 6 presents findings from the Poisson regression
analysis. After adjustment for age, gender, smoking
status and years worked, employment in job exposure
classes 1 and 2 was associated wiuh an approximately
twofold, non-significant risk of all LHC combined. The
largest risk was observed for persons employed in job
exposure class 3 — the lowest non-baseline exposure
class. There was also a statistically significant fivefold
increased risk associated widi working 15 or more years,
relative to persons with less than 15 years employment
Analyses of leukaemia deaths suggested an increased
risk among persons in job exposure class 2, altiiough this
finding was not statistically significant. Smokers, persons
of unknown smoking status, and those employed 15
or more years had non-significant increased risks of
Table 5. Observed and expected" deaths, standardized mortality ratios (SMR), and 95% confidence intervals (CI) for all active males combined
and active males employed in job exposure class 3, by time period
All active males
Cause of death
Original study period
(1964S6)
Obs/Exp'
All lymphopoietic cancer
Lymphosarcoma, reticulosarcoma
Hodgkin's disease
Leukaemia, aleukaemia
All other lymphopoietic
19/22.5
2/4.0
1/2.7
5/8.6
11/7.2
CI
84 51-132
58 19-136
152 76-272
Active males employed In job exposure class 3
Update study period
(1987-92)
Obs/Exp' SMft
14/13.7
0/0.6
0/0.7
9/5.0
5/7.4
CI
102 56-172
181 83-344
68 22-158
Original study period
(1964-86)
Obs/Exp' SMR
14/8.9
2/1.6
1/1.3
4/3.4
7/2.7
CI
156
86-263
256 b 103-528
Update study period
(1987-92)
Obs/Exp"
SMR
CI
6/5.9
0/0.3
0/0.3
2/2.1
4/3.2
102
_
37-222
_
_
• Obs/Exp •> observed/expected deaths, where expected deaths are based on New Jersey mortality rates, males, all races combined.
6
95% CI does not Include 100.
R. J. Lewis et a/.: Petrochemical researcher mortality update 75
Table 6. Lymphopoietic cancer mortality — assuming a 10-year latency — by job exposure class, smoking status and years worked using
Polsson regression analysis
Variable
Job exposure class1
Job exposure class 1
Job exposure class 2
Job exposure class 3
Smoking status2
Smoker
Ex-Smoker
Unknown smoking status
Years worked 3
15 or more years employed
All lymphopoietic cancers combined
Leukaemla/aleukaemia
All other lymphopoietic cancers
Rate ratio" (95% Cl)
Rate ratio' (95% Cl)
Rate ratio' (95% CO
1.93
1.62
2.55
(0.41-9.10)
(0.29-8.95)
(0.59-11.12)
1.47 (0.36-6.03)
2.12 (0.51-8.82)
1.03 (0.16-6.90)
2.19
1.05
2.28
(0.80-5.99)
(0.29-3.79)
(0.75-6.88)
1.76 (0.42-7.46)
0.50 (0.05-4.83)
1.96 (0.39-9.90)
1.26 (0.25-6.37)
1.77 (0.35-9.08)
2.80 (0.61-12.84)
3.55 (0.38-33.62)
3.41 (0.60-19.28)
5.04b (1.33-19.05)
No deaths
1.59 (0.28-8.92)
• AD rate rattos are adjusted for the other variables noted in the table as well as age and gender (except leukaemia analyses which were limited to males due to
the lack of female leukaemia deaths).
b
95% confidence Interval does not Include 1.00.
0
Reference group was job exposure class 3 due to the lack of leukaemia deaths among subjects in Job exposure class 4.
Reference groups: 1 Job Exposure Class 4; 2Never smoked; 3Less than 15 years employment duration.
leukaemia. Finally, risk for 'all other LHC was not
increased among persons employed in the highest job
exposure class (class 1). Risk was elevated among persons employed in job exposure class 3, but this finding
was not statistically significant. There were no deaths in
the subcategory 'all other LHC among persons employed in job exposure class 2.
DISCUSSION
This investigation provides an additional six years of
mortality rates follow-up to an earlier study9 of petrochemical R&D workers. Overall, the results suggest a
favourable mortality profile, with deficits being observed
for all causes, all malignant neoplasms, ischaemic heart
disease and external causes of death. These findings are
likely the result of the Healthy Worker Effect13 and the
relatively high socioeconomic status of the workers, as
there are no obvious methodological flaws that might
explain the low SMRs.
Slight to moderate increases in deaths due to suicides
(primarily among women) and prostate cancer were observed, but thesefindingswere not statistically significant,
generally inconsistent across comparison populations
(national vs. state) and limited to certain subgroups of
workers. Several previous studies of chemical researchers
and engineers have observed increased deaths for
suicides5'14 and prostate cancer2'6'15 but the findings
have generally been inconsistent, based on small numbers, and/or not statistically significant. For prostate
cancer, it is notable that the largest SMR was observed
in the least exposed job exposure class, which suggests
non-occupational factors may be responsible for this
excess.
We identified six mesothelioma deaths among males in
the cohort, and these decedents had relatively long
latency and durations of employment (35-40 years) and
were hired in earlier time periods (between 1937 and
1956). The approximated SMR for mesothelioma
among active males was non-significantly elevated.
A more detailed analysis of a subset of these deaths is
presented elsewhere.16
We also found a slight, non-significant increase in
deaths due to breast cancer among active females. This
cause of death was also elevated in the previous study
(SMR = 1.37; Cl = 71-240). 9 The number of breast
cancer deaths among retired women was too small for
meaningful analyses (four observed; 0.9 expected). Most
importantly, there was no trend of increasing SMRs with
increasing duration of employment, and most deaths
occurred in the least exposed group (i.e., job exposure
class 4). Few previous studies of female R&D workers
have been conducted to date, although Walrath et cd.5 and
Li et al.3 reported elevated breast cancer rates among
female chemists. A recent review of breast cancer among
working women observed consistent increases in breast
cancer deaths among clerical and professional women.17
The reviewers suggest that these excesses may be due to
uncontrolled confounding given the lack of identifiable
exposures. One possible confounder in this cohort may
be delayed childbearing among northeastern females,18
although other factors such as diet, alcohol intake, and
heredity are also possible confounders. In general, there
is little evidence to suggest that the increased breast
cancer deaths observed in this study were related to
employment.
The main finding of this study is a twofold increase in
deaths for the subcategory 'all other L H C among active
males in job exposure class 3 (e.g., scientists and engineers). The earlier study9 also found the subcategory
'all other L H C to be elevated among employees in job
exposure class 3 (SMR = 246; Cl = 98-507). Increased deaths due to 'all other L H C have frequently
been observed in other petroleum cohorts,19 although
recent analysis shows that multiple myeloma (a part of
the 'other L H C category) is not elevated in over 200,000
petroleum workers.20
Poisson regression analyses showed a statistically significant relationship between all LHC combined and
working 15 years or more, assuming a 10-year latency
period. This finding, when combined with the literature
76 Occup. Med. VDI. 49, 1999
reporting excess LHC-related deaths among chemical
researchers and engineers 1 " 7 could be viewed as suggestive of a possible work-related hazard. However,
several factors argue against an occupational relationship
for the subcategory 'all other LHC'. First, analyses among
employees in job exposure class 3 did not indicate a trend
of increasing SMRs with increasing duration of employment. Second, there were no excess deaths due to any
LHC-related cause of death among retirees who had
long latencies and durations of employment, although
this was a small cohort. Third, the disease category 'all
other L H C was not increased in the update period.
Fourth, Poisson regression analyses did not indicate a
relationship of increasing risk with increasing job exposure class for any LHC-related cause of death. In fact,
a 'reverse' exposure-response trend was generally observed, with higher risks tending to occur in job exposure classes with lower estimated exposures (e.g., job
exposure class 3). Finally, the two previous studies of
chemical researchers and engineers that examined risk
by duration of employment have not found a trend of
increasing LHC-related mortality with increasing duration of employment. 6 ' 21
Our study has several limitations, which must be considered in interpreting the LHC findings described above.
First, the job exposure class categories are relatively nonspecific; they are based on the employee's job with the
highest level of exposure, and they do not provide a
measure of exposure to specific chemical and physical
agents. With regard to the assignment of job exposure
class based on highest exposure class, this may be problematic for individuals who held jobs in different job
exposure class categories. However, individuals whose
original job tides were in job exposure classes 1, 2 or 4
generally kept job titles in the same class. In other words,
relatively few research technicians and mechanics (jobs
typically included in job exposure classes 1 and 2,
respectively) became scientists and engineers Gob exposure class 3) or administrators Gob exposure class 4).
Conversely, it was common for professionals in job
exposure class 3 to move into administrative positions
Gob exposure class 4). Thus, it is unlikely that the
method of assigning workers to job exposure classes
biased the findings to a large degree.
An additional limitation is the small number of deaths
in the specific LHC subcategories assessed in the
Poisson regression analyses. Finally, this study could not
discern the specific chemical/physical agents and/or lifestyle factors that may be associated with excess deaths
due to 'all other L H C . Non-occupational factors that
may play a role in the development of non-Hodgkin's
lymphoma include familial factors, immunodeficiency
syndromes, immunosuppressing agents, 22 viruses, hair
dyes 23 and exposure to pesticides.24 Immunodeficiency
syndromes are unlikely to explain the 'all other L H C
excess given that a review of death certificates for all
LHC-related deaths indicated no mention of AIDS or
AIDS-related conditions. Unfortunately, we did not have
information on patterns of exposure or lifestyle factors.
With regard to possible occupational exposures, a
case-control study of LHC deaths 10 nested within the
previous cohort 9 suggested no relationship between work
place exposures to benzene, ionizing radiation, other solvents, cigarette smoking and diagnostic x-rays and LHC.
However, the study was based on a relatively small number of LHC cases (n = 24), and only qualitative judgements of exposure were available. As such, the cause of
the marginal excess of deaths in the subcategory 'all
other L H C currently remains unknown. Further studies
of research workers are needed to clarify the aetiology of
excess LHC-related deaths in this and other chemical
researcher/engineer cohorts.
SUMMARY AND CONCLUSIONS
The overall mortality profile of employees in this cohort
was favourable. The main finding was a marginal
increase in deaths due to 'all other L H C among active
males from an exposure group containing scientists and
engineers (e.g., job exposure class 3). A slight, nonsignificant increase in female breast cancer deaths
showed no relationship with duration of employment,
and the deaths occurred primarily among women in the
lowest job exposure class. Based on the literature and this
study's findings, evidence for a work-related LHC
hazard for this population has not been identified.
ACKNOWLEDGEMENTS
The authors wish to acknowledge the computer programming support provided by Celia A. Milano. We also
appreciate the data entry support and assistance in
manuscript preparation provided by Victoria Fowler.
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