the mortality of coke workers in britain

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American Journal of Industrial Medicine 4:691-704 (1983) The Mortality of Coke Workers in Britain J.F. Hurley, MA, R.MCL. Archibald, MB, ChB, DIH, FFOM, P.L. Collings, MSc, D.M. Fanning, OBE, MFOM, M. Jacobsen, PhD, and R.C. Steele, BSC Two studies of coke workers in Britain, comprising 6,767 men, gave similar results. The proportion of lung cancer deaths was about 20% higher than in manual workers generally. The excess occurred primarily among younger men. The ratio of lung cancers to all other cancers was also higher than expected, with limited data showing no evidence of ex- cessive tobacco consumption. Death rates from other causes were generally favorable. Overall the lung cancer death rates in oven workers were similar to those in non-oven men, but in both studies some indications of a job-specific excess were noted. These findings are compared with results from earlier studies in the United States and Canada where a much higher excess lung cancer mortality was found in oven men, particularly those with longer exposure times. We discuss possible reasons for the differences, and conclude that the results reported now contribute further evidence that exposure to coal carbonization fumes can cause lung cancer. Key words: occupation, lung cancer, proportional mortality, coke ovens, selection effects INTRODUCTION It is more than 200 years since an occurrence of scrota1 cancers in London chimney sweeps was reported [Pott, 17751, the first of many findings of occupational cancers involving the carbonization or distillation of bituminous coal. Lloyd [ 19711 provided a comprehensive review of these studies. Regarding coke workers in partic- ular, he noted that evidence of lung cancer mortality at that time was ambiguous. However, in the same paper he clearly showed an excess lung cancer mortality associated with work on the oven tops among coke plant employees at two large Institute of Occupational Medicine, Edinburgh (J.F.H., P.L.C., M.J., R.C.S.). National Coal Board, London (R.McL.A.). British Steel Corporation, Landon (D.M.F.). D.M. Fanning’s present address is Chloride Group, London. England. Address reprint requests to J F Hurley, Statistics Branch, Institute of Occupational Medicine, Roxburgh Place, Edinburgh EHS 9SU, Scotland. Accepted for publication April 11, 1983. 0 1983 Alan R. Liss, Inc.

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Page 1: The mortality of coke workers in Britain

American Journal of Industrial Medicine 4:691-704 (1983)

The Mortality of Coke Workers in Britain

J.F. Hurley, MA, R.MCL. Archibald, MB, ChB, DIH, FFOM, P.L. Collings, MSc, D.M. Fanning, OBE, MFOM, M. Jacobsen, PhD, and R.C. Steele, BSC

Two studies of coke workers in Britain, comprising 6,767 men, gave similar results. The proportion of lung cancer deaths was about 20% higher than in manual workers generally. The excess occurred primarily among younger men. The ratio of lung cancers to all other cancers was also higher than expected, with limited data showing no evidence of ex- cessive tobacco consumption. Death rates from other causes were generally favorable.

Overall the lung cancer death rates in oven workers were similar to those in non-oven men, but in both studies some indications of a job-specific excess were noted. These findings are compared with results from earlier studies in the United States and Canada where a much higher excess lung cancer mortality was found in oven men, particularly those with longer exposure times. We discuss possible reasons for the differences, and conclude that the results reported now contribute further evidence that exposure to coal carbonization fumes can cause lung cancer.

Key words: occupation, lung cancer, proportional mortality, coke ovens, selection effects

INTRODUCTION

It is more than 200 years since an occurrence of scrota1 cancers in London chimney sweeps was reported [Pott, 17751, the first of many findings of occupational cancers involving the carbonization or distillation of bituminous coal. Lloyd [ 19711 provided a comprehensive review of these studies. Regarding coke workers in partic- ular, he noted that evidence of lung cancer mortality at that time was ambiguous. However, in the same paper he clearly showed an excess lung cancer mortality associated with work on the oven tops among coke plant employees at two large

Institute of Occupational Medicine, Edinburgh (J.F.H., P.L.C., M.J., R.C.S.). National Coal Board, London (R.McL.A.). British Steel Corporation, Landon (D.M.F.). D.M. Fanning’s present address is Chloride Group, London. England. Address reprint requests to JF Hurley, Statistics Branch, Institute of Occupational Medicine, Roxburgh Place, Edinburgh EHS 9SU, Scotland. Accepted for publication April 11, 1983.

0 1983 Alan R. Liss, Inc.

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United States steel works. An extension of Lloyd’s work to other geographical regions of the United States and Canada confirmed this result [Redmond et al, 19721 and a high mortality from kidney cancers was also noted. Further analyses [Redmond et al, 19761 pointed to excess deaths from cancers of the digestive organs among non-oven men. These and other results were reviewed in the most recent report of these studies [Redmond et al, 19811.

There followed several reports of more limited studies by various workers [Sakabe et al, 1975; Radford, 1976; Manz, 1976; Okubo and Tsuchiya, 19781 giving results consistent with an excess of deaths from lung cancer, but not in themselves convincing evidence of an occupational hazard.

COKE WORKERS IN BRITAIN

With regard to conditions in Britain specifically, Reid and Buck [1956] had described mortality over a 6-year period among men employed in National Coal Board coke works. They found little evidence of excess mortality from cancers in general or from respiratory cancers in particular, but recommended a large-scale prospective investigation in view of methodological limitations of their study. More recently, Davies [1977] studied coke workers in two integrated steel works in South Wales, over an 11-year period. Death rates from cardiovascular illness were favora- ble. Mortality from other causes, including lung cancers, was not unusual. On the other hand national statistics referring to occupational mortality in England and Wales [Registrar General, 1957, 1971, 19781 have consistently shown an excess of lung cancers among those groups of occupations that included coke workers. Some limi- tations of official statistics on occupational mortality have been noted [Heasman et al, 1958; Alderson, 1972; Registrar General, 19781, but the most recent report on a prospective study of British gas workers [Doll et al, 19721 again indicated that exposure to products of coal carbonization can cause lung cancer.

It was against this background of suggestive but conflicting evidence of excess lung cancers among coke workers, dominated by the results from Lloyd and his colleagues, that about 10 years ago two large-scale studies of coke workers in Britain were begun. One was of men employed by the British Steel Corporation (BSC) and was part of a larger mortality study of steelworkers. The other included employees of National Smokeless Fuels Limited (NSF), a subsidiary of the National Coal Board. These two industries together employ almost all coke workers in Britain. As well as surveying mortality generally, the objective was to establish whether or not a specific association between lung cancer deaths and work on the coke ovens, as reported in the United States, was also evident in Britain and if so to estimate the magnitude of the risk.

This paper reports analyses of the mortality of BSC and NSF coke workers over 12- and 13-year follow-up periods, respectively.

POPULATIONS, MATERIALS, AND METHODS

separate administrative control initially, and so procedures were not identical. Although there was liaison between the two studies throughout, they were under

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Mortality of British Coke Workers 693

Study Definitions The BSC study was designed to include all male manual workers employed

continuously in coke works or departments from January 1, 1966 to July 31, 1967. Maintenance workers were excluded, because in general it was not possible to confirm their employment as workers in the coke departments as distinct from duties elsewhere in associated steel production complexes.

The NSF study was designed to include all male manual workers employed by the company on January 1, 1967. Most of the factories were clearly defined, separate units, so that it was operationally convenient, and relevant, to study all industrial workers, including maintenance men; and this was done.

Cohort Identification and Collection of Data During 1971 to 1973, staff at BSC establishments were asked to identify the

cohort, to collect the necessary occupational data, and (for those men still employed in the steel works at that time) to administer a questionnaire about smoking habits and respiratory symptoms. Different standard forms were used by staff at NSF works to identify the men, and to obtain information for follow-up and on work histories. No attempt was made to administer a questionnaire on respiratory symptoms or smoking habits. However, in a separate exercise, cross-sectional surveys of respiratory symp- tom prevalence were carried out during 1968 and 1973 at four of the NSF works, including information on smoking habits using methods described by Walker et a1 [1971].

Processing and editing of the NSF data during 1974, and the BSC information in 1976, revealed gaps in the number of men identified and inadequacies in some of the occupational histories provided. A second phase of data collection was carried out. Broadly similar procedures were now used in the two studies. Staff at the works were briefed at a series of special meetings. They used a variety of methods to supplement company records, including interviews with some men and with former colleagues of others no longer employed. Where appropriate, information recorded previously was checked and incorporated into the revised work histories. Close liaison was maintained between staff at the works and the main research workers. In this way, the major data problems were resolved, but there remained some variations in the completeness and reliability of the work histories recorded.

Follow-up Methods Arrangements were made with the Office of Population, Censuses and Surveys

for England and Wales, and with the Registrar General in Scotland, to follow up all men identified in the two studies. Men were classified as dead, alive, or untraced on the basis of the information thus provided. Primary cause of death, coded according to the eighth revision of the International Statistical Classification of Disease, Injuries and Causes of Death (ICD), was used in the analyses.

Classification of Occupational Histories Senior operations staff at the two companies agreed that the same classification

of work histories might be applied sensibly throughout, because the carbonization processes were essentially similar at all the works. All job names recorded, including

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local variations, were classified initially into one of five groups: coal handling, oven jobs (from service bunker to wharf,), screens, by-products, and other work. It became evident that jobs classified as “other” included some that involved oven work at least on an occasional basis. In the NSF study, jobs such as maintenance worker, general worker, and shift laborer were reallocated to a sixth, part oven, category, provided definite reference had been made to the oven as a place of work. In the BSC study a wider definition of “part oven” was used, with “other” work reserved for those jobs where it was clear that no oven work had been involved.

Further subdivision of the oven jobs into work on the oven tops or sides could not in general be done reliably, and this was not attempted.

Statistical Methods Comparisons with external reference populations were made by indirect age

standardization leading to SMRs and Relative SMRs [RSMR; Kupper et al, 19781, using year-by-year published data on age-specific mortality of men in Scotland, and in the appropriate standard administrative regions of England and Wales. Compari- sons with men in Social Classes mm, IV, and V (England and Wales) used death rates for the period 1970 to 1972 [Registrar General, 19781, which were published only for men less than 65 years old. A “man-years at risk” approach was used throughout. Mortality of occupational subgroups was investigated using both direct and indirect age standardization, with the study group as a whole as reference. The two approaches gave similar summaries of the data. For brevity, results using the indirect method only are given here.

Summary chi-squared tests of the statistical significance of an SMR, based on the likelihood ratio principle, were used throughout. These were analogous to the more conventional approaches that Breslow [ 19751 considered in the context of score tests. Likelihood ratio analogues of tests of homogeneity of an SMR for a particular cause over various groups of men [Peto and Pike, 1973; Beaumont and Breslow, 19821 were used similarly. In addition, the homogeneity tests were adapted to examine the equality, in a given group of men, of the SMR from various causes, ie, a test of the statistical significance of an RSMR.

Conventional methods for matched pairs were used in analyzing a case-control study among NSF workers. Advantages and drawbacks of using the case-control approach within a prospective study of occupational mortality have been discussed by, for instance, Liddell et a1 [ 19771 and Breslow and Patton [ 19791.

RESULTS General and Cause-specific Mortality

The 6,767 men studied were employed at 14 BSC works in England, Scotland, and Wales, and at 13 NSF works in England and Wales only. Losses to follow-up led to the exclusion of 159 men (2.3%) from the mortality analyses. Men aged 25 to 64 years when follow-up began were well represented in both studies (Table I).

Observed and expected deaths among the BSC and NSF workers, over 12- and 13-year follow-up periods, respectively, are shown in Table II. The 1,137 deaths observed in total were fewer than expected among men in Britain generally (SMR = 89). The overall mortality of BSC workers was more favorable than their NSF counterparts, but not significantly so statistically. The 167 deaths from lung cancer

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Mortality of British Coke Workers

TABLE I. Follow-UD Status* of All Men Studied. and Age Distribution** of Traced Men

695

Not Traced men Study 15-24 24-34 35-44 45-54 55-64 65+ All traced

BSC 58 400 778 866 632 19 2753 89 NSF 245 545 900 1120 1045 0 3855 70

All 303 945 1678 1986 1677 19 6608 159

*BSC: after 12 years; NSF: after 13 years. **At start of the follow-up periods (in years).

TABLE II. Observed and Expected Deaths Over 12 and 13 Years, Respectively, Among Traced Men in the BSC and NSF Studies; Summary Indices for the Studies Combined

Number of deaths ICD code BSC NSF (8th Revision) Certified primary cause of death (Obs) EX^)^ (Obs) EX^)^ SMRb RSMR‘

151

162 (140-209) less

152-154

(151-154, 162)

410-414 430-438 490-493

800-999 000-999

Malignant neoplasms Stomach Intestine and rectum Trachea, bronchus, and lung Other

Ischemic heart disease Cerebrovascular disease Bronchitis, emphysema, asthma Other internal (nonviolent) causes External (violent) causes All causes

14 7

63 36

146 28 24 71 14

403

14.2 13.4 52.8 40.4

172.0 41.2 32.1 95.4 18.7

480.2

24 17

104 70

263 53 55

120 28

734

23.6 22.4 89.5 71.9

278.9 72.7 58.0

158.0 27.5

802.5

101 113 67 76

117 132 94 106

91 102 71 80 88 99 75 85 91 103 89 100

aExpected number of deaths calculated from the calendar year and age-specific published death rates for all men in Scotland and the administrative regions of England and Wales concerned. bSMR = standardized mortality ratio = (100 X Obs/Exp). ‘RSMR = relative SMR = (cause-specific SMRlSMR all causes).

were 24 more than expected (SMR = 117, p < 0.05). Because the general mortality of the coke workers was better than expected, this corresponded to a clear excess in the proportion of deaths from lung cancer in the two studies combined, compared with the fraction expected (RSMR = 132, p < 0.001). There was no evidence of excess mortality from any other of the causes listed in Table 11, with the slightly elevated RSMR from stomach cancers easily attributable to chance. An excess of mortality from bladder cancers among NSF workers, based on 12 deaths (SMR = 161), was not found in the BSC workers. Excesses were also found among NSF workers in deaths attributed to “other” diseases of the urinary system (ICD 590-599: SMR = 178) and to cancer of the buccal cavity and pharynx (ICD 140-149: SMR = 144). These, however, were based on very small numbers (five and four deaths, respectively) and may, on this evidence, be easily attributed to chance.

Data from the two studies were analyzed separately, partly in view of the differences in cohort definitions and follow-up periods. However, selected analyses of the NSF study (Table 111) showed that neither the exclusion of maintenance workers

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TABLE 111. Mortality of NSF Coke Workers According to Various Study Definitions and Follow-up Periods

Deaths Cause of death Cohort” Obs Exp’ SMR RSMR

I 734 802.5 91 All causes I1 603 655.6 92

111 558 620.8 90 Malignant neoplasms I 104 89.5 I I6 127 Trachea, bronchus, I1 87 73.2 I I9 129 lung (ICD 162) 111 81 71.0 1 I4 128

“I: All men. 13-year follow-up (as in Table 11). 11: All men, excluding maintenance workers, 13-year follow-up. 111: All men, including maintenance workers, alive and in the industry on 7, 31. 1968, followed up until 12, 31, 1979. hExpected number of deaths calculated from the calendar year and age-specific published death rates for all men i n the administrative regions of England and Wales concerned.

TABLE IV. The SMR and RSMR of Coke Workers,* for Selected Causes, Using Reference Populations Described by Social Class**

Cancers Trachea, bronchus, and lung All causes

Number of deaths 639 103 in comparison with SMR SMR RSMR

1. All men (England and Wales) 94 127 136 2. Men (England and Wales)

Social Class IIIm’ 93 I 0 0 I I8 Social Class I V ~ 89 I05 I I8 Social Class V’ 75 90 12 1

*In England and Wales, at ages 15-64 years. in the two studies combined. **Social Class IIIm: skilled manual workers; IV: partly skilled manual workers; V: unskilled manual workers.

nor the requirement of at least 19 months continuous prior employment had any noticeable effect on the SMRs overall. Thus insofar as mortality patterns from the two studies were similar, it is acceptable as well as convenient to consider results from the studies jointly.

Further analyses were made of the mortality of coke workers in England and Wales, at ages less than 65 years, compared with deaths among manual workers. The majority of the coke workers studied accord with the Registrar General’s criterion for Social Class IV (partly skilled manual workers), though men from Classes IIIm (skilled) and V (general workers) were also included. The SMRs for deaths from all causes, and from lung cancer, reflect the well-known variations in mortality by Social Class (Table IV). However, the lung cancer RSMR consistently showed an excess of about 20% in all three comparisons.

An effort was also made to compare the mortality of coke workers with that of a group of BSC steelworkers who had never worked in the coke oven departments. Analyses of proportional mortality among the steelworkers appeared plausible and supported the main findings, reported above, of an excess proportion of lung cancer

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among the coke workers. However, there were extreme and inconsistent patterns in the numbers of deaths per calendar year among the steelworkers considered. These were quite unlike the death rates among BSC steel workers generally, from whom the sample was selected. We think that these patterns are artefacts, but we have not yet investigated the methodological issues involved; and so in view of our reservations about their reliability, we are not including these results now. *

Age, Tobacco Smoking, and Differences Between Works

There were definite age-specific variations in the mortality patterns found. Thus, while excess lung cancer deaths relative to men in Britain generally were found at all ages (Table V), the SMR was clearly highest among younger men, with more than 50% excess at age 54 or less. Mortality from ischemic heart disease showed a similar age-specific trend in the SMRs, while a converse trend was found (but less markedly so) for deaths from internal causes other than cancers or IHD.

Available data on the smoking habits of coke workers are summarized in Table VI, suggesting that the proportions of smokers and the amounts smoked by them were

TABLE V. Observed and Expected* Deaths Among All Traced Coke Workers,** by Age a t Death

Cause of death Age (years)

< 45 45-54 55-64 65 + All

Malignant neoplasms Trachea, bronchus Obs 5 28 73 61 167

Other Obs 5 23 71 69 168

lschernic heart Obs 18 74 I66 151 409

Other nonviolent Obs 8 37 1 I3 I93 351

and lung EXP 2. I 19.4 62.6 58.2 142.3

EXP 6.5 25. I 73.1 81.2 185.9

disease Exp 11.9 72.5 178.1 188.4 450.9

Exp 13.4 50.9 157.2 235.8 457.4 All nonviolent Obs 36 162 423 474 1095

Exp 34.0 167.9 471.0 563.7 1236.5

*Expected deaths calculated as in Table 11. **BSC: 2753 men over 12 years: NSF: 3855 men over 13 years.

TABLE VI. Smoking Habits of Coke Workers at Time of Surveys

Average number of Classification by (equivalent)’ smoking habits cigarettes per day

Dates of Number Non- EX- All men Study surveys examined smoker smoker Smoke? Smokers examined

BSC 1972173 1089 I3 1 253 705 (65%) 19.7 12.5 NSF‘ 1968or 1264 191 20 1 872 (69%) 16.6 11.4

1973

aPositive reply to the question “Do you smoke?”. ‘One ounce tobacco equivalent to 37.5 cigarettes, BSC men, England and Wales; 27.2 cigarettes, BSC men, Scotland; 30.0 cigarettes: NSF men. ‘Of the 1264 men examined, 25% were not in the mortality study.

*Fuller details of these and other methods and results are available on request: Institute of Occupational Medicine Report No. TM/82/6.

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698 Hurley et al

similar among BSC and NSF workers. These results are useful because the BSC men are drawn from all 14 works considered, and the NSF data, though restricted to four works, refer to most men employed at the time of the surveys. Nevertheless the two sets of results may be unrepresentative. On average data were available for only 40% of the BSC workers, a proportion that varied considerably works by works. Further- more, later analyses showed that all four NSF works studied were ones with relatively favorable lung cancer mortality.

The lung cancer SMR ranged over the 27 works studied from zero (no deaths observed, two expected) to 202 (10 deaths observed, five expected). Overall, the variations in lung cancer SMR between works in the BSC and NSF studies could be due to chance (p > 0.2 and p > 0.3, respectively). No attempt was made to group together different works according to (necessarily subjective) assessments of the levels of coke oven fume emissions over the periods when men were exposed. There was some evidence of real differences between works in mortality from nonviolent causes generally and from ischemic heart disease in particular.

Mortality and Occupation Within the Industry Seventy-six NSF workers for whom no reliable work histories were available

were excluded from further analyses. Of the remainder, 30% had worked in the ovens jobs at some time prior to the start of follow-up (Table VII), compared with 60% of the BSC men. The part oven group was identified primarily to clarify the contrast between oven and non-oven men.

Summary mortality results (Table VII) showed no evidence from either study that lung cancer risks in oven men were really higher than in the non-oven group, either in absolute terms or relative to all nonviolent causes. However, differences were found among NSF workers aged less than 45 years when follow-up began, where subsequent lung cancer mortality in non-oven men (1 death observed, 5.2 expected) was less (p < 0.05) than among oven workers (5 observed, 3.5 expected) with further excess in the small part oven group (4 observed, 1.4 expected).

Lung cancer rates were not correspondingly high among younger BSC oven workers. Non-oven men in the NSF study showed less favorable stomach cancer mortality than men in the oven group (p < 0.01), but this result was not supported by the BSC findings. Combined results from the two studies showed more deaths than expected from ischemic heart disease (SMR = 111) and from bronchitis, emphysema, and asthma (SMR = 123) in the oven group. Compared with other men, these excesses were not significant statistically at the conventional 5% level (p = 0.06 and 0.11, respectively).

NSF workers were reclassified according to occupations up to January 1, 1976. There were 35 lung cancer deaths in a larger group of 1,338 oven men. Mortality analyses showed no important differences from the patterns described in Table VII. These 35 cases were used to investigate a possible association between years worked on the ovens and mortality from lung cancer relative to other causes. Each case was matched with another (control) oven man from the same works, who had died from nonviolent causes other than cancer (i.e., ICD [OOl to 7991 less [ 140 to 209]), selected so that age at death of men in each pair was as close as possible. Two pairs of men were not considered further because of inadequate work history records. In 16 of the remaining 33 pairs (Fig. I), the case had spent more time on the ovens than his matched control, and conversely for the other 17 pairs of men. On average there was

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Mortality of British Coke Workers 699

TABLE VII. Mortality Over 12 Years (BSC) or 13 Years (NSF), by Occupation

Occupational groupa BSC NSF

Non-oven 394 2113 Part oven 742 508

Oven 1617 1158

Numbers of men

Cause of death (Obs) EX^)^ (SMR) (Obs) EX^)^ (SMR)

Malignant neoplasms Stomach

Trachea, bronchus, lung. Other

Ischemic heart disease

Bronchitis, emphysema, asthma. Other nonviolent

All nonviolent

Non-oven Part oven Oven Non-oven Part oven Oven Non-oven Part oven Oven Non-oven Part oven Oven Non-oven Part oven Oven Non-oven Part oven Oven Non-oven Part oven

I 5 8

11 18 34

7 9

27 26 32 88 3 3

18 15 31 53 63 98

3.0 33 3.1 161 7.9 101

11.9 92 14.9 121 36.2 94 7.9 89

10.3 87 24.8 109 26.7 97 34.8 92 84.5 104 5.0 60 5.3 57

13.7 131 20.0 75 22.4 138 56.7 93 74.5 85 90.8 108

19 3 2

54 15 32 51 8

27 136 23 94 29 7

19 94 18 49

383 74

14.0 136 2.9 103 7.2 28

58.3 93 12.1 124 30.6 105 48.7 105 10.6 75 26.6 102

142.3 96 31.7 73 79.1 119 32.2 90 6.6 106

16.3 117 93.8 100 19.1 94 48.0 102

389.3 98 83.0 89

Oven 228 223.8 102 223 207.8 107

aBased on lifetime work prior to the follow-up period; 76 NSF workers were unclassifiable because of inadequate occupational data. bExpected number of deaths calculated from aggregate rates observed among men in the studies separately.

less than 1 year’s difference between the two groups in time worked in the ovens jobs, and among younger men also there were no clear-cut differences between cases and controls.

Years of work on the oven jobs up to August 1967 were determined for 1,472 (91%) of the 1,617 BSC ovens workers. As expected, those with longer experience of oven work were on average older men than their co-workers (Table VIII). Men with less than 5 years in the oven jobs showed fewer lung cancer deaths than expected, significantly less (p < 0.05) than men with at least 10 years of oven work. Mortality from other cancers also increased with length of time on oven work, but this could be attributed to chance.

DISCUSSION

Mortality from all causes among the coke workers was lower than expected, most clearly so when compared with other manual workers. An excess number of deaths from lung cancer (ICD 162) was found in some, but not all, of the comparisons

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700 Hurley et al

Frequency

I I

R i

R I

-20 -10 0 +10 +20 +30 Years

Fig. 1. Differences (case minus control) in years worked in oven jobs among 33 matched pairs of NSF oven workers. Cases had died from lung cancer (ICD 162), controls from nonviolent causes other than malignant neoplasms. Matching was on works and attained age. Shaded boxes refer to the nine pairs where the attained age of the case was less than 55 years.

TABLE VIII. Mortality of 1472* BSC Oven Workers According to Time Worked on the Characteristic Oven Jobs

Years of ovens work Age" < 5 5-9 10+

< 35 160 78 14 Numbers 35-44 I34 14 1 157 of 45-54 122 129 2 13 men 55 + 55 67 202

All 47 1 415 586

Cause of death Obs Expb SMR Obs Expb SMR Obs Expb SMR

Neoplasms: trachea, 2 7.0 29 10 8.4 119 20 18.0 111

Neoplasms: other 4 6.3 63 5 6.3 79 18 15.1 119 Ischemic heart 16 16.2 99 25 18.4 136 34 38.7 88

All other causes 13 13.7 95 13 16.0 81 47 40.2 117

bronchus and lung

disease

*The other 145 oven workers were not classifiable. "At August 1, 1967 (in years). %lased on a standard population of 2608 coke workers (all traced BSC men less 145 excluded as unclassifiable).

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Mortality of British Coke Workers 701

made. However an excess of deaths from lung cancer relative to other causes was found consistently for workers in both studies in relation to all comparison groups considered. (This includes the control group of BSC steelworkers, although as noted earlier those results may be unreliable.) The RSMR of 132 compared with men in similar geographical regions of Britain was highly significant statistically.

It is known [Registrar General, 19781 that relative mortality from lung cancer among manual workers generally in England and Wales is higher than among other men. However, this alone does not explain the relatively high lung cancer rates found, because comparisons with various groups of manual workers all showed an excess lung cancer RSMR of about 20%, which was still statistically significant at the 10% level.

Favorable overall death rates among the coke workers had been anticipated, as it is well known [Goldsmith, 1975; Fox and Collier, 1976; Waxweiler et al, 19811 that long-term studies of industrial workers usually show fewer deaths than those expected, based on general population rates. This is partly because of the employment initially of relatively healthy men, and partly because some workers whose health deteriorates may leave the industry as a result. In the present studies, it may be that the more susceptible young entrants of many years previously had left the industry before 1967, and so were not included in these analyses. This would explain why the SMRs (notably from lung cancer and ischemic heart disease) declined clearly with age. If this is so, then the real death rates among older workers will have been underestimated. Whatever the causes of the high SMRs among younger men, they imply a shorter life expectation among coke workers than if the SMR for all ages had applied throughout, and this simple consequence of the results has implications for hazard assessment.

The desirability of adjusting cause-specific SMRs to take account of the favor- able overall health of industrial workers was noted by Goldsmith [1975]. An RSMR may, therefore, give a clearer indication of a particular occupational health risk than would the corresponding cause-specific SMR [McMichael et al, 19751. It has been suggested, however, [Enterline, 1975; Gaffrey, 19751 that the influence of such selection factors on deaths for malignant neoplasms is less than their effect on other causes of death, and if so the interpretation of a high cancer RSMR is not straighfor- ward. This seems reasonable, though the evidence of such cause specificity in the operation of selection factors is by no means clear-cut [Fox and Collier, 1976; McMichael, 1976; Waxweiler et al, 19811. In the present studies, we note that the lung cancer excess was found not only in relation to all other causes, but also relative to deaths from other malignant neoplasms (ICD 140-209, less 162). This supports the view that the lung cancer excess is a real work-related effect rather than an artefact arising from cause-specific patterns of inclusion in the studies.

It is also necessary to consider whether or not the excess lung cancer mortality was attributable partly or primarily to the smoking habits of coke workers. The Registrar General [ 19781 has reported a smoking score (the age-standardized propor- tion of those currently smoking cigarettes as a percentage of the expected proportion) of 117 among gas, coke, and chemical workers. The results of the present studies indicated that the proportions of smoking among the BSC and NSF coke workers and the amounts smoked by them (Table VI) were similar to the levels among other manual workers in Britain reported by Lee [ 19761. Recall, however, that the available data on smoking habits referred to only about one-third of all coke workers consid-

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702 Hurley et sl

ered, and it is possible that this subgroup is unrepresentative of the total. Thus, while the results showed no evidence of unusually heavy smoking habits in the men examined, an influence of excess smoking on the relatively high lung cancer mortality cannot be excluded.

Deaths from a few other, relatively rare, causes were greater than expected, but on the evidence available these patterns could have arisen by chance. Excess mortality attributed to cancers of the buccal cavity or pharynx (ICD 140-149) in the NSF study, while consistent with the results of Redmond et a1 [1981], was based on just four deaths. Contrary to the United States findings, death rates from malignant neoplasms of the kidney were not high, and excess mortality from bladder cancer was found in only one of the studies. It will be of interest to monitor these patterns as the follow- up period is extended.

Analyses of lung cancer deaths in relation to occupation at the coke works gave ambiguous results. Two main results pointed to a job-specific excess. Among younger NSF workers, those with no recorded experience of work in oven jobs had much more favorable lung cancer mortality than those with at least part oven experience, while among BSC oven workers higher rates were found among those who had worked longer in the oven jobs. Other analyses produced negative findings. There was no evidence of unfavorable lung cancer mortality among older NSF oven workers or among BSC workers in general. Nor was there any evidence among NSF oven men of higher lung cancer death rates among those with longer experience in the oven jobs. As for other causes, an excess of stomach cancers (ICD 151) among NSF workers in the non-oven group was consistent with results from the United States studies [Redmond et al, 19761, but among BSC workers the converse was found.

More clear-cut findings might have been anticipated if the lung cancer excess arose from a work-related hazard, but factors such as health-related job changes, long latent periods, or the influence of peak exposures may have blurred any real relationships. A further possibility is implicit in the findings of Lloyd [1971] and Redmond et al [1981]. They showed that only a small proportion of the total workforce was particularly at risk. This suggests that very specific and accurate classifications of men in relation to occupation are necessary in order to detect an exposure-response relationship, if one exists. Further work is desirable, particularly in the NSF study, to distinguish within the non-oven group those men who positively did not work on the oven jobs from others where information was vague. In addition, the oven worker groups used in the BSC and NSF studies may have been too wide to identify those men who experienced particularly high exposures to emissions from the ovens.

Thus, evidence of exposure-response relationships among coke workers in Britain is weak. This is an important difference between the results from the present studies and the work of Lloyd [1971] and Redmond et a1 [1981], where gradients in lung cancer mortality according to workplace and length of employment were unam- biguous. Furthermore, the excess lung cancer among BSC and NSF coke workers, particularly oven men, was clearly less than reported from the United States and Canada. As noted above, issues arising from the classification of work histories may partly explain these differences but other factors also need to be considered. We do not know of any reports pointing to differences in the composition of the fumes emitted from coke ovens in the countries concerned. Levels of exposure among oven workers may, however, have been higher in the United States studies. Follow-up in the United States and British studies started in 1953 and in 1967, respectively, and so

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a greater proportion of workers in the United States study were exposed in the years prior to and during World War I1 when the concentrations of fumes may have been higher than those in later years. It seems likely too that socioeconomic and cultural differences, possibly race related, caused some United States workers to remain for longer periods than their counterparts in Britain in those jobs where the concentrations were highest. Other influences, such as differences in smoking habits, may have played a part. Finally, the higher lung cancer rates among men in Britain generally compared with those in the United States may have made an occupational excess more difficult to detect in our data. Further work is in progress to try to resolve these issues.

In the meantime, we note the similar patterns of results from the two studies. Both showed more lung cancer deaths than would have been expected among British men generally, especially in the younger age groups. Both showed higher mortality from lung cancers relative to other causes of death when compared with male manual workers in Britain; and in both studies, the ratio of lung cancers to all other cancers was also higher than expected. In addition both studies showed some, limited, evidence of an association between lung cancer mortality and work on the coke ovens, though overall the lung cancer mortality of oven and non-oven men was similar. The limited data available indicate that neither excessive tobacco consumption nor the cause-specific operation of study selection factors was a major influence on the high proportion of lung cancer deaths. We interpret these consistent findings as further evidence supporting the view that workers exposed to coal carbonization fumes are at relatively high risk of developing lung cancer. It is clear, however, that the excess lung cancer mortality among oven workers in Britain was substantially lower than that found in earlier studies in the United States and Canada, particularly for men with the longest exposure times.

ACKNOWLEDGMENTS

We gratefully acknowledge the assistance of many people whose cooperation was essential to the conduct of these studies: BSC Medical Officers and NSF Training and Safety personnel for their part in data collection; senior operations staff of the two companies for advice on classifying occupations; the Office of the Registrar General (Scotland) and the Offices of Population Censuses and Surveys (England and Wales) for information on mortality; and many colleagues in the Statistics Branch of the Institute of Occupational Medicine for their involvement in some aspect or other of the work.

This research work was carried out with financial aid from the European Coal and Steel Community, the National Coal Board and the British Steel Corporation.

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