a mortality study of workers manufacturing friction ...members.optusnet.com.au/geoffberry1/gb...

Ann. occup. Hyy.. Vol. 26, Nos. I -4. 899-909. 1982. 0O03^t878/82/O[0899-l IS03.00/0Printed in Great Britain. Pergamon Press Ltd.Inhaled Panicles V c 1982 British Occupational Hygiene Society.

A MORTALITY STUDY OF WORKERS MANUFACTURINGFRICTION MATERIALS WITH CHRYSOTILE ASBESTOS

M. L. NEWHOUSE

TUC Institute of Occupational Health, London WC1E 7HT, U.K.

G. BERRY and J. W. SKIDMORE

MRC Pneumoconiosis Unit, Penarth, Glamorgan, S. Wales, U.K.

Abstract—This is a study of workers at a friction materials factory founded in 1898. Crocidolite wasused only during two well defined periods before 1945. Apart from this, only chrysotile asbestos hasbeen used. The mortality study dates from 1942 and extends to 1979. Over 99% of the population havebeen traced. Ten deaths from mesothelial tumours occurred. An analysis contrasting the observednumber of deaths with those to be expected showed a significant excess of deaths from cancer of thelung and pleura in those employed before 1942. Among these deaths were seven from pleuralmesothelioma. A case control study of the ten workers dying from pleural mesothelioma showed thatall but two of these had definite exposure to crocidolite asbestos. Allowing for concomitant exposureto chrysotile asbestos among both cases and controls, the evidence of an association of the deathsfrom pleural mesothelioma with an exposure to crocidolite asbestos had a statistical significance levelof 0.06.

INTRODUCTION

THIS mortality study has been carried out on the employees of a factory which has beenproducing friction materials since the turn of the century. Apart from two well definedperiods when crocidolite asbestos was used for a specific order, only chrysotile asbestoshas been used in the factory. The objectives of the study are, therefore, to establish theeffects of exposure to chrysotile asbestos as used in this factory and to contrast this withthe effects on subjects who were also exposed to crocidolite asbestos. The workprograms were so arranged that none of the workers had a pure crocidolite exposure. Itis also intended to establish dose-response relationships to chrysotile asbestos. Thepresent communication deals with the first two objectives.

After several years of experimentation on brake blocks and brake and clutchlinings, the factory was founded by Herbert Frood in 1898. At that time cast iron, elmwood, leather, hemp, colliery wire and flax were all in use. Michelin in France hadpatented a brake block made of canvas laminated with rubber. However, gritty roadswore them away rapidly and at 7-10 shillings a pair they were reckoned to be expensive.Eventually Frood carried out experiments on cotton textile belting hardened andglazed with mineral oil, and patented the process in 1901. He continued to experimentwith bonding materials and in patent of 1910 specified: '65 parts by weight of resinspirit, 25 parts rock bitumen, and 25 parts gum copal brought to a suitable condition bylinseed oil with litharge or other drier added as required.' (FERODO, 1957). In 1910 hedecided to experiment with woven asbestos, which proved to have far superior qualityand had virtually supplanted cotton belting by 1920. No manufacture of cotton or

899

900 M. L. NEWHOUSE. G. BERRY and J. W. SKIDMORE

asbestos textiles took place in the factory. The essential features of the productionprocess remain the coating of the fibre in resin, forming to the required shape, bakingand machining.

In 1922 oil-based resins were replaced by phenolic resins and non-woven asbestosfibre came into use when a patent was taken out on a method of making brake blocksusing chrysotile fibre and resin cast into a die. Products based on woven materialcontinued to dominate, but the proportion based on fibre gradually increased to aboutone sixth of the total output by 1940 and, as a result of further growth and the growth oftotal output, space was provided in 1960 for further expansion by transferring theproduction involving woven asbestos goods to a new factory.

Chrysotile was found, technically, to be the most suitable asbestos for the productsand Canadian and Rhodesian chrysotile have been used almost exclusively. There weretwo occasions in the factory history, 1929/1933 and 1939/1944, when crocidolite fibrewas used to produce specially ordered railway blocks. On both occasions these blockswere produced in one well defined area of the workshop. Additionally a very smallquantity of crocidolite has been used in the experimental workshop.

ENVIRONMENTAL CONDITIONS

There have been large changes in the environmental conditions within the factoryover the years. Prior to 1931 the only stimulus to improvement was productionefficiency requirements. Machines were heavily concentrated and all operatives in theopen plan of the factory were similarly exposed. The Asbestos Regulations (1931)provided the first mandatory dust suppression requirements. These called for exhaustventilation of some machining operations and the use of respirators for some dry fibrehandling operations. Enforcement was less rigid during the war years. In the post-waryears there was a gradual introduction of further dust suppression of machiningoperations and also of product cleaning. The impending Threshold Limit Value forchrysotile prompted an environmental assay of all major operations in 1968/1969 todetermine the necessary action for compliance by 1970. Engineering records from theearliest days noting the expansion of the factory and the applications of each new blockand its contained plant are held at the factory. These records include details of theapplication of exhaust ventilation to the various machines. Personnel recordscommenced in 1941 recorded the occupational details of all employees at that time andall subsequent changes have been noted.

Environmental measurements have been made at the factory only since 1967 withthe introduction of the membrane filter method. At this time air was drawn through thefilter by means of a water aspirator and the filter holder was held at head height close tothe operator. Personal sampling, with the sampler mounted on the operative,commenced in 1968 when the battery-operated pump unit became available. At thattime the asbestos clouds associated with the majority of the operations had beenessentially the same since 1950. The exceptions were the application of exhaustventilation to processes not requiring it under the Asbestos Regulations (1931). Thedates of such applications were available from engineering records. Since nomeasurements were available for earlier periods it has been necessary to simulateearlier working conditions. The simulation has been made possible by the availability

A mortality study of workers manufacturing friction materials with chrysotile asbestos 901

of the original machines, similar spatial situations and long-serving operatives withknowledge of working practices.

During the simulation exercises personal measurements were made on theoperative and any associated serviceman together with static measurements of thesurrounding atmosphere.

For the purposes of this paper the environmental history of the factory has beendivided into four periods:

(1) Pre-1931 Before the Asbestos Regulations and when all operations werecarried out in one open-plan area.

(2) 1932-1950 When exhaust ventilation was applied to many machining opera-tions and larger premises provided for greater separation betweenthe stages of production.

(3) 1951-1969 A period of gradual improvement including product cleaning andthe application of exhaust ventilation to machines not included inthe Asbestos Regulations.

(4) 1970-1979 Following the introduction of 2 f ml"' TLV.

The variability of individual methods of working and product sizes resulted in arange of fibre concentrations for each type of operation; the mean concentrationsassociated with the main steps in the production process have been placed in seven fibreconcentration ranges and are shown in Table 1, for the four time periods.

POPULATION IN STUDY AND SEARCH PROCEDURE

In 1941 the factory started to maintain a systematic alphabetical file of allemployees. This recorded identification details including date of birth and date ofstarting work and leaving employment. The actual job was not recorded, but the

TABLE 1. MEAN CONCENTRATIONS OF ASBESTOS (fibres ml"1) IN FOUR PERIODS

Office/laboratoryStorage and

distributionPreparationImpregnationFormingGrindingDrilling, boring,

turningInspection,

stencillingPacking,despatch

Pre-1931Woven

10-20

>20>20>20>20>20

>20

>20

>20

Fibre

10-20

>20>20>20>20>20

>20

>20

>20

1932/1950Woven

<0.5

1-21-21-21-25-10

2-5

2-5

1-2

Fibre

<0.5

2-510-202-52-55-10

2-5

2-5

1-2

1951/1969Woven

<0.5

1-21-21-21-22-5

1-2

2-5

1-2

Fibre

<0.5

2-52-51-21-22-5

1-2

1-2

0.5-1

1970/1979Woven

<0.5

0.5-10.5-1

1-21-21-2

1-2

1-2

1-2

Fibre

<0.5

0.5-11-2

0.5-10.5-10.5-1

1-2

0.5-1

<0.5

902 M. L. NEWHOUSE, G. BERRY and J. W. SKIDMORE

number of the 'cost centre' of the particular project was entered in a numerical code.These changed frequently and, although it was possible for personnel staff at the factoryto interpret these codes, it was too laborious to undertake for the entire file of over13 000 records and detailed job histories were only built up for workers of specialinterest. The population included in the study are those employed in or after 1941. Thestudy terminated on 31 December 1979.

Special forms in duplicate were used to abstract the identification details and datesof employment. National Health Service numbers and National Insurance numberswhen recorded were included. One copy was sent to the National Health ServiceCentral Register who, by searching their files, were able to identify the subject as eithercurrently on a general practitioner's list and therefore presumably alive, deceased,emigrated or untraceable. For the deceased, a copy of the death entry was sent toourselves, together with the original form. In those presumed to be alive, the subjectwas flagged so that future deaths could be reported. One copy of the form was retainedand the details entered into a computer. Among the 'untraced' were over 300 forms ofpast workers. Where the National Insurance number was recorded, these were sent tothe National Insurance office of the DHSS who were also able to identify the living andthe dead and forward copies of the death entries.

Table 2 shows the trace status up to 31 December 1979. Among the records were 35where either the date of birth or the years worked at the factory were unknown. Thesewere excluded from the study. Apart from these 35, 99.3% of the population understudy were accounted for. The date of embarkation was given for those who hademigrated and they were classified as traced up to that date.

TABLE 2. TRACE STATUS TO 31 DECEMBER 1979

AliveD e a d 'EmigratedUntracedTotal

No.

71641640254

559113

Men%

(78.6)(18.0)(2.8)(0.6)

(100.0)

No.

3873346

9434

4347

Women%

(89.1)(8.0)(2.2)(0.8)

(100.1)

No.

11037198634889

13460

Total%

(82.0)(14.8)

(2.6)(0.7)

(100.0)

A higher proportion of men had died than women, 18% compared with 8%. Thereare two reasons for this. First, the men were, on average, born about 5 yr earlier thanthe women, e.g. 17% of the men but only 11 % of the women were born before 1910 andonly 43% of the men compared with 69% of women started work at the factory agedyounger than 25. Secondly, the mortality experience in the national population is morefavourable for women than for men. The expectation of life of women aged 25 is 52.0 yrcompared with 46.3 for men.

Two main analyses have been undertaken. First, a comparison of the observedmortality with that expected, using the sex, age and period specific death rates forEngland and Wales. The number of expected deaths was calculated by the subject yearsmethod. (CASE and LEA, 1955). Secondly, a case control study of the deaths frommesothelial tumours following the method of LIDDELL et al. (1977).


THE MORTALITY STUDY

Three main factors have been examined: the period of first employment at thefactory, the length of time worked in the factory and the length of follow-up since firstemployment. In addition, to maximize the possible effects, those with 20 years'employment or longer were identified and their mortality examined after completion of20 years' service. Men and women have been separately analysed. As well as the totalnumber of deaths the separate causes considered were cancer of lung and pleura (ICD162 and 163), cancer of the gastro-intestinal tract (ICD 150-159) and all other cancers(ICD 140-149, 160-161, 170-207) using the 8th Revision of the InternationalClassification of Diseases (WHO, 1969).

The first three analyses have been restricted to the period following the 10thanniversary of first employment at the factory. A total of 2243 were excluded. Themajority of these were men and women who did not start their employment in thefactory until 1970 or later, but there were 343 deaths which occurred within 10 years ofstarting work at the factory which were also excluded. This left 7474 men of whom 1339had died and 3708 women of whom 299 had died.

The periods of employment (Table 3) were divided into short service up to 2 yr,moderate lengths of service of 2-5 yr and long periods of 5 or more years. The mainimpression is.that there is little evidence of any excess mortality either from all causes or

TABLE 3. OBSERVED AND EXPECTED MORTALITY AFTER 10 YR FROM FIRST EMPLOYMENT BY LENGTH OF

EMPLOYMENT

(a) MenNo./subject-yearsCause of death

All causesLung and

pleuralcancer

Gastro-intestinalcancer

Other cancersOther causes

(b) WomenNo./subject-years

All causesLung and

pleuralcancer



Less than 2 yr

3195/39240Obs

407

43

3430

300

Exp

398.9

41.9

31.226.8

299.0

1705/23 294

128

6(1)

101993

130.4

4.6

10.924.290.7

2

Length of employment!-S yr

1218/17099Obs

212

24

1412

162

Exp

211.3

20.1

16.213.6

161.4

908/14 393

59

6'8

1140

72.8

2.5

6.013.450.9

More than 5 yr

3061/47 855Obs

720

84(8)

5535

546

1095/21

112

2(1)

112178

Exp

751.6

77.6

59.847.3

566.9

129

124.8

4.2

10.522.587.6

Total

7474/104 193Obs

1339

151

10377

1008

Exp

1361.8

(8) 139.5

107.287.7

1027.4

3708/58 816

299

8

2951

211

328.0

(2) 11.3

27.460.0

229.3

( ) = no. of pleural mesotheliomas.

904 M. L. NEWHOUSE, G. BERRY and J. W. SKIDMORE

from any particular cause and notably lung cancer. Among the men there were eightdeaths from mesothelial tumours and two among the women. All but one of thesedeaths occurred in the long service group.

In Table 4 the mortality is examined according to the period when first employed.There is a significant excess of deaths due to cancer of the lung and pleura (42 obs: 30.4exp p < 0.05) in the group of men starting work before 1942. The more detailed analysisshowed that this was found among those with more than 5 years' service and 25 or moreyears follow-up. Omitting the seven deaths from pleural mesothelioma, the excess wasnot statistically significant.

The mortality experience of workers with very long service analysed by the length offollow-up after completing 20 yr at work is shown in Table 5. There is an excess ofdeaths from cancer of lung and pleura among both men and women with more than10 yr follow-up, statistically among men (32 obs: 19.2 exp p<0.05). However, againthis result was heavily influenced by the seven deaths from mesothelial tumours whichoccurred in this group.

No analysis showed an excess of deaths from gastro-intestinal tumours or othercancers. Cancer of larynx and cancer of ovary have been attributed to exposure toasbestos. There were no deaths from cancer of larynx among women and two deaths in

TABLE 4. OBSERVED AND EXPECTED MORTALITY RATES AFTER 10 YR FROM FIRST EMPLOYMENT BY YEAR STARTED

IN FACTORY

Before 1942Year started

1942-1950 1951-1969

(a) MenNo./subject-yearsCause of death

641/17064Obs Exp

2212/43 703Obs Exp

4621/43425Obs Exp

All causesLung and

pleuralcancer



298

42 (7)*

1213

231

335.7

30.4

26.319.9

259.1

661

73

5236

500

644.8

67.7

51.041.0

485.1

380

36(1)

3928

277

381.3

41.4

30.026.8

283.1

(b) WomenNo./subject-years 368/11049 1234/27062 2106/20706

All causesLung and

pleuralcancer



66

2(1)

81244

68.6

2.3

5.812.747.8

174

• 3 ( 1 )

1627

128

188.2

6.2

15.832.7

133.5

59

3

51239

71.2

2.8

5.914.647.9

* p < 0.05 = Significance of excess (one-sided test).( ) = number of pleural mesotheliomas.


TABLE 5. OBSERVED AND EXPECTED MORTALITY AFTER COMPLETING 20 YR EMPLOYMENT

Follow up after 20 yr exposure0-10 yr 10+ yr

(a) MenNo./subject-years 1328/10822 750/5667Cause of death Obs Exp Obs Exp

All causesLung and pleural cancerGastro-intestinal cancerOther cancersOther causes

12915101094

161.418.813.810.4

118.4

19532 (7)f107

146

193.519.215.211.8

147.3

(b) WomenNo./subject-years 197/1706 146/1699

All causesLung and pleural cancer

-Gastro-intestinal cancerOther cancersOther causes

90009

10.30.41.02.26.7

402(1)48

26

32.50.92.74.1

24.8

fp< 0.01= Significance of excess (one-sided test).( ) = number of pleural mesotheliomas.

males compared with an expectation of 3.6; eight of the women died of cancer of theovary, the expectation was 8.1.

Chronic respiratory diseases including asbestosis have not been separatelyconsidered in this analysis; only one death was certified as due to asbestosis (ICD515.2). He was employed between 1946 and 1958 and died in 1965.

CASE CONTROL STUDY OF DEATHS FROM PLEURAL MESOTHELIOMA

This study was carried out in order to assess the significance of crocidolite exposurein the aetiology of the 10 pleural mesothelial tumours which occurred during the studyperiod. Details of the 10 workers are given in Table 6. All but case I had worked in thefactory during one or both periods (1929-1933 and 1939-1944) when crocidolite was inuse. Each case was matched with four controls. Matching was for the following:

(l)Sex.

(2) Year started work in factory (+1 yr).

(3) Year of birth (±4yr).

(4) Control still alive when case died.

(5) Control employed at factory during crocidolite periods for same time as case.

A detailed history of jobs within the factory was then completed for each case andcontrol and for each job, the exposure to crocidolite and the level of exposure tochrysotile established. During this process there was no indication of whether a


TABLE 6. DETAILS OF WORKERS DYING FROM PLEURAL MESOTHELIOMA

(CROCIDOLITE IN USE IN FACTORY 1929-1933, 1939-1944)

Reference

ABCDEF

GHIJ

Sex

MMMMMM

MFFM

Years atfactory

1923-19601924-19681924-19641928-19671929-19711928-1941,1946-19661930-19661938-19731943_1944*1957-1963*

Yeardied

19691968196419671974

19761966197319711976

Ageat

death

7060655767

6152526172

Latentperiod

4644403945

483635

uncertain55

*Case I employed in factory 9 yr earlier.*Case J employed at an asbestos cement factory 1921-1944.

particular individual was a case or control, but as most of the mesotheliomas werepreviously known about in the factory the names may have been remembered. Everyeffort was made to produce the job history in as much detail as possible. Exposure tocrodicolite was classified in three categories:

(1) Definite exposure—working with crocidolite.

(2) Nearby exposure—working in the same shed within 15 m of the crocidolite work(in fact none of the cases and controls was in this category).

(3) Fringe exposure—working in the same shed but more than 15 m from thecrocidolite work or for some reason may have had chance intermittent exposure tocrocidolite.

Eight of the mesotheliomas had definite exposure to crocidolite, one had fringeexposure and the other no exposure in the factory. In contrast, only three of thecontrols had definite exposure, seven had fringe exposure and 30 no exposure (Table 7).

The two mesotheliomas without a definite history of exposure to crocidolite were Iand J. Case I, a woman, was recorded as working at the factory for just 1 yr and duringthis time she was a planning clerk with only incidental exposure to chrysotile. However,when she started in 1943 it was recorded that she had previously worked at the factoryfor 9 yr before the personnel file system was set up in 1941. The job she did during these9 yr may have involved fringe exposure to crocidolite. Case J did not start work at thefactory until he was 53 yr old. He had previously worked at an asbestos cement factorybetween 1921 and 1944 as a cost clerk. His duties may have taken him into productiondepartments and crocidolite was in use at the time in pipe manufacture. Forcompensation purposes his mesothelioma has been attributed to the earlier exposure atthe cement factory. Thus, there was only one mesothelioma (Case I) that was attributedto working at the factory and also definitely did not work with crocidolite. The verydifferent pattern of crocidolite exposure of the controls suggests a strong association,


TABLE 7. EXPOSURE OF MESOTHELIOMA SUBJECTS AND MATCHED CONTROLS TO CROCIDOLITE AND CHRYSOTILE IN

THE FACTORY

Reference Mesothelioma Controls

5c 5cABCDEFGHIJ

+ 5c+ 5c+ 5c+ 5c+ 5c+ 5c+ 5c+ 5cf

- 5c

f5c+ 5cf

+ 5cf5c+f5c

_

ff5cf--5c-____ - 5 c

Crocidolite exposure: + = definite, f= fringe exposure, - = no known exposure.Chrysotile exposure: 5c to a level of 5 fibres ml"1 or higher.

but this association is weakened by a correlation between chrysotile and crocidoliteexposure. Those who worked with crocidolite were production workers and in otheryears would be processing chrysotile and be exposed to relatively high levels. Incontrast, some of the controls were office workers and were not exposed, or exposedonly to low levels of chrysotile. All the cases except Case I had been exposed at sometime to a level of chrysotile of at least 5 f ml"', but only 10 out of the 40 controls hadbeen exposed to such a level (Table 7). The possible confounding effect of chrysotileexposure has been removed by restricting attention to those with controls who wereexposed to at least 5 f ml ~ i . This left six mesotheliomas, five of whom had been exposedto crocidolite, with 10 controls, only two of whom had been exposed to crocidolite.Analysing these results taking account of the matching (PIKE and MORROW, 1970), thenthe evidence of an association with crocidolite exposure has a statistical significancelevel of 0.06.

Three other mesotheliomas are known to have occurred at the factory but are not inour study population, having left the factory before the filing system was set up in 1941.They all worked at the factory during the first period when crocidolite was beingprocessed, but there are no records available to determine their actual jobs at the time.

DISCUSSIONTo summarize our current findings, there is no excess of deaths from all causes, the

ratio of observed to expected deaths is 0.98 in males and 0.91 in females, for deaths fromcancer of lung and pleura the ratio is 1.08 in males and 0.71 for females and is near orbelow unity for other groups of cancers examined and for all other causes. However,establishment of dose response relationships may reveal an excess of deaths from lungcancer at the higher cumulative dose levels as was found by MCDONALD et al. (1980).The study population is heavily weighted by short service workers, many of whom didnot have high exposure. It is notable, however, that, although environmentalmeasurements showed very high levels before 1931, these were much reduced in the1932-1950 period. By 1952 except in certain well-defined jobs, preparation and


grinding, the level of asbestos in air in the factory was at or below the current TLV of2 f ml"1. Examination of the mortality of workers first employed in the 1951-1960period may give much-needed information on the mortality of workers exposed to lowlevels of asbestos in air.

There is firm documentary evidence that eight of the 10 deaths from mesothelialtumours occurred in workers who were employed in the manufacture of the particularengine brake blocks where the use of crocidolite asbestos was specified. As they wereproduction workers, employed in the earlier periods, they were also exposed to heavyconcentrations of chrysotile asbestos. Over 4000 men and women were first employedin the 10 yr following the completion of the crocidolite job, that is, between 1946 and1955. Deaths from mesothelial tumours with a latent period of between 25 and 30 yr arenot uncommon (NEWHOUSE and THOMPSON, 1965), yet no deaths from mesothelialtumours occurred in this group. The evidence of an association between crocidoliteexposure and death from a mesothelioma had a significance level of 0.06 in the casecontrol study. More information on the role of crocidolite asbestos as an aetiologicalfactor may be obtained from electron microscope studies of lung tissue ofmesothelioma cases and controls, for which material is currently being collected. Nodeaths from peritoneal mesothelioma have been identified in this study. Deaths fromthis cause have been certified as due to cancer of pancreas, cancer of ovary, or due tocarcinomatosis (NEWHOUSE and WAGNER, 1969; SELIKOFF and SEIDMAN, 1980). Deathsfrom these causes in our study which occurred in hospitals are now being followed upand post-mortem reports and histological material obtained for validation of the causeof death on the death certificate. The pleural mesotheliomas recorded in this study haveall had histological validation.

This report may be considered as a preliminary communication, but it is apparentthat the mortality experience in the friction materials factory is very different from theexperience in other sections of the asbestos industry. Among the friction workers therewas no excess mortality, mesothelial tumours accounted for 0.5 % of the total mortality.In the East End of London factory manufacturing textiles and some other products(NEWHOUSE and BERRY, 1979) the excess overall mortality among men amounted to24%, with mesothelioma deaths accounting for 10% of the total mortality. Among theNew York insulation workers studied by SELIKOFF et al. (1979) the excess mortality was45 % and the proportion of all deaths due to mesothelial tumours amounted to 8 %. Themortality experience was more favourable among chrysotile miners and millers inCanada (MCDONALD et al. 1980); the excess mortality from all causes amounted to 9%,but the proportion of deaths due to mesothelial tumours was 0.2%.

Environmental conditions have been generally satisfactory at this factory for thelast 25-30 yr. The comparatively favourable mortality figures may be due to threefactors: the limited use of crocidolite asbestos, the nature of the actual process inmaking friction materials which favours good environmental control and goodhousekeeping on the part of the factory.

Acknowledgements—This research was supported by a grant from the Medical Research Council.We would like to thank Ferodo Limited for the opportunity to carry out the project, and particularly Mr

Bernard Dufficy, without whose comprehensive knowledge of the factory the investigation would not havebeen possible.


R E F E R E N C E S

ASBESTOS INDUSTRY REGULATIONS (1931) S.R. and O. 1931, No. 1140.

CASE, R. A. M. and LEA, A. J. (1955) Mustard gas poisoning, chronic bronchitis and lung cancer. Br. J. prev.soc. Med. 9, 62-72.

FERODO (1957) The Ferodo Story. Privately published by Ferodo Ltd.INTERNATIONAL CLASSIFICATION OF DISEASES (1969) World Health Organization.LIDDELL, F. D. K.., MCDONALD, J. C. and THOMAS, D. C. (1977) Method of cohort analysis: appraisal by

application to asbestos mining. J. R. Stat. Soc. Ser. A 140, 469-491.MCDONALD, J. C, LIDDELL, F. D. K.., GIBBS, G. W., EYSSEN, G. F. and MCDONALD, A. D. (1980) Dust

exposure and mortality in chrysotile mining 1910-1975. Br. J. ind. Med. 37, 11-24.NEWHOUSE, M. L. and BF.RRY, G. (1979) Patterns of mortality in asbestos factory workers. Ann. N.Y. Acad.

Sci. 330, 53-66.NEWHOUSE, M. L. and THOMPSON, H. (1965) Mesothelioma of pleura and peritoneum following exposure to

asbestos in the London area. Br. J. ind. Med. 22, 261-269.NEWHOUSE, M. L. and WAGNER, J. C. (1969) Validation of death certificates in asbestos workers. Br. J. ind.

Med. 26, 302-307.PIKE, M. C. and MORROW, R. H. (1970) Statistical analysis of patient-control studies in epidemiology. Factor

under investigation an all-or-none variable. Br. J. prev. soc. Med. 24, 42-44.SELIKOFF, I. J., HAMMOND, E. C. and SEIDMAN, H. (1979) Mortality experience of insulation workers in the

United States and Canada. Ann. N.Y. Acad. Sci. 330, 91-117.SELIKOFF, I. J. and SEIDMAN, H. (1980) Cancer of pancreas among asbestos insulation workers. Pro. Int.

Meeting Pancreatic Cancer, New Orleans, 10 March 1980.

D I S C U S S I O N

F. D. K. L IDDELL : The authors are to be congratulated on the really beautiful selection of referents in the casecontrol study of deaths from pleural mesothelioma.

Dr NEWHOUSE: Thank you. Mr Berry was responsible and he alone deserves those congratulations.

R. S. J. DU TOIT: YOU stated that the evidence of an association of the 10 deaths from pleural mesotheliomawith an exposure to crocidolite reached the 0.06 level of significance. Is it not usual to say that a level is notsignificant unless it reaches the 0.05 level?

Dr NEWHOUSE: Yes, that is a common convention, but is not as informative as stating the actual level.Allowing for chrysotile, the observation that 5 of the mesotheliomas had worked with crocidolite was themost unlikely outcome, if there was no association with crocidolite, and has a probability of 1 in 32; this is theone-sided significance level.

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