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Laryngeal and hypopharyngeal cancers andoccupational exposure to formaldehyde andvarious dusts: a case-control study in France

Laurent Laforest, Danile Luce, Paquerette Goldberg, Denis Bgin, Michel Grin,Paul A Demers, Jacques Brugre, Annette Leclerc

Abstract

Objectives A case-control study was

conducted in France to assess possible

associations between occupational expo-

sures and squamous cell carcinomas of

the larynx and hypopharynx.

Methods The study was restricted to

men, and included 201 hypopharyngeal

cancers, 296 laryngeal cancers, and 296

controls (patients with other tumour

sites). Detailed information on smoking,

alcohol consumption, and lifetime occu-

pational history was collected. Occupa-tional exposure to seven substances

(formaldehyde, leather dust, wood dust,

flour dust, coal dust, silica dust, and

textile dust) was assessed with a job expo-

sure matrix. Exposure variables used in

the analysis were probability, duration,

and cumulative level of exposure. Odds

ratios (ORs) with their 95% confidence

intervals (95% CIs) were estimated by

unconditional logistic regression, and

were adjusted for major confounding fac-

tors (age, smoking, alcohol, and when rel-

evant other occupational exposures).

Results Hypopharyngeal cancer was

found to be associated with exposure tocoal dust (OR 2.31, 95% CI 1.21 to 4.40),

with a significant rise in risk with prob-

ability (p

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hypopharyngeal and laryngeal tumours amongwomen, only men were recruited. The studyinitially identified 664 patients meeting thesecriteria. Of these, 136 (20.5%) could not beinterviewed because of health problems(n=40), death before an interview could beconducted (n=11), refusal to participate(n=22), or because they could not be located(n=63). In all, the study finally included 206men with hypopharyngeal cancer, 315 with

laryngeal cancer, and seven with cancers ofboth the larynx and hypopharynx. Controlswere patients with primary cancers of diVerentsites, selected by frequency matching on ageand recruited between 1987 and 1991 in thesame hospitals as the cases or in similar hospi-tals nearby. The aim was to obtain a controlgroup of the same size as the group of cases oflaryngeal cancer. Of the 355 controls identi-fied, 50 (14%) could not be interviewed,because of medical conditions (n=14), refusal(n=14), or because they could not be located(n=22). To obtain comparable catchmentareas, controls were patients with cancersrequiring the same medical environment as the

cases.The following sites were selected:rectumor anal canal (n=30), liver or gall bladder(n=15), pancreas (n=11), haematopoietic sys-tem (n=34), bones and cartilage (n=7), skin(melanoma) (n=18), soft tissue (n=11), pros-tate (n=63), testis (n=17), bladder (n=29),other urinary organs (n=27), brain and nerv-ous system (n=18), thyroid (n=18), colon(n=5), and stomach (n=2).

Occupational physicians specially trained forthis study interviewed both cases and controlsand collected detailed data about demographiccharacteristics, alcohol and tobacco consump-tion, and lifetime occupational history. A morecomplete description of the study design and ofthe characteristics of cases and controls haspreviously been published.25

Of the 833 subjects included in the study,some were excluded from the analysis: thosewith cancer at more than one site (n=7), thosewho did not answer the questions on alcohol(five cases of laryngeal cancer), and those whodid not drink at all (nine controls, 14 cases oflaryngeal cancer, five cases of hypopharyngealcancer, because it was unclear whether theywere teetotal or former drinkers who hadstopped for health reasons). The final analysisthus included 793 subjects: 296 controls, 201men with hypopharyngeal cancer, and 296with laryngeal cancer. Those with laryngealcancer were divided into anatomical subsites

according to the classification used by Tuyns etal.4 These sites were the epilarynx (n=97), theglottis (n=102), the supraglottis (n=80), thesubglottis (n=3), and larynx unspecified(n=14).

OCCUPATIONAL EXPOSURES

Each job was coded with standard classificationfor occupation (ISCO)27 and industry (ISIC).28

Occupational exposures were assessed througha job exposure matrix (JEM), developed earlierby three of the authors (PD for wood andleather dust, DB and MG for other substances)

for a study on sinonasal cancer29 (the JEM isavailable from the authors).

For each ISCO-ISIC combination and foreach substance, the JEM gives the probabilityof exposure, and the level of exposure incategories. The cut oVpoints used for the cat-egories of level and probability, according tothe substance, are presented in table 1. Whenthe probability of exposure was less than 10%(for wood dust and leather dust) or less than

1% (for the other substances), the job was con-sidered to be unexposed.

For a given substance, for each job i of eachsubject, the JEM provides the probability ofexposure Pi and the level of exposure Li. Thejob duration Di was also available. Subjectscould have had several successive jobs iexposed to the same substance throughouttheir working life. Several exposure variableswere calculated to summarise the subjects life-time exposure: maximum probability ofexposure=max (Pi); total duration of exposure(in years)=Di; cumulative level of exposure=Li Pi Di.

To calculate the cumulative level, quantita-

tive values (midpoints of the intervals) wereassigned to each category of level and probabil-ity (table 1).

DATA ANALYSIS

Each substance was studied independently.Separate analyses were conducted for cancersof the larynx and hypopharynx. Each analysisused the same control group.

Occupational exposure was first assessed bya dichotomous variable ever versus neverexposed. Quantitative exposure variables wereconverted into categorical variables accordingto their distribution among all subjects. Thenumber of classes ranged from two to four,depending on the frequency of exposure. The

reference category for each analysis was thenever exposed subjects.

Table 1 Categories for level and probability of exposuredefined in the JEM for the studied substances and assignedvalues

Substances

Level categories Probability categories

Cut oVpoints*

Assignedvalue

Cut oVpoints(%)

Assignedvalue

Leather dust Low 1.00 1050 0.30Medium 2.00 5090 0.70High 3.00 >90 1.00

Wood dust 5 9.00 >90 1.00

Formaldehyde 1.00 3.00 5090 0.70

>90 1.00Flour dust 5 10.0 >90 1.00

Coal dust 2 5.00 >90 1.00

Silica 0.1 0.200 >90 1.00

Textile dust 0.5 1.500 >90 1.00

*Wood dust, flour dust, coal dust, silica, textile dust (mg/m3),and formaldehyde (ppm).

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The analysis used multivariate unconditionallogistic regression, performed with SAS soft-ware. Adjusted odds ratios (ORs) are reportedwith their 95% confidence intervals (95% CIs).All ORs were adjusted for age (60),alcohol (04 glasses/day; >5 glasses/day) andsmoking (30 pack-years).Preliminary analysis showed that with a finerstratification for age (69), alcohol (13) and smoking (non-smoker, 129 pack-years, 3044, >45) did not substantially alterthe adjusted ORs for occupational exposures.

Other occupational exposures (includingasbestos and man made mineral fibres) andeducation level (primary school or less, higherthan primary school) were considered to bepossible confounders for any given substanceand were included in the final model if morethan 10 subjects (cases or controls) wereexposed to both the substance under consid-eration and the potential confounder, and if theconfounder changed the OR estimate by morethan 10%. When a dose-response pattern of

interest was found for a categorised exposurevariable, a test for trend was performed byassigning the score k to the kth level ofexposure (the number 0 was assigned to thereference category) and entering the score intologistic models. We also examined morecomplex logistic models including interactionterms between occupational exposures andage, and alcohol and tobacco consumption.

For substances other than wood dust andleather dust, further analyses were performedby excluding subjects considered to be exposedwith a probability of exposure less than 10%.

We also analysed the data according tolaryngeal subsites, and we compared, for each

substance, the risk of cancer of the epilarynxwith that of cancer of other parts of the larynx(endolarynx) and the hypopharynx.

To allow for a possible induction period,successive analyses excluded the 5, 10, and 15years of exposure immediately before the refer-ence date (cancer diagnosis).

In all additional analyses, the ORs wereadjusted for the variables found to be con-founders in the earlier analyses.

ResultsTable 2 summarises the principal characteris-tics of each of the study groups for educationlevel, age, daily alcohol consumption, andsmoking.

Table 3 reports the number of exposed sub-jects and the adjusted OR for each substance.Only 19 workers (2.4% of all subjects) wereexposed to leather dust; therefore no furtheranalysis was performed for it. Exposure was

most frequent for formaldehyde (n=270,34.0%) and silica dust (n=326, 41.1%). Thefrequency of exposure to the other substanceswas 8.1% for textile dust (n=64), 13.4% forcoal dust (n=106), 14.6% for wood dust(n=116), and 7.2% for flour dust (n=57).

An increased risk of hypopharyngeal cancer,although not significant, was found for expo-sure to formaldehyde. There was a clear dose-response pattern with the probability ofexposure (trend test p

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Table 3 Risk of cancer of the larynx and hypopharynx, by occupational exposure to each substance

Larynx Hypopharynx

Cases/ controls Adjusted OR 95% CI Cases/ controls Adjusted OR 95% CI

Leather dust:Never exposed 288/288 1.00 198/288 1.00

Ever exposed 8/8 0.89 0.62 to 1.27 3/8 0.80 0.16 to 4.07Wood dust*:

Never exposed 249/260 1.00 168/260 1.00

Ever exposed 47/36 1.00 0.58 to 1.72 33/36 0.89 0.47 to 1.68Probability of exposure (%):

70 27/18 1.08 0.53 to 2.21 22/18 1.06 0.48 to 2.31Duration of exposure (y):

10 19/13 1.03 0.46 to 2.30 13/13 1.19 0.48 to 2.95Cumulative level:

Low (42) 12/9 0.86 0.33 to 2.28 15/9 1.52 0.59 to 3.94Formaldehyde:

Never exposed 194/211 1.00 118/211 1.00

Ever exposed 102/85 1.14 0.76 to 1.70 83/85 1.35 0.86 to 2.14Probability of exposure (%):

50 21/15 1.04 0.44 to 2.47 26/15 3.78 1.50 to 9.49Duration of exposure (y):

20 30/32 0.96 0.52 to 1.76 28/32 1.51 0.78 to 2.92

Cumulative level:Low (0.09) 29/30 0.87 0.45 to 1.67 28/30 1.51 0.74 to 3.10Flour dust:

Never exposed 274/282 1.00 180/282 1.00 Ever exposed 22/14 1.52 0.69 to 3.34 21/14 1.92 0.82 to 4.50

Probability of exposure (%):90 17/13 1.11 0.47 to 2.60 16/13 1.16 0.46 to 2.91Duration of exposure (y):3 13/8 1.15 0.43 to 1.28 13/8 1.53 0.53 to 4.44Cumulative level:

Low (20) 12/6 1.59 0.54 to 4.70 8/6 1.51 0.42 to 5.38

Textile dust:Never exposed 272/275 1.00 182/275 1.00 Ever exposed 24/21 1.22 0.61 to 2.43 19/21 1.63 0.74 to 3.60

Probability of exposure (%):90 16/11 1.41 0.58 to 3.44 9/11 1.21 0.39 to 3.72Duration of exposure (y):

7 9/14 0.74 0.28 to 1.95 8/14 1.26 0.43 to 3.65Cumulative level:

Low (0.3) 13/8 1.37 0.50 to 3.81 9/8 1.24 0.38 to 4.05

Coal dust:Never exposed 254/270 1.00 163/270 1.00 Ever exposed 42/26 1.67 0.92 to 3.02 38/26 2.31 1.21 to 4.40

Probability of exposure (%):90 20/13 1.75 0.75 to 4.08 19/13 3.12 1.22 to 8.01Duration of exposure (y):

20 17/12 1.52 0.64 to 3.58 11/12 2.37 0.86 to 6.52

Cumulative level:Low (25) 15/9 1.78 0.66 to 4.78 14/9 3.73 1.23 to 11.3

Silica dust**:Never exposed 173/181 1.00 113/181 1.00 Ever exposed 123/115 0.96 0.65 to 1.40 88/115 0.81 0.50 to 1.31

Probability of exposure (%):90 38/27 1.23 0.62 to 2.44 28/27 0.98 0.43 to 2.20

Duration of exposure (y):36 30/29 1.07 0.57 to 1.99 19/29 0.87 0.41 to 1.87

Cumulative level:Very low (0.100) 32/24 0.95 0.45 to 2.01 23/24 0.73 0.31 to 1.76

ORs adjusted for age, smoking, and alcohol (see text).*Also adjusted for exposure to formaldehyde (yes/no) and mineral fibres (yes/no) for the larynx, and for exposure to formaldehyde (yes/no) , mineral fibres (yes/no)and asbestos (yes/no) for the hypopharynx.Also adjusted for exposure to coal dust (yes/no) for the larynx, and for exposure to coal dust (yes/no) and asbestos (yes/no) for the hypopharynx.Also adjusted for exposure to formaldehyde (yes/no) for the larynx and hypopharynx.Also adjusted for exposure to formaldehyde (yes/no) and asbestos (yes/no) for the hypopharynx only.Also adjusted for exposure to formaldehyde (yes/no) for the larynx and hypopharynx.**Also adjusted for exposure to coal dust (yes/no) for the larynx, and for exposure to coal dust (yes/no), asbestos (yes/no), and education level (only primary schoolor more) for the hypopharynx.

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risk increased with cumulative level of expo-sure (p

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discordant as to laryngeal11 13 4345 and oropha-ryngeal and hypopharyngeal7 35 46 cancers.Similarly, although some authors have foundan increased risk of laryngeal or pharyngealcancers among textile workers,79 47 other stud-ies have failed to confirm these results.13 35 39 48

Moreover, most textile or wood workers areexposed not only to wood dust or textile dust,but also to numerous other substances, includ-ing solvents, varnishes, dyes, asbestos, and

formaldehyde.37 49 50 These other substancescan all be potential confounders in studiesbased on job titles.

Our study has several limitations. Controlswere recruited during a longer period than thecases, but controls diagnosed in 19878 weresimilar to those recruited in 198991 (period ofcases recruitment) for the number of jobs, thelevel of education, the consumption of alcoholand tobacco, and the frequency of occupationalexposures.

Interviewers were aware of case-controlstatus, which could introduce a bias. However,they were blind to the study hypotheses andcomparisons between interviewers concerning

the main characteristics of the subjects did notshow important diVerences. This suggests thatan interviewer bias is probably not a majorlimitation in this study.

The use of a cancer control group limits apossible recall bias. On the other hand, somecancer sites among controls may be associatedwith the occupational exposures under study. Asthe proportion of each cancer site was limited, itis unlikely that such associations seriously biasedthe results. Furthermore, the proportion ofexposed subjects did not diVer between the sites,for all the substances studied.

The occupational exposures were assessedwith a JEM, which could have entailed

non-diVerential misclassification and biasedthe ORs towards unity. Such non-diVerentialmisclassification may also be responsible forthe lack of a dose-response pattern in our data.Dosemici et al51 showed that in some situationsnon-diVerential misclassification when expo-sure is assessed by polytomous variables canbias the ORs estimated for intermediate levelsaway from the null and invert the slope of thedose-response trend. Furthermore, the fewsubjects with high levels of exposure may beresponsible for some negative results.

The few subjects exposed to some sub-stances also necessitated a broad categorisationof the confounders, but as a more stringentcontrol for age, alcohol, and smoking did notaVect the ORs substantially, there is probablyno important residual confounding.

In this study, we could not consider theimpact of some substances known or suspectedto be carcinogenic to the larynx and pharynxsuch as sulphuric acid and polycyclic aromatichydrocarbons. Future work will expand expo-sure assessment to other substances.

On the whole, our data suggest an associ-ation between hypopharyngeal cancer andexposure to formaldehyde and coal dust.Further epidemiological investigations are re-quired for confirmation.

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