Respiratory symptoms and occupation: a cross-sectional study of the general population.

BACKGROUND: This study focused on respiratory symptoms due to occupational exposures in a contemporary general population cohort. Subjects were from the Dutch Monitoring Project on Risk Factors for Chronic Diseases (MORGEN). The composition of this population enabled estimation of respiratory risks due to occupation from the recent past for both men and women.
METHODS: The study subjects (aged 20-59) were all inhabitants of Doetinchem, a small industrial town, and came from a survey of a random sample of 1104 persons conducted in 1993. A total of 274 cases with respiratory symptoms (subdivided in asthma and bronchitis symptoms) and 274 controls without symptoms were matched for age and sex. Relations between industry and occupation and respiratory symptoms were explored and adjusted for smoking habits and social economic status.
RESULTS: Employment in the 'construction' (OR = 3.38; 95%CI 1.02 - 11.27), 'metal' (OR = 3.17; 95%CI 0. 98 - 10.28), 'rubber, plastics and synthetics' (OR = 6.52; 95%CI 1.26 - 53.80), and 'printing' industry (OR = 3.96; 95%CI 0.85 - 18.48) were positively associated with chronic bronchitis symptoms. In addition, the 'metal' industry was found to be weakly associated with asthma symptoms (OR = 2.59; 95%CI 0.87 - 7.69). Duration of employment within these industries was also positively associated with respiratory symptoms.
CONCLUSION: Respiratory symptoms in the general population are traceable to employment in particular industries even in a contemporary cohort with relatively young individuals.

Introduction

Respiratory diseases such as asthma, bronchitis and emphysema form a major health problem in the general population in many western countries [1,2]. The evidence that these diseases are caused by exogenous factors other than tobacco smoke, and that occupational exposures are amongst these causes, is growing [3]. As the working population smokes less, the relative importance of occupational exposures causing respiratory diseases is likely to increase. The contribution of occupational exposure to respiratory diseases has been estimated at 11–19% for males and 4–5% for females [1,2,4,5].

The relationships between occupation and occupational exposures and respiratory symptoms in community-based studies have been studied since the late 1970s. In many of these general population studies, an association with exposure to dust, gases and fumes has been found with odds ratios ranging from 1.3 to 2.5 for exposed versus non-exposed workers [4-14]. However, working conditions are continually changing, as are the characteristics of the workforce itself. Therefore, it is questionable whether the known results of the older population-based studies reflect the respiratory health risks in contemporary workforces. Hence, recent population based studies have shown that some not previously identified occupations and occupational exposures are possibly associated with asthma [2,5,15-17].

The aim of this study was to investigate relations between occupations and respiratory symptoms in a cross sectional study of the general population. The composition of the population, both male and female and its relatively young age could provide information on respiratory health risks due to occupational exposures from the present and recent past.

Material and methods

Data were collected as part of the Monitoring Project on Risk Factors for Chronic Diseases (MORGEN-Study). Its general purpose is to determine the prevalence of risk factors for chronic diseases as well as the prevalence of some specific chronic conditions in a general population sample.

Subjects

The present study is a nested case control study of the population of Doetinchem, one of the three cities included in the MORGEN study. Doetinchem is an old industrial town in the eastern part of the Netherlands with about 38,000 inhabitants.

A sample of 1104 subjects, aged 20–59 years were included. The study population originates from a random sample of the inhabitants of Doetinchem stratified by aged and sex drawn in 1987 who participated in an earlier survey (n = 1780). No inclusion or exclusion criteria were applied in the present study, but because a more extensive protocol had to be followed not everybody could be re-invited. Therefore a random sample of 1383 of this original study population was invited to participate in the present study. Of those, 1104 agreed to participate (response 80%). These subjects were asked to complete a self-administered shortened questionnaire on respiratory symptoms. The questionnaire contained also information on present smoking habits, socio-economic status (SES) and sectors of industry and occupation in which a respondent worked for more than one year. Asthma was defined as a positive answer to at least one of the following questions: 'Have you had an attack of shortness of breath while wheezing at any time in the last 12 months', 'Have you ever had an attack of asthma?'. Bronchitis was defined as a positive answer to at least one of the following questions: 'Do you cough daily for more than three months a year?', 'Do you bring up phlegm daily for more than three months a year?', 'Have you had episodes in the last three years in which you coughed and brought up phlegm which lasted for more than three weeks?', and 'Have you had attacks of shortness of breath while walking on a flat terrain at normal speed with other people?'.

A total of 274 subjects (24.8%) who reported one or more asthma or bronchitis related symptoms in the shortened questionnaire on respiratory symptoms were selected from the original sample (symptomatic). As controls, 274 subjects were selected among the 688 subjects, who reported no respiratory symptoms at all (asymptomatic) and were frequency matched for age (+/- 1 year) and sex at group level. The symptomatic sub sample was subsequently subdivided into two groups with symptoms of bronchitis (including chronic cough and chronic phlegm and shortness of breath) and asthma (including ever asthma, and wheezing), respectively.

Coding of industry and occupation

Information on type of industry (2-digits) and occupation (3-digits) was coded blindly by one of the authors (RV) based on the coding scheme by Hoar et al. [18]. When a specific company name was mentioned in the self-administered questionnaire additional information was gathered to confirm the classification of this industry from the local occupational health service, Chamber of Commerce and the local authorities.

Statistical analysis

The relation between occupation, based both on the complete occupational history and longest held occupation, and asthma and bronchitis symptoms was initially studied bivariately using the Cochran-Mantzel-Haenszel test. Associations between respiratory symptoms as outcome variables and risk factors such as 'industry', and 'occupation' were further investigated by means of multiple logistic regression analysis adjusting for SES and smoking. In the regression analyses the prevalence of respiratory symptoms of subjects who ever worked in a particular industry was contrasted to the prevalence of respiratory symptoms of subjects with exclusive employment in the category "occupations with few chemical exposures" (n = 97). This group comprises occupations such as business, law, communications, sales, etc. Smokers were categorised as never, ex and current smokers. SES was measured as a variable with three categories (high, intermediate and low) based on the status of the occupation of the subject. Analyses that involved only a part of the population and consequently departed from the matching criteria were also adjusted for age and sex.

Results

Table 1 gives an overview of the general characteristics of the study population for the symptomatic and asymptomatic group. Of the 548 cases and controls, 547 subjects reported a complete occupational history. One person refused to report his occupational history and was therefore excluded from the analyses. The population consisted of 47% men and 53% women. Of men and women, 38.5% and 28.6%, respectively, were current smokers and 38.5% and 33.1% were former smokers at the time of the survey. Of the 547 participants only 20 (3.7%) subjects never had a job. The mean number of jobs held by the subjects was 2.5 in 1.7 sectors of industry over an average working life of 20.8 (s.d. 12.0) years.

Table 1

General characteristics of the study population (n = 547)

		SYMPTOMATIC GROUP (n = 273)		ASYMPTOMATIC GROUP (n = 274)
		n	%	n	%
Smoking	never	70	25.6	100	36.5
	current	120	44.0	62	22.6
	ex	83	30.4	112	40.9
SES	low	174	63.7	156	56.9
	intermediate	57	20.8	54	19.7
	high	42	15.4	64	23.1
Sex	male	128	46.9	129	47.1
	female	145	53.1	145	52.9
Age	years (s.d.)	48.1 (10.4)		48.2 (10.5)
Job duration	years (s.d.)	20.5 (12.1)		21 (12.0)

Table 2 shows the distribution of the population over the different sectors of industry for the complete job history and the longest held occupation. The distribution over the sectors of industry was very similar for the complete job history and the longest held job. Initial exploratory analyses were based on the longest held occupation (Table 3). Increased risks for bronchitis symptoms were observed for the 'construction' (OR = 3.38; 95% CI 1.02 – 11.27), 'metal' (OR = 3.17; 95% CI 0.98 – 10.28), 'rubber, plastics and synthetics' (OR = 6.52; 95% CI 1.26 – 53.80), and 'printing' industry (OR = 3.96; 95% CI 0.85 – 18.48). The 'metal' industry was also weakly associated with asthma symptoms (OR = 2.95; 95% CI 0.77 – 11.23).

Table 2

Distribution of the symptomatic and asymptomatic population by industry based on total and longest held occupation.

	Complete occupational history			Longest held occupation
Industry	Symptomatic (n)	Asymptomatic (n)	Total1(%)	Symptomatic (n)	Asymptomatic (n)	Total2(%)
Agriculture, forestry, fishing	13	18	3.3	4	8	2.2
Construction	25	17	4.4	18	10	5.1
Paper and wood	19	27	4.9	9	12	3.8
Glass, clay and stone	7	2	1.0	3	0	0.5
Metal	35	22	6.0	19	8	4.9
Machinery	25	24	5.2	14	15	5.3
Shipbuilding, motor vehicles, aircraft and other transportation methods	22	24	4.9	7	9	2.9
Food and tobacco	45	40	9.0	18	16	6.2
Textiles	33	32	6.9	18	18	6.6
Chemicals, drugs and paints	9	11	2.1	3	7	1.8
Rubber, plastics and synthetics	16	9	2.6	7	3	1.8
Fuel	3	6	1.0	2	3	0.9
Leather	7	8	1.6	3	4	1.3
Medicine and science	42	59	10.7	36	40	13.9
Entertainment and recreation	10	11	2.2	7	6	2.4
Printing	22	11	3.5	10	4	2.6
Occupations with few chemical exposures	126	147	28.8	81	101	33.3
Never employed	10	10	2.1	10	10	3.7

1 calculated as proportion of the total number of jobs (n = 957); 2 calculated as proportion of the total number of longest held occupations (n = 547)

Table 3

Relationships between asthma, and bronchitis symptoms and longest held occupation in a particular industry adjusted for age, sex, smoking and SES expressed as Odds Ratios (OR) with 95% confidence interval in parenthesis (95% CI) 1.

Industry	Asthma symptoms OR (95% CI)	Bronchitis symptoms OR (95% CI)
Agriculture, forestry, fishing	0.24 (0.07 – 2.38)	0.58 (0.14 – 2.43)
Construction	1.27 (0.28 – 5.81)	3.38 (1.02 – 11.27)*
Paper and wood	1.01 (0.28 – 3.67)	0.78 (0.24 – 2.57)
Glass, clay and stone	NC	NC
Metal	2.95 (0.77 – 11.23)#	3.17 (0.98 – 10.28)#
Machinery	0.99 (0.25 – 3.94)	1.58 (0.52 – 4.86)
Shipbuilding, motor vehicles, aircraft and other transportation methods	0.45 (0.08 – 2.53)	0.70 (0.20 – 2.40)
Food and tobacco	0.79 (0.24 – 2.55)	1.08 (0.41 – 2.84)
Textiles	0.51 (0.14 – 1.78)	1.02 (0.40 – 2.60)
Chemicals, drugs and paints	0.41 (0.04 – 4.03)	0.57 (0.12 – 2.64)
Rubber, plastics and synthetics	4.44 (0.69 – 28.59)	6.52 (1.26 – 53.80)*
Fuel	1.01 (0.08 – 13.19)	1.11 (0.14 – 9.07)
Leather	1.08 (0.17 – 7.04)	0.51 (0.08 – 3.35)
Medicine and science	0.81 (0.31 – 2.08)	0.73 (0.35 – 1.52)
Entertainment and recreation	0.82 (0.16 – 4.25)	1.26 (0.32 – 4.96)
Printing	1.30 (0.15 – 11.32)	3.96 (0.85 – 18.48) #

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