Chronic Obstructive Pulmonary Disease in Finland: Prevalence and Risk Factors

Abstract

Chronic obstructive pulmonary disease (COPD) is one of the leading causes of morbidity and mortality worldwide. In Finland, the current prevalence of COPD is unknown. This population-based study aimed to assess the prevalence of COPD in Finland in adults 21–70 years of age by using internationally accepted criteria for COPD, and to evaluate risk factors for COPD. A random sample (n = 959) of questionnaire responders was invited to participate in a clinical study, which included a structured interview, skin-prick tests, and spirometry. Complete data were received from 683 subjects. According to the criteria of the British Thoracic Society and the Global Initiative for Chronic Obstructive Lung Disease, the prevalence of COPD was 5.4% and 9.4% respectively. The risk of COPD increased significantly with age, amount of smoking, and family history of obstructive airways disease, but not with skin-prick positive allergy, recreational skiing, or outdoor work. The smoking-adjusted risk of clinically relevant COPD did not differ significantly between men and women. Among subjects with a smoking history of more than 40 pack-years, the prevalence of COPD was over 50%. Early start age of smoking was a strong risk factor.

Key words: :

• COPD
• Epidemiology
• Spirometry

Introduction

According to common opinion, chronic obstructive pulmonary disease (COPD) is associated with smoking in 80–90% of cases, but only 10–20% of smokers develop clinically significant COPD [1]. Pharmacological treatment may relieve symptoms of COPD [2&3], but does not improve prognosis or slow down the decline of ventilatory function [4&5].

The vast burden of COPD has recently been recognized globally [6-8]. Standards for the diagnosis and care of patients with COPD have been updated [9], and international [10&11] and national [12-14] COPD programs and initiatives have been started. Recent studies have shown some progress in the treatment of COPD [15-17], and new epidemiological data are consistently emerging. The age-adjusted COPD mortality rate in the United States from 1979 to 1993 has increased by 47.3% [18]. In Europe, the morbidity and mortality rates vary substantially between countries, but according to a recent review [19], 4–6% of the adult population in Europe suffers from clinically relevant COPD. A population-based multi-center study in Spain [20] reported that the prevalence of COPD in ever-smoking men in the age group 40–49 years was 8.6% and 30.6% in the age group 60–69 years. The corresponding prevalence rates in ever-smoking women were 4.3% and 6.1%. The majority of COPD cases had not been diagnosed previously. The prevalence of COPD, estimated in the United States and in Europe by using age-specific and smoking-specific rates, revealed that in the United States only 14–46% of all COPD cases had been formerly diagnosed [21].

In Finland, the prevalence of emphysema with airflow limitation, reported over 40 years ago, was 8.7% in men and 3.7% in women [22]. Another pioneer Finnish study found the prevalence of emphysema (with FEV₁/FVC ratio < 0.70) to be 10.0% among men and 2.3% among women aged 40–64 years [23]. Knowledge about the current COPD prevalence in Finland is, however, lacking. A report did assess the prevalence of COPD among elderly people [24], but the criteria used did not conform to published guidelines [11-14]. In a recent Finnish study among a general adult population aged 30 years and older, airways obstruction defined as FEV₁/FVC ≤ 0.69 was found in 11% of men and 5% of women [25].

The main aim of this study was to assess the prevalence of COPD in a general adult population in Northern Finland by using the Global Initiative for Chronic Obstructive Lung Disease (GOLD) [11] and the British Thoracic Society (BTS) [12&13] criteria. The associations between COPD and age, gender, smoking as cumulative life-time pack-years number, skin prick positive allergy to common allergens, outdoor work, regular cross-country skiing, passive smoking during childhood, patient-reported chronic bronchitis and emphysema, and family history of obstructive airways disease (OAD) were also evaluated.

Methods

Study Area and Population

During the winter 1995–96 a postal questionnaire study was conducted in Lapland, the northernmost province of Finland. A random sample of 7,937 subjects aged 20–69 years was invited to participate in a postal questionnaire study; 6,633 responded. The results of this study have been reported [26-30]. Of the responders in the southern part of the study area (n = 3420) a random sample of 959 subjects was invited to a clinical study. This study included a structured interview, skin-prick tests, and pulmonary function measurements. At the time of the clinical study, the participants were 21–70 years of age.

Questionnaire and Structured Interview

Both the Finnish FinEsS questionnaire [26][31] and FinEsS interview used in this study were developed from the OLIN questionnaire [32], and contain questions originating from the BMRC [33], IUATLD [34] and ECRHS questionnaires [35]. Pack-years data for the clinical study participants was taken from the interview, while all other data about allergy, heredity, respiratory symptoms, asthma, chronic bronchitis, symptoms under special circumstances, use of asthma medication, profession, outdoor work, and skiing habits were obtained from the postal questionnaire [26]. For example, working conditions were assessed by the question “Are you nowadays working mainly at a) outdoor or b) indoor work?,” and skiing habits by the question “Are You, or have You been a regular recreational skier?”

Lung Function Tests

Spirometric measurements, using a flow-volume spirometer (Sensor-medics VMAX 22), included flow-volume spirometry, a bronchodilation test, and slow vital capacity. At least three successive measurements were performed according to the following American Thoracic Society (ATS) standards [36]:

• exhalation time at least 6 seconds,

• the largest FVC and the largest FEV₁ may come from different curves, but all other parameters from the curve that has largest sum of FVC plus FEV₁ (best curve method),

• the two largest FVC within 200 ml of each other,

• the two largest FEV₁ within 200 ml of each other.

The Finnish reference values were used [37]. The spirometer was calibrated daily. Nose clips and standard bacterial filters were used. Bronchodilation was assessed 10 minutes after inhalation of salbutamol 2 × 200 mcg (Ventoline MDI 200 mcg/puff with Volumatic, Glaxo Pharmaceuticals).

Skin Prick Tests

The standard duplicate method [38] was used to test skin prick reactivity to 12 allergens: Dermatophagoides pteronyssinus, Acarus siro, cat, dog, horse, cow, birch, timothy, mugwort, Cladosporium herbarum, latex, and reindeer. A positive (histamine HCL, 10 mg/ml, in 50% glycerol) and a negative control (50% glycerol) were included. The reactions were read 15 minutes after application of allergens on the forearm. A reaction was considered positive if the mean wheal diameter ([d1 + d2]/2) was at least 3 mm.

Definitions and Diagnostic Criteria

Airway obstruction in COPD is defined as FEV₁/VC < 0.7 by BTS and FEV₁/FVC < 0.7 by GOLD criteria, post-bronchodilator results. The BTS definition of COPD states: “Unlike asthma, airflow limitation in COPD as measured by the FEV₁can never be returned to normal values.” Thus only those who also had post-bronchodilator FEV₁< 80% of predicted were included in BTS COPD cases. According to the BTS criteria, FEV₁ ranges (of predicted) in mild disease from 50% to 80%, in moderate from 30% to 49% and in severe COPD FEV₁ is below 30%, whereas GOLD considers FEV₁≥ 80% as mild COPD, 50% ≤ FEV₁< 80% as moderate, 30% ≤ FEV₁< 50% as severe, and FEV₁< 30% as very severe.

In order to detect clinically relevant COPD cases among subjects with a smoking history ≤ 20 pack-years, they were classified with the following arbitrary criteria:

1. other respiratory conditions as they were reported

2. asthmatics: those who had BOTH a self-reported asthma or had a positive skin-prick test AND had reported attacks of respiratory obstruction or wheezing.

3. subjects with clinically relevant COPD: those who had nothing listed above but had post-bronchodilator FEV1 < 80% of predicted

4. chronic bronchitis: those who had nothing listed above but had reported chronic cough or chronic productive cough

5. healthy if subject had nothing listed above

Analyses

To distinguish COPD from pure asthma, the following adjustment was performed: those who had both physician-diagnosed asthma and smoking history of less than 10 pack-years were not included in the COPD group. Chi-square test was used for bi-variate comparisons. One-way ANOVA (analysis of variance) was used to test trends. Odds ratios (OR) for various independent variables were calculated using multiple logistic regression analysis. Variables included gender, age, smoking habits, allergy, family history of OAD, smoking parent, outdoor work, and recreational skiing. An interaction analysis between age, smoking, and family history of OAD was performed by using an additive model with multiple logistic regression analysis.

Results

Participation and Smoking Habits

Responses to the questionnaire study and participation in the clinical study with lung function results and smoking habits by age and gender are shown in Table 1. A total of 959 subjects were invited to the clinical study; 695 attended. Spirometry was repeated for 63 subjects, 51 of them were accepted after repetition, but for 12 subjects the result did not meet the criteria and they were excluded from the final analysis; thus 683 subjects (71.2% of invited) remained in the study sample. The total participation rate of the clinical study was 60.1% (0.844 × 0.712). Men in the youngest age group had the lowest participation rate both in the questionnaire and in the clinical study.

Table 1.  Response, participation, and smoking habits in the questionnaire and clinical study, and accepted spirometries by age and gender

Age cohort, years	20–29		30–39		40–49		50–59		60–69		20–69	20–69	all
	m	f	m	f	m	f	m	f	m	f	m	f	m + f
Questionnaire
Forms sent	441	344	461	449	557	504	365	365	293	305	2087	1967	4054
Forms returned	303	282	371	387	441	441	313	338	264	280	1692	1728	3420
Respondents %	68.7	82.0	80.5	86.2	79.2	87.5	85.8	92.6	90.1	91.8	81.1	87.8	84.4
non-smokers (%)	43.6	50.3	41.0	43.9	32.9	54.1	33.0	67.9	34.3	73.9	36.9	56.9	47.2
ex-smokers (%)	8.2	8.7	12.4	17.5	23.1	15.6	34.9	10.6	34.1	9.7	21.6	12.9	17.2
Current smokers (%)	48.2	41.0	46.6	38.7	44.0	30.3	32.1	21.6	31.7	16.4	41.4	30.2	35.6
Clinical study
Invited	73	71	91	88	117	128	97	97	102	95	480	479	959
Attended	31	39	58	69	90	103	74	83	83	65	336	359	695
Accepted spirometry	31	38	58	67	87	103	71	83	80	65	327	356	683
% of invited	42	54	64	76	74	80	73	86	78	68	68	74	71
non-smokers (%)	29	53	38	49	29	54	39	65	34	80	34	61	48
ex-smokers (%)	16	8	17	15	29	17	33	13	36	9	28	13	20
Current smokers (%)	55	39	45	36	42	29	28	22	30	11	38	26	32

(m=males, f=females).

Distribution of age, gender, asthma, chronic bronchitis, smoking habits, and respiratory symptoms among the questionnaire respondents (n = 3420), those who were invited (n = 959), or those who participated (n = 683) in the clinical study are shown in Table 2. The proportion of ex-smokers was higher among the clinical study participants than among the questionnaire responders or the invited subjects. Differences in respiratory diseases and symptoms between the clinical study participants and the questionnaire respondents or the invited persons were, however, small and not significant.

Table 2.  Age, gender, respiratory conditions, and smoking habits (% of group) among postal questionnaire respondents (n=3420), subjects invited to the clinical study (n=959), and participants with accepted lung function (n=683).

	Physician-diagnosed asthma	Physician-diagnosed chronic bronchitis	Using asthma medication	Attacks of SOB/last 12 months	Chronic productive cough	Recurrent wheeze	Dyspnea grade II	Non-smokers	Ex-smokers	Current smokers	Mean age (years)	Men
Clinical study participants	6.3	3.7	7.0	12.9	13.9	7.0	14.1	47.7	20.4	31.9	45.7	47.9
Questionnaire respondents	5.8	3.4	6.1	11.6	11.5	6.7	14.6	46.9	17.5	35.6	44.1	49.5
Subjects invited to clinical study	6.2	3.3	6.3	11.8	12.5	7.5	14.3	45.9	18.5	35.7	44.2	50.1

No significant differences between respondents and participants or between respondents and those invited were found by chi-square or rank test.

SOB=shortness of breath.

Among the clinical study participants, male current smokers (n = 112) had an average smoking history of 20.8 pack-years, and ex-smokers (n = 138) 17.7 pack-years. The corresponding figures for females were 11.7 (n = 95) and 6.9 (n = 75) pack-years. The average smoking history for ever-smoking (ex- or current smokers) men was 19.1 pack-years, and for women 9.6 pack-years.

Prevalence of COPD and Severity Classification

The overall prevalence of COPD was 5.4% by BTS criteria and 9.4% by GOLD criteria. In men, it was 8.6% according to BTS and 15.6% according to GOLD criteria, and in women 2.5% and 3.7%, respectively (Table 3). Adjustment of prevalence by excluding those with both physician-diagnosed asthma and smoking history of less than 10 pack-years had only a minor effect on prevalence (Figure 1). The GOLD criteria classified 50% of the COPD cases as mild, 44% as moderate, and 6% as severe.

Table 3.  COPD cases in the study population (n=683) by BTS or GOLD criteria and pack-years classification.

Pack-y category	Number	Mean age	Mean pack-y	COPD by BTS criteria		COPD by GOLD criteria
				Cases (n)	Prevalence (%)	Cases (n)	Prevalence (%)
Men
≤ 1	102	46.7	0.09	2	2.0	10	9.8
1 < – ≤ 20	128	44.6	9.9	2	1.6	4	3.1
20 < – ≤ 40	67	54.5	27.7	8	11.9	17	25.4
>20	97	56.6	36.1	24	24.7	37	38.1
>40	30	61.4	54.8	16	53.3	20	66.7
all	327	48.8	14.6	28	8.6	51	15.6
Women
≤ 1	217	48.6	0.06	3	1.4	7	3.2
1 < – ≤ 20	120	43.3	8.9	2	1.7	2	1.7
20 < – ≤ 40	17	50.4	26.7	3	17.6	3	17.6
>20	19	52.4	28.6	4	21.1	4	21.1
>40	2	69.0	44.5	1	50.0	1	50.0
all	356	47.0	4.6	9	2.5	13	3.7
All
≤ 1	319	48.0	0.07	5	1.7	17	5.3
1 < – ≤ 20	248	44.0	9.4	4	1.6	6	2.4
20 < – ≤ 40	84	53.7	27.5	11	13.1	20	23.8
>20	116	56.0	34.8	28	24.1	41	35.3
>40	32	61.9	54.1	17	53.1	21	65.6
all	683	47.9	9.4	37	5.4	64	9.4

The prevalence of COPD correlated significantly (p < 0.001, test for trend) with the number of pack-years.

Persons having both physician-diagnosed asthma and pack-years < 10 were excluded. (pack-y=pack-years).

Figure 1 Prevalence (%) of COPD in men and women by the BTS and the GOLD criteria. Note: in every pair all cases included versus cases with asthma and < 10 pack-years excluded, the difference was not statistically significant.

Effect of Smoking Habits on Prevalence of COPD

The prevalence of COPD increased with age and number of pack-years. Among subjects who were lifelong non-smokers or had a smoking history of 1 pack-year or less, the prevalence of COPD was 1.7% according to BTS and 5.3% according to GOLD criteria; but among those with a smoking history of more than 20 pack-years the respective prevalence was 24.1% and 35.3% (Table 3).

COPD was associated with a smoking history of at least one pack-year in 75% of GOLD cases and in 86% of BTS cases. By GOLD criteria, the proportion of non-smokers was 41% among the mild COPD cases and 16% among moderate cases. In mild COPD as defined by GOLD criteria, the mean amount of smoking was 18.8 pack-years. The corresponding figures in moderate and severe COPD according to GOLD criteria were 31.6 pack-years and 71.5 pack-years.

Of the 567 subjects with a smoking history of ≤ 20 pack-years, 27 had spirometric obstruction. Nine of them fulfilled the BTS criteria for COPD and 22 fulfilled the GOLD criteria. The analysis of these 27 subjects revealed that according to our arbitrary criteria:

• all 7 healthy subjects were classified as healthy by BTS, but GOLD labeled all of them as having COPD

• both BTS and GOLD found 5 of the 8 subjects having COPD

• of 8 subjects having asthma, 3 were classified as COPD by BTS and 6 by GOLD

• of the 9 cases having COPD by the BTS criteria, 4 might be false positives (3 asthmatics and 1 sequelae after pulmonary tuberculosis), but of the 22 cases having COPD by the GOLD criteria, only 5 may have a clinically relevant COPD.

Multivariate Relationships and Interaction Analysis

According to multiple logistic regression analysis, the odds ratios for COPD increased significantly with age and amount of smoking (Table 4). COPD was more prevalent among men than women. However, when adjusted for age and smoking, the risk of clinically relevant COPD (BTS criteria) did not differ significantly between men and women. Family history of obstructive airways disease was associated with a significantly higher risk for COPD. Positive skin-prick test, smoking parents, outdoor work, and recreational cross-country skiing did not affect the odds ratios significantly.

Table 4.  Risk factors for COPD by BTS or GOLD criteria calculated by multiple logistic regression analysis

Independent variables	Categories	Dependent variables
		COPD by BTS			COPD by GOLD
		OR	95% CI	p-value	OR	95% CI	p-value
Sex	Male	1			1
	Female	1.08	0.41–2.83	ns	0.47	0.22–0.98	< 0.05
Age	OR increase per one year	1.10	1.05–1.15	< 0.001	1.13	1.09–1.17	< 0.001
Smoking	OR increase per one pack-year	1.07	1.04–1.09	< 0.001	1.06	1.05–1.08	< 0.001
Skin-prick positivity	No	1			1
	Yes	0.62	0.19–2.06	ns	1.59	0.60–4.19	ns
Family history of OAD	No	1			1
	Yes	3.07	1.32–7.16	< 0.01	3.77	1.86–7.65	< 0.001
Smoking parent	No	1			1
	Yes	0.82	0.36–1.87	ns	1.05	0.54–2.05	ns
Outdoor work	No	1			1
	Yes	0.89	0.24–3.33	ns	1.63	0.64–4.14	ns
Recreational skiing	No	1			1
	Yes	0.72	0.27–1.91	ns	0.64	0.29–1.42	ns

Odds ratios (OR), 95% confidence intervals (95% CI) and p-values given.

COPD cases with both physician-diagnosed asthma and pack-years < 10 were excluded.

OAD = obstructive airway disease, ns = not significant.

The probability of COPD by BTS and GOLD criteria in relation to age and family history of OAD was calculated for non-smokers and current smokers with the assumption that smoking 1 pack per day was started at the age of 18 years (Figure 2). After 50 years of age, the probability of COPD increased considerably in subjects who continued to smoke or had a family history of OAD. The probability at the age of 60 years was 2 times higher among continuing smokers with a family history of OAD than among those with no family history of OAD.

Figure 2 Probability of COPD in correlation to age, family history of obstructive airways disease, and number of pack-years.

A logistic regression analysis was performed among ever-smokers (all current and ex-smokers included), with COPD (by BTS criteria) as the dependent variable, and age, gender, and starting age of smoking as independent variables. A significant increase in the risk of COPD was present with earlier start of smoking (p = 0.0182). The risk of COPD increased by 14% per year (95% CI 2–28%), while the corresponding increase per pack-year was 7% (95% CI 4%–9%).

COPD in Relation to Patient-Reported Previous Diagnoses or Symptoms

The prevalence of COPD according to GOLD criteria was 27.9% among subjects who reported having asthma and 40.0% among those reporting physician-diagnosed chronic bronchitis or emphysema. According to BTS criteria, the corresponding figures were 23.3% and 32.0%. In subjects with chronic productive cough, the prevalence of COPD by GOLD criteria was 21.1%, and by BTS criteria 12.6%. Men generally had higher prevalence rates than women.

Only 15.6% of GOLD COPD cases and 21.6% of BTS COPD cases reported having physician-diagnosed chronic bronchitis or emphysema, and 31.2% and 32.4%, respectively, had chronic productive cough. Among COPD cases diagnosed by GOLD criteria, 64.1% had smoked more than 20 pack-years, and 26.6% had a smoking history of one pack-year or less. The corresponding figures for cases diagnosed by BTS criteria were 75.7% and 13.5%.

Discussion

The results revealed that age, family history of obstructive airways disease, and number of pack-years were independent risk factors for COPD, and their effect was additive. The prevalence of COPD in Northern Finland in the general population aged 21–70 years was 9.4% by the GOLD and 5.4% by the BTS criteria. These prevalence rates are in line with recent international studies and reviews [18-20][39][40][41]. Men had a higher prevalence of COPD than women, but according to multivariate analysis by the BTS criteria, the risk for COPD did not differ significantly between the sexes. The common opinion of higher risk of COPD among men may merely reflect the difference in smoking habits between men and women. In Finland, a population-based study assessing the prevalence of COPD in a general adult population was published about four decades ago [23], but little current data on this topic have been published. Moreover, divergence from internationally accepted COPD criteria weakens the validity of recent Finnish studies. In a study among elderly Finns, COPD was defined as FEV₁/FVC ratio ≤ 0.65 [24], which may be clinically relevant but is not in agreement with national or international guidelines [11-14].

In our study, BTS criteria found 37 COPD cases (28 men and 9 women), and GOLD criteria found 64 cases (51 men and 13 women). Low numbers in sub-groups weaken the precision and significance, therefore an extensive statistical analysis in Table 3 was considered not meaningful. The original random sample was 4054 subjects. Of those 3420 subjects responded in the postal questionnaire, 959 subjects were invited to the clinical study and results were obtained from 683 subjects. Thus these 683 participants represent 20.0% of those responding in the questionnaire or 16.8% of the original sample. We feel that our results are well representative for the original study population because, firstly, standard study methods and criteria were used; secondly, all population samples were randomised; thirdly, the participation rates were high: 84.4% in the questionnaire study and 71.2% in the clinical study; fourthly, the proportion of subjects with respiratory symptoms and conditions did not differ between the questionnaire and the clinical study; and finally, our separate non-response analysis [28] revealed no significant difference in the prevalence rates of respiratory diseases or symptoms in this study population. Thus, in spite of low numbers, prevalence rates shown in this report may apply in the general adult population in Northern Finland.

The cross-sectional design in our study could also bring about some limitations. For example, at the time of quitting smoking no data of atopy, lung function or symptoms are available and subjects with respiratory symptoms or allergy to pollen could have changed from working outdoor to indoor. Moreover, in our statistical analysis all pack-years were considered equal, but it may be argued that a certain number of pack-years is more harmful for a current smoker than for an ex-smoker. However, our interaction analysis for the risk of COPD (including both current and ex-smokers) revealed that the effect of an earlier start of smoking by one year, a 14% increase, was two times stronger than the effect of 1 pack-year.

The prevalence rates and distribution of severity of COPD in our study have a good correlation with recent studies in both Europe [42] and the United States [43]. The prevalence of COPD among smokers with a smoking history of more than 40 pack-years was 53% by BTS and 66% by GOLD criteria. Our results support recent findings of high prevalence of COPD among elderly smokers [21][42][44]. However, it should be kept in mind that both BTS and GOLD use a fixed cut-off value of 0.7 for FEV₁/VC or FEV1/FVC ratio, which may result in an overestimation of airway obstruction with advancing age [45].

We have previously reported a comparison of asthma, chronic bronchitis, and respiratory symptoms between Northern and Southern Finland [29]. According to those results, the prevalence of chronic productive cough was approximately the same in Northern (11.0%) and Southern (12.1%) Finland and the prevalence rates of asthma and chronic bronchitis did not differ significantly. Differences in smoking habits were small; in Northern Finland 36.6% of adults were current smokers vs. 37.7% in Southern Finland (not significant), for ever-smokers, the corresponding figures were 53.8% and 54.3% (not significant). Based on these findings and the homogeneity of the Finnish population, we assume that our findings of COPD in Northern Finland apply also to other parts of the country.

The BTS and GOLD guidelines consider COPD to be distinct from asthma, but the differentiation between COPD and chronic asthma with persistent airflow limitation is difficult. A consensus exists in international guidelines concerning the definition of COPD: “chronic obstructive pulmonary disease is a disease state characterized by airflow limitation that is not fully reversible. The airflow limitation is usually both progressive and associated with an abnormal inflammatory response of the lungs to noxious particles or gases” [11]. However, despite overall agreement, the wording used between different COPD guidelines varies, and more importantly, the limit values for spirometry results differ [11-14]. In practice, the cases classified as mild COPD by GOLD criteria are not labelled as COPD at all by BTS. This is an important point if early diagnosis and intervention are considered essential for COPD management.

On the other hand, among COPD cases found by the GOLD criteria, many false positives may be included. This was underlined by our sub-analysis among the COPD cases having a smoking history ≤ 20 pack-years. The results revealed that many asthmatics, subjects with other respiratory conditions and also apparently healthy subjects may be included among the COPD cases found by the GOLD criteria. In comparison to the other COPD criteria that include declined FEV₁, the GOLD criteria may more often include other conditions among the COPD cases, e.g., undiagnosed asthma. Thus the clinical significance and prognostic value of stage 1 (mild) and stage 0 (“at risk”) COPD by the GOLD criteria may be questioned [46].

We have earlier reported that the risk for chronic bronchitis and associated symptoms is higher among outdoor than indoor workers [29&30]. However, pack-year data were not available in these reports. The multivariate analysis in the present study shows that the risk for COPD increases with age, pack-years, and family history for OAD, but not with positive skin prick test, outdoor work, or cross-country skiing.

Less than 25% of the COPD cases had an earlier diagnosis of chronic bronchitis or emphysema. The prevalence of COPD in subjects with chronic productive cough was 16–28% in men and 6–9% in women. Two-thirds of the COPD cases did not report chronic productive cough. This is supported by a recent paper that revealed that only 12% of subjects reporting a diagnosis of chronic bronchitis met the criteria of chronic bronchitis [47]. These findings underline the diagnostic difficulties: many subjects with COPD do not report chronic symptoms and only a small proportion of subjects with chronic productive cough hve COPD. Women report less chronic productive cough than men, and thus under-diagnosis seems to be high in women, as reported by others [48].

It does not seem justified to assess the prevalence of COPD by using patient-reported diagnosis or respiratory symptoms alone as has been done in some studies [49]. Spirometric measurements have been recommended as the best means to find COPD cases at an early stage [50] and to reduce the risk of under-diagnosis and gender bias [48]. The problems with clinical practice guidelines targeting COPD have been recognised: the methodological quality is limited, and there are disparities among many of their recommendations [51]. Valid comparisons of COPD prevalence between different studies are not possible unless smoking data are given in pack-years and similar spirometric and diagnostic criteria are employed. Obviously, a more specified international consensus about COPD criteria is needed. The assessment of COPD prevalence should be based on both relevant patient information [47] and lung function measurements which pay attention to the normal distribution of lung function in different populations and age groups [45].

When smoking started earlier in life, the risk of COPD increased considerably. Young teenagers may have a higher biological susceptibility to harmful effects of tobacco smoke than adults. The probability of COPD increased very steeply after the age of 50 years in subjects with a smoking history of more than 30 pack-years who continued to smoke, and in those who had a positive family history of OAD and a smoking history of more than 20 pack-years. Thus, the development of new COPD cases could be effectively diminished if starting age of smoking increases and if most smokers quit before 20 pack-years.

In conclusion, a high prevalence of COPD was found in a general adult population of smoking men and women in Finland. Among those having smoked more than 20 pack-years, the prevalence of COPD was 24% by BTS and 35% by GOLD criteria. The risk for COPD rose with increasing age and number of pack-years, especially if smoking was started early in life. Also a family history of OAD was a risk factor for COPD. Although outdoor work in a cold climate may increase respiratory symptoms and chronic bronchitis, it seemed not to have an impact on the risk for COPD.

Acknowledgments

The authors thank Länsi-Pohja Central Hospital for assistance in data collection, and Statistician Anders Odén, Sweden, and Docent Matti Kataja, Tampere, Finland, for help in performing statistical analyses.