Results

ATTENDANCE

A total of 73.9% women and 65.6% men completed the main questionnaire (total 71.3%) (table 1). The response rates were 48.9% (20–29 years), 69.7% (30–39 years), 80.6% (40–69 years), 76.6% (70–79 years) and 51% (80+ years).

NON-RESPONDER STUDY

Of 685 subjects randomly selected to the non-responder study, 326 subjects (47.6%) responded either to a telephone interview or the questionnaire. The most important reason for non-attending in age group 20–69 was lack of time/moved away (54%), while in those aged 70 years or more immobilising disease (21%) and follow up by medical doctor (28%) were important reasons.

The only significant difference in reported symptoms and diseases was lower prevalence of coughing in non-responders compared with responders, even though a significant higher prevalence of smoking was found in the non-responder group (table 2).

SMOKING HABITS

A total of 29.7% of men and 30.6% of women reported current smoking, while 32% of men and 19.7% of women were ex-smokers (table1). In age group 20–59 years smoking was more prevalent in women than in men (35.8% versus 31.0%, p<0.001), in contrast with subjects aged 60 or more (18.4% versus 26.5% in women and men respectively, p<0.001). On average men started to smoke at younger age than women (18.4 versus 19.8 years, p<0.001), and had higher average daily cigarette consumption (13.1 versus 9.7 cigarettes, p<0.001). In “ever smokers” the average number of pack years were 16.0 and 10.3 for men and women respectively (p<0.001).

SYMPTOMS AND RESPIRATORY DISEASE

Episodes of wheezing or breathlessness

In all, 12.7% of men and 12.1% of women reported episodes of wheezing or breathlessness during the past 12 months. Smokers reported these symptoms twice as prevalent as never smokers, and the prevalence increased by smoking burden (p for trend<0.001) (table 3, fig 1). Among never smokers more women than men (9.2% versus 8.6% when adjusted by age and BMI, p < 0.001), reported wheezing or breathlessness and the difference between sexes increased by increasing smoke burden (pack years) (table 4, fig1).

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Figure 1

Age adjusted prevalence (%) of self reported episodes of wheezing or breathlessness in the past 12 months and current asthma by number of pack years in men and women (95% confidence intervals). Nord-Trøndelag, Norway 1997.

Ever asthma and current asthma

Some 8.8% of men and 8.4% of women reported “ever asthma” (table 3), and 4.8% of men and 5.1% of women was categorised as having current asthma. The lowest cumulative prevalence of asthma was reported in the group aged 40–49 (7.9%) as compared with the groups aged 20–29 (9.3%, p<0.001) and 70–79 (9.6%, p<0.001). In those reporting ever asthma, 97.5% of women and 96.9% of men reported that they had been given the diagnosis by a medical doctor.

“Ever smoking” women reported current asthma more frequently than “ever smoking” men (6.4% versus 5.7%, p=0.005). A similar pattern was also found among current smokers (fig 1). The prevalence of current asthma increased with increasing number of cigarettes per day in women, amounting to 10.4% in those smoking more than 20 cigarettes per day. In men, however, no such increase with increasing cigarette consumption was found. With increasing smoke burden and current smoker's daily cigarette consumption, more women than men reported current asthma (table 4, fig 1).

Coughing, phlegm and chronic bronchitis

Persistent coughing was reported more than twice among smokers compared with never smokers throughout all age groups except in the oldest group of women. Men reported higher crude prevalence than women did in all cough related questions. With increasing smoking burden and daily cigarette consumption the prevalence of coughing increased, and even for this symptom we found higher susceptibility in women compared with men (table 4, fig 2).

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Figure 2

Age adjusted prevalence of persistent coughing, coughing with phlegm and chronic bronchitis by number of pack years in men and women (95% confidence intervals). Nord-Trøndelag, Norway 1997.

SMOKING CESSATION IN RELATION TO SYMPTOMS AND SEX

Among women who reported coughing, there was an increase of 40% that had quit smoking compared with those without symptoms (table 5). Among men, no such relation was seen. Among reporters of asthma related complaints, twice as many women and 1.7 as many men had quit smoking compared with those without respiratory symptoms. However, more “ever smoking” men than women had quit smoking, regardless of whether they reported respiratory symptoms at the screening or not. In subjects with current asthma 23.3% women and 39% men were ex-smokers and 38.5% women and 32.8% men were current smokers.

Discussion

In this population based cross sectional study we found higher prevalence of self reported asthma and use of asthma medication than previous studies in eastern Norway, including the city of Oslo 1972,12 Northern Sweden 1986,13 Southern Sweden 199214 and Denmark 1997.15 The reporting of “ever asthma”, ever having used asthma medication and current asthma had the lowest prevalence in the middle aged group (40–49 years). There was a strong association between tobacco smoking and respiratory symptoms, and smokers reported symptoms more than twice as frequent compared with never smokers. With increasing smoking burden, more women reported symptoms such as episodes of wheezing or breathlessness, persistent coughing, in addition to current asthma than men did. In a cross sectional study, however, causation cannot be directly inferred.

The questions of asthma, asthma medication and coughing used in this study were similar to those used by others.16 17 Unlike others, we combined the question of wheezing and breathlessness in one question, limiting the comparison of these symptoms with other studies.

PARTICIPANTS AND QUESTIONS

The participation rate was about 10% lower in all age groups compared with a similar health survey in 1984–86,18 with lowest rates in younger age groups, especially among men. Some people are sceptical of such studies and actively choose not to participate. This could explain that only about 50% answered the non-responder study. Other reasons for non-attendants in this survey were time consuming examinations and absence from work. A high prevalence of smokers in non-responders or late responders compared with early responders is in accordance with other studies where smoking habits have been examined.18-20 Equal distributions of respiratory symptoms do not preclude non-responder bias,21but we have no reason to believe that our study overestimates the prevalence of respiratory symptoms and diseases.

RESPIRATORY SYMPTOMS AND DISEASES BY SEX

The study confirms a clear relation between tobacco smoking and respiratory symptoms like wheezing, breathlessness and coughing.22-25

More women reported respiratory symptoms compared with men in never smokers and smokers with comparable smoke burden or number of cigarettes. Generally, women seem to have lower threshold for reporting such symptoms compared with men.24 26 It has however, been proposed that respiratory symptoms are more sensitive markers of change in general health, but less specific markers regarding respiratory diseases in women compared with men.1 This is confirmed by studies showing that respiratory symptom as shortness of breath is equally predictive of mortality in both sexes.27 28 Although, this does not explain the increasing sex difference with increasing smoke burden.

Regarding coughing with phlegm, no sex difference by increasing smoke burden was reported. This symptom is probably underestimated in women as they are less likely to report phlegm than men.1

Higher prevalence of respiratory symptoms and current asthma in women compared with men with the same smoke burden or daily cigarette consumption indicate that women are more susceptible to tobacco smoking than men. Cyclical hormonal variations1 and possible anti-oestrogenic effect of smoking29 have been discussed as reasons. Given the same degree of inhalation per cigarette, female airways probably are exposed to higher relative concentration of gases and particles compared with men. In Copenhagen comparable sex difference in smoking effect was found both on lung function and risk of hospitalisation for patients with COPD.8 Higher prevalence of coughing as well as wheezing and breathlessness attributable to tobacco smoking in women than men could indicate future increase of COPD, especially among women.

DISEASE AND SYMPTOM PREVALENCE

There is a wide geographical variation in prevalence of asthma and use of asthma medication.30 The prevalence of self reported asthma and use of asthma medication was higher than found in earlier Scandinavian studies12 13 15 31 and in crude agreement with more recent studies.14 32 This could reflect an increasing incidence of asthma, but also increased focus on asthma and asthma treatment in Norway. A high prevalence of asthma in the group aged 20–29, consistent with other studies,14could support the hypothesis of increasing asthma incidence in younger age cohorts, but this is under debate.33

Lack of increase in prevalence of “ever asthma” or current asthma with age in never smokers, are in agreement with the ECHRS study30 and a study from Southern Sweden.14In a chronic disease like asthma, one could expect increase in cumulative prevalence. The absence of this age effect could partly be explained by increasing recall bias with age, low symptom perception, and under-diagnosis of asthma in elderly.34 35 A change of diagnostic criteria during the past decades could also mask an increasing prevalence with age.

SMOKING AND SMOKING CESSATION

According to the WHO the overall smoking prevalence in Norway is 35.5% in women and 36.4% in men, and only Danish women have higher proportion of smoking than Norwegian women have. Norwegian men are ranked 62 in the world.36 We found that men reported smoking cessation more frequently than women did, regardless of reporting respiratory symptoms at the screening or not.

Even though women feel more pressure from family and society to quit smoking, and are equally likely to quit smoking compared with men, they are less likely to remain abstinent than men are. One important problem in motivating women to cease smoking is that they more probably view weight control as a benefit of smoking.37-39

A higher prevalence of asthma among ex-smokers compared with smokers have been found in several studies.40 41 We confirmed this among men, but not among women. There was a similar sex difference in reported use of asthma medication. These findings might support the view that mostly men without asthma-like symptoms go on smoking (the healthy smoker effect).42 A previous study has indicated a higher tendency for smoking men to be given the diagnosis of chronic obstructive lung disease compared with women.43 This study could not identify differences in use of diagnosis as chronic bronchitis and asthma between the sexes.

In conclusion, the respiratory symptoms were clearly associated with tobacco smoking. At the same smoking burden, more women reported respiratory symptoms and current asthma than men did. Even if symptoms seemed to motivate more women than men to cease smoking, more men had quit smoking regardless of whether they had or did not have respiratory symptoms. High prevalence of smoking in young women combined with a possible increased susceptibility to tobacco smoking compared with men, might contribute to future increase in serious smoke related diseases in Norwegian women. To prevent this scenario it is necessary to develop programmes that encourage non-smoking in adolescents and effectively aid adults in ceasing to smoke. Different strategies seem to be necessary for men and women.44