Higher occurrence of asthma-related symptoms in an urban than a suburban area in adults, but not in children

The prevalence of respiratory symptoms and asthma is increasing worldwide, especially in westernized countries. It therefore remains important to identify risk factors responsible for this increase. Studies comparing asthma prevalence in different countries and regions, such as the European Community Respiratory Health Survey (ECRHS) 1 in adults and the International Study of Asthma and Allergies in Childhood (ISAAC) 2, have identified large differences between countries. Higher prevalence rates of asthma and allergic disorders found in children 3 and adults 4 in cities in West compared to East Germany, have led to the suggestion that increasing prevalence is associated with a Western lifestyle. In Belgium, differences in occurrence of respiratory symptoms have recently been reported between young adults in an urban and suburban area 5, 6, that were of a similar magnitude to those found in the same age group between Hamburg and the formerly East-German city of Erfuhrt 4.

This study further investigated these urban-suburban differences in respiratory symptoms by repeating the survey in young adults and by extending it to other age groups. It was considered whether these differences would be age-related and reproducible, in a homogeneous Flemish population of subjects with an age range 5–75 yrs, living in a small geographical area.

Material and methods

Results

As shown in tables 1 and 2⇓⇓, for males and females, respectively, there were no consistent differences in occurrence rates of respiratory symptoms and asthma between the urban and suburban areas in the 5–8 and 12–15 yr old children, whereas these rates were consistently higher in urban Antwerp for both younger (20–44 yrs) and older adults (45–75 yrs). Boys aged 5–8 yrs had higher prevalence rates for respiratory symptoms than girls, but in 12–15 and 20–44 yr old subjects, sex differences were smaller, or even inverse, except for the occurrence of asthma. In the older study group, males again more often reported respiratory symptoms and asthma.

Comparisons of prevalence rates for respiratory symptoms and asthma in 20–44 yr old adults in both the 1991–1992 and 1996 surveys (fig. 1⇓) shows the urban-suburban differences to be of similar magnitude and significance (p<0.05), except for asthma in the last 12 months, in 1996. P‐values for the differences between 1991–1992 and 1996 were all below 0.05, except for nocturnal dyspnoea and nocturnal cough in urban Antwerp, and asthma in the last 12 months, where the p‐value>0.1. Furthermore, reported prevalence rates were on average 32% higher in 1996 than in 1991–92, for both areas.

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Fig. 1.—

Prevalence rates (95% condidence intervals) of respiratory symptoms and asthma in the last 12 months in 20–44 yr old subjects, living in urban (□) and suburban (└) Antwerp, as reported in two surveys: one in 1991–1992 and one in 1996. WD: wheezing with dyspnoea; WNC: wheezing without a cold; NT: nocturnal tightness; ND: nocturnal dyspnoea; NC: nocturnal coughing; 91: 1991–2 study; 96: 1996 study.

The response rate in the group aged 5–8 yrs was 86%, in the 12–15 yr olds group it reached 97%. In the sample of nonresponders in these study groups, occurrence rates were only slightly lower than in responders. For both the 20–44 and 45–75 yr old study groups, the response rates were markedly lower, 57%. In initial nonresponders, occurrence rates for wheezing and “ever having had asthma” in the 20–44 yr study group were lower than in responders (table 3⇓), but they were still higher in the urban area, and differences with the suburban area were of a similar magnitude. In the 45–75 yr group, rates of wheezing and “ever having had asthma” in initial nonresponders were quite similar to those in responders (table 3⇓). The increase in the occurrence of respiratory symptoms in adults observed between 1991–1992 and 1996 was not sensitive to various assumptions regarding symptoms in nonresponders. Table 4⇓ presents the estimated occurrence rates for the total population, based on the assumption that the occurrence in nonresponders would be half the rate of the responders.

Adjustment for the recorded potential risk factors hardly changed the associations between area and symptoms in the 5–8 and 12–15 yr old subjects (not shown). In the 20–44 and 45–75 yr study groups, adjustment weakened most associations (table 5⇓), but subjects living in urban Antwerp still had a higher risk for the occurrence of most respiratory symptoms and asthma. In the 20–44 yr age group, most associations were slightly stronger in females than in males, while in the older adults there was an inverse trend (i.e. stronger in males than females).

Crude associations between area of residence and wheezing in the last 12 months, within groups of increasing age, are shown in figure 2⇓ for males and females separately. The magnitude of the associations tended to be slightly larger in adults than in children.

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Fig. 2.—

Associations (expressed as odds ratios (OR's) on a logarithmic scale) for males (—) and females (- - -) between area of residence and wheezing in the last 12 months in several age groups. CI: confidence interval.

Discussion

The main finding of the study is the higher occurrence of asthma-related symptoms in the urban compared to the suburban area, in both younger (20–44 yrs) and older (45–75 yrs) adults, whereas such consistent differences were not found in 5–8 and 12–15 yr old children. Even after adjustment for recorded risk factors, younger and older adults living in urban Antwerp still had a higher risk for most respiratory symptoms. The 1996 survey in 20–44 yr old subjects confirmed the authors’ previous findings 1, 5, 6; not only were prevalence rates again higher in the urban area, but the differences between the areas were also of a similar magnitude and all recorded rates had on average increased by 32%, in both areas, between 1991 and 1996, a time-related increase similar to that found by others 10, 11.

Response rates, a critical issue in population-based surveys, were high in the two child groups and there were only minor differences in occurrence of symptoms between responders and initial nonresponders. In both older age groups, response rates were only 57% in 1996, but differences between areas were the same for responders and nonresponders, both in direction and magnitude, although most occurrence rates tended to be lower in the latter. Moreover, adjustment for the number of reminders hardly influenced the differences in symptom-reporting between the areas. The authors are therefore confident that, with respect to the area differences, response bias did not affect the results. Response rates in the 20–44 yr old adults in 1991–1992 were higher than in 1996, 75% versus 57% respectively. Sensitivity analyses, based on different assumptions of prevalence rates of nonresponders, showed that the lower response rate in 1996 may only partly explain the higher prevalence rates found in 1996.

This cross-sectional study retrospectively collected data on exposure and life-time occurrence of symptoms. Differences in occurrence rates between the different age groups could therefore result from exposures to different risk factors at different times (cohort effect). In the youngest group (5–8 yrs) information was given by their parents, while in all other age groups symptoms were self-reported. The exact phrasing of some questions also slightly differed between the childrens' and adults' questionnaires, especially between “wheezing apart from a cold” and “asthma in the last 12 months”. However, the urban-suburban comparison within age groups is not thought to be influenced by cohort effects, nor by the self- versus parental-reporting, nor by slightly different phrasing of questions.

To explore any possible bias resulting from slight differences in methods used, associations between area and respiratory symptoms were assessed in the different age groups within the study groups. A trend towards increased odds ratio was found for the associations between area and wheezing in the last 12 months, in adults, in comparison with children, in both males and females. This suggests that the area effects differ with age, and that they are consistently found in the age-groups of younger and older adults, thus supporting the validity of the present findings.

A gradient in asthma-related symptoms between urban and rural areas has already been reported in several studies in Africa 12, 13 and in comparisons in USA “inner cities” 14. However, a gradient in the occurrence of respiratory symptoms between neighbouring areas, only a few kilometres apart, with a homogeneous, and in the present case Flemish, population has not been reported before. Differences in genetic background cannot be expected to be responsible for the observed area differences; neither is it expected that these would be greatly confounded by differences in personal risk factors. Nonetheless, a number of determinants listed by Becklake and Ernst 15 were explored for possible confounding.

Age, smoking habits, asthma in siblings and response, partly explained the differences between urban and suburban Antwerp, but even after adjustment for these factors, adults living in urban Antwerp still had a higher risk for most respiratory symptoms than those in the suburban area. For estimation of social economic status (SES), the authors had to use as a proxy “age at completing full time education” in the 20–44 yr old study group. A confounding effect of SES seems unlikely since 35% and 37% of subjects in the urban and suburban areas, respectively, completed education be-fore or at age 18. In the same age group in 1991–1992, other personal risk factors, such as “having had a severe respiratory infection before the age of 5” and “smoking of the mother during pregnancy or childhood” as well as some housing factors (“an open fire for heating” and “moulds in the last 12 months”) failed to explain area differences in respiratory symptoms 5. No evidence could be found for a confounding effect of family size or having older brothers or sisters. Information regarding dietary risk factors 16, such as fruit, fish and salt intake, was only available in the 5–8 and 12–15 yr old children, but no relevant differences in such habits were recorded between the urban and suburban areas.

In the 1991–1992 surveys, house dust mite (HDM) allergy was found to explain most of the urban-suburban differences in childhood asthma in young adults, but not the difference in recent occurrence of asthma symptoms 5. In the recent surveys, skin allergy testing was only performed in a subgroup of the 5–8 yr old children, and higher prevalence rates of HDM allergy were only found in boys of the urban area 17; this allergy could precede an increased occurrence of respiratory symptoms later in life. This also weighs against a cohort effect between the different study groups.

Area differences in occurrence of asthma-related symptoms probably reflect differences in life-time exposure to the environment. Area of residence was the proxy variable used for assessing this exposure. However, this could have induced misclassification bias, since the official residence is not necessarily the place where subjects spend most of their time. Indeed, approximately one third of the subjects' time is spent at work or at school. Migration of the subjects may be another obvious cause of misclassification with respect to life-time exposure. However, in repeated modelling of these effects, such misclassification, whether differential or nondifferential, was more likely to attenuate than increase observed area differences.

Although the area differences in respiratory symptoms and asthma in adults seem to be real, it is still not clear which causative factors linked to the urban area could be responsible for an “urban asthma” effect. A progressive effect of higher levels of exposure to air pollution from traffic in the urban area would be a cause to consider. Exposures to gaseous pollutants and ultrafine particles in diesel exhaust fumes have been associated with atopy and asthmatic airway inflammation 18–20, even though their effects on symptoms are still controversial 21. Except for diesel vehicles constituting approximately 50% of all motor vehicles in Belgium and the observation that traffic congestion is high in our city centres, there are at present, no further data associating our area-differences with exposure to traffic-related air pollution.

In conclusion, this systematic comparison of asthma occurrence in people whose ages span 70 yrs, carried out in an urban and an adjacent suburban area of a medium size cosmopolitan city, suggests a progressive effect of long-term exposure to the urban environment. A number of factors that have been associated with “urban asthma” in other studies cannot explain this effect in the present populations. Follow-up studies, in an advanced planning stage, of the cohort that participated in the European Community Respiratory Health Survey and the International Study of Asthma and Allergies in Childhood could provide more support for a hypothesis of a long-term “urban asthma” effect.