The present study confirmed a lower prevalence of asthma symptoms among Chinese adolescents born in mainland China and a significantly higher prevalence among Chinese adolescents born in Hong Kong and Canada. These results suggest that early environmental exposure reduced the subsequent development of asthma, despite similar genetic background.21
However, the environment continues to be an important factor influencing the prevalence of asthma and atopy even after the early years of sensitization.22,23
Among Chinese adolescents in Vancouver, we observed increasing prevalence of ever wheezing and ever having had asthma with longer duration of residence in Canada, although most trends did not achieve significance when stratified by sex, possibly because of sample sizes. A trend toward higher prevalence of asthma and allergic disease among Chinese adolescents living in affluent communities was evident in phase 1 of the International Study of Asthma and Allergies in Childhood.2,24
The prevalence rates of current wheeze in 13- and 14-year-old schoolchildren, as identified by written questionnaires, were 12.4% in Hong Kong and 4.2% in mainland China.2
Data from phase 2 of the study suggested that environmental factors and diet could partly explain the difference in prevalence of asthma between children living in Hong Kong and those living in mainland China (Beijing and Guangzhou). Phase 2 showed that factors significantly associated with current wheeze were cooking with gas (OR 2.04, 95% CI 1.34–3.13), use of foam pillows (OR 2.58, 95% CI 1.66–3.99) and living in damp housing (OR 1.89, 95% CI 1.26–2.83), whereas factors protecting against current wheeze were use of cotton quilts and the consumption of fruit and raw vegetables.25,26
In contrast, current nocturnal coughing and current exercise-induced wheezing showed no significant gradients among the different groups of Chinese adolescents. In a previous report, nocturnal coughing had the lowest predictive value for bronchial hyperresponsiveness.27
This symptom may reflect respiratory conditions other than asthma,2
yet exercise-induced wheezing was the second-best predictor for bronchial hyperresponsiveness.27
Among the Chinese adolescents in our study, exercise-induced wheezing was more common than ever wheezing or current wheezing or ever having had asthma. This phenomenon was also seen in phase 1 of the International Study of Asthma and Allergies in Childhood2,21
and in a survey of adolescents in schools in Shanghai, China.28
The reason for the discrepancy remains unclear. It may be real, it may represent overreporting by adolescents or it may reflect participants' interpretation of the questions about wheezing.2
The validity of exercise-induced wheezing in epidemiologic studies of Chinese adolescents needs further investigation.
Improved hygiene and health care in western countries has altered the pattern of exposure to infectious agents, including Mycobacterium tuberculosis
, parasites and endotoxins, in early life, possibly predisposing the immune system toward an atopic response.29,30
Although the prevalence of current wheezing showed an increasing gradient with greater “westernization” (Appendix 2 and Appendix 3, available at www.cmaj.ca/cgi/content/full/179/11/1133/DC2
), this trend was less evident for severe wheezing among boys. However, girls had higher rates of frequent wheezing episodes, nocturnal awakening and wheeze that limited speech, which suggests a potential effect of sex or other factors, including treatment, on the severity of asthma, rather than only a “westernization” effect.
Migrating populations with stable genetics provide an opportunity to observe changes in disease with changes in environment, just as genetically different groups living in the same region allow study of the effects of genetic diversity. Increases in asthma symptoms related to migration from developing to industrialized areas has been reported from several countries and regions, including New Zealand,31
the United Kingdom,39–41
and the United States.44,45
The prevalence of asthma in Asian populations has repeatedly been shown to be lower than that in the West, but variations exist within Asian countries.1,19,46
Cultural, dietary, economic and geographic factors, as well as the variability in distributions of characteristics related to country of birth, residential history, linguistic differences and degree of acculturation to a western lifestyle, may account for differences between ethnic subgroups.46,47
Interactions among genetic, environmental and social factors have also been postulated to explain the observed differences.19
There are many sociologic and cultural differences among the populations that we studied. More than 80% of native Chinese adolescents living in mainland China, but only 16.6% of those in Hong Kong and about 10% of those in Canada, had no siblings. Of mothers in Canada, 40% to 60% had received a college education, compared with 30% to 40% in mainland China and only 8.2% in Hong Kong. Nearly 70% of native Chinese adolescents in mainland China, but only 25% to 30% of those in Hong Kong and Canada, had exposure to environmental tobacco smoke in the home. More adolescents in China than in Canada had frequent exposure to trucks. Pet ownership was most common among adolescents in Beijing and non-Chinese adolescents in Vancouver and least common for those living in Hong Kong. However, the differences in prevalence rates of asthma for adolescents in mainland China, Hong Kong and Canada remained significant after adjustment for these factors.
The use of “race” and “ethnicity” as predictive variables for describing populations is associated with problems because of the inherent difficulties of defining the terms.48
Using “birthplace” as a variable could also be criticized because of differences between different groups of people from the same geographic region.49,50
Using a combination of these 2 variables to define populations may better reflect both genetic and environmental (residential) influences. Davis and associates46
suggested that, when possible, asthma surveys involving a substantial number of Asian respondents should use a more detailed categorization of race and ethnicity than the traditional category of “Asian.” We used “Chinese” in our study, as this has been the principal group among the growing number of immigrants to Canada since 1986.15
Strengths and limitations
This comparative study had several strengths. Canada and China represent almost the extremes of asthma prevalence among 58 countries included in phase 3 of the International Study of Asthma and Allergies in Childhood. The data were collected almost contemporaneously using identical methodology in each centre. Also, we were able to examine the duration of residence in the new country (Canada). Differences in several demographic characteristics, including number of siblings, also provided unique data for further study.
Our study also had limitations. The ethnicity of participants was not recorded directly, but rather was determined by surname; as a result, 7.2% of participants in Canada were excluded because of uncertainty in classification. There may also have been misclassification bias. A Chinese surname would likely mean that the adolescent had at least a Chinese father but perhaps not a Chinese mother, whereas a child classified as “non-Chinese” by surname might have had a Chinese mother. Second, the original birthplace of Chinese immigrants in Vancouver was not collected, which might have introduced misclassification of years of residence in the West. All participants were born around 1987–1988, when Hong Kong was the major source of Chinese immigrants to Canada, but many of those with less than 7 years' residence in Canada might have come from mainland China. We excluded those with uncertainty regarding birthplace (). Third, some important potential confounders were not recorded, including family history of asthma, family income, history of infectious disease and personal smoking. Finally, self-reported heights and weights might have been subject to reporting bias.
Further studies in China and Canada, including objective measures such as allergen skin prick test, bronchial hyperresponsiveness and environmental measurements (e.g., endotoxins and diesel exhaust particles) are necessary to identify the risk factors or protective factors associated with asthma. Moreover, the reason for differences between the sexes in the prevalence rates for native Chinese living in Guangzhou and in Beijing needs further investigation.
In conclusion, asthma symptoms in Chinese adolescents were lowest among those living in mainland China, were greater among those who lived in Hong Kong or who had immigrated to Canada during childhood, and were highest for those born in Canada. These findings strongly suggest that environmental factors and duration of exposure, in addition to genetic factors, influence the prevalence of asthma.