Children of Chinese ancestry who have lived only in Metro Vancouver are more likely to have asthma than those living in Hong Kong or mainland China, says a study published today in the Canadian Medical Association Journal.
The study of more than 10,000 13- and 14-year-olds found that asthma and wheezing were most prevalent among Chinese adolescents who have lived their whole lives in Vancouver, second-highest among native Chinese adolescents living in Hong Kong, and lowest among native Chinese adolescents living in the mainland China cities Beijing and Guangzhou.
Among Chinese adolescents of both sexes in Metro Vancouver, the prevalence of wheezing—a major symptom of asthma—was 14.5 per cent for immigrants who had lived in Canada fewer than seven years, 18.8 per cent for those with who lived here seven or more years and 20.9 per cent for those who had lived their whole lives in Canada.
The prevalence of a confirmed asthma diagnosis was 7.7 per cent for immigrants with fewer than seven years in Canada, 11.2 per cent for those with seven or more years in Canada and 15.9 per cent for those born in Canada.
“These findings suggest that environmental factors and duration of exposure influence asthma prevalence,” the study states, without pinpointing what those factors are.
In earlier research, factors significantly associated with wheezing included living in a house where cooking is done with gas, the use of foam pillows, and living in damp housing. Protective factors against wheezing included the use of cotton quilts and fruit and vegetable consumption.
Dr. Alexander Ferguson, a now-retired Vancouver co-author of the study could not be reached. Study leader Dr. Hong-Yu (Wendy) Wang, a respirologist from China doing post-doctoral fellowship training in Hamilton, said the study appears to reinforce the “westernization” effect on asthma. Improved sanitation and hygiene, less exposure to rural environments, fewer infections in infancy and increased use of antibiotic and vaccination are associated with an increased risk of childhood asthma.
Other research has pointed to the “hygiene hypothesis” to explain higher asthma rates. In that theory, exposure to infections and contact with older siblings are believed to reduce the risk of allergic diseases, of which asthma is one type.
Wang and her team of researchers from China, Hong Kong and Canada worked with questionnaires filled out by children and/or their parents from 2001 to 2003. There were 7,794 adolescents from China in the study and 2,235 from Vancouver.
The study found that asthma rates among Metro Vancouver-born Chinese children were higher than among children in mainland China, even though a greater proportion of children in China were exposed to parents smoking at home and to truck fumes.
Wang said she still can’t pinpoint the reason Chinese children who have lived their whole lives in Vancouver had the highest prevalence of asthma and wheezing.
“We don’t know exactly what it is about the environment there,” she said. “It may be that there are children who are genetically predisposed and then, when they live in a certain environment, they have an immune system response that causes the asthma. Maybe it is that Vancouver is a wet-climate coastal city, but we need to do more research on this.”
Wang discounted the notion that it may be that Vancouver doctors simply diagnose asthma more than doctors in China.
She said that as a doctor from China, she can attest that Beijing and Guangzhou are very similar to Vancouver when it comes to access to medical care and provision of medical services.
The study, which can be read on the Internet at http://www.cmaj.ca, was financed by Health Canada and a grant to Chinese University of Hong Kong.
A study in the same issue of CMAJ, on Canadian adults diagnosed with asthma, suggests doctors may be overdiagnosing the condition. It found that 30 per cent of adults who enrolled in the study after being diagnosed with asthma actually did not have it, based on more detailed lung and bronchial assessments.
Few studies on the prevalence of asthma among immigrants have been done in Canada, the researchers say, and this is the first to collect such data on Chinese people living in Canada. It is part of the ongoing International Study of Asthma and Allergies in Childhood which began in 1991.
[Editors Note: “Prevalence of asthma among Chinese adolescents living in Canada and in China,” by Hong-Yu Wang, MD PhD, et al. can be read on-line here.]