Thorax 1994;49:347-351

Prevalence of asthma and risk factors amongChinese, Malay, and Indian adults in Singapore

T P Ng, K P Hui, Wan C Tan

AbstractBackground - The prevalence and mor-bidity of asthma vary greatly among dif-ferent ethnic communities and geo-graphical locations, but the roles ofenvironmental and genetic factors arenot fully understood. The differences inprevalence of adult asthma among Chi-nese, Malay, and Indian ethnic groups inSingapore were examined, and the extentto which these could be explained by per-sonal and environmental factors wereinvestigated.Methods - A stratified disproportionaterandom sample (n = 2868) of Chinese(n = 1018), Malays (n = 967), and Indians(n = 883) of both sexes was drawn fromhouseholds in five public housing estates,and an interviewer administered ques-tionnaire was used to determine cumu-lative and current prevalence of "phys-ician diagnosed asthma" (symptomswith a physician diagnosis of asthma).Results - Lifetime cumulative prevalence(standardised to the general population)of "physician diagnosed asthma" was4 7% in men and 4 3% in women; 12month period prevalences were 2-4% and2-0%, respectively. Cumulative preval-ence of asthma was significantly higherin Indians (6-6%) and Malays (6-0%) thanin Chinese (3-0%); period prevalences ofasthma were 4-5% in Indians, 3*3% inMalays, and 0-9% in Chinese. Ownershipof cats or dogs was more frequent inMalays (15-4%) and Indians (11-2%) thanin Chinese (8-8%). Rugs and carpets werealso more frequently used by Malays(52-2%) and Indians (40-7%) than by Chi-nese (8-9%). Current smoking preval-ences were higher in Malays (27-3%) thanin Indians (19-4%) and Chinese (23-0%).Malays and Indians did not have higherrates of atopy (11-1% and 15-2%, respect-ively) than Chinese (15-4%). Adjustmentfor these factors in multivariate analysesreduced the greater odds of asthma inMalays and Indians, but not to a signific-ant extent.Conclusions - There are ethnic dif-ferences in the prevalence of asthma inSingapore which are not entirelyexplained by differences in smoking,atopy, or other risk factors. Otherunmeasured environmental factors orgenetic influences are likely to accountfor residual differences in the prevalenceof asthma.

(Thorax 1994;49:347-351)

The prevalence of asthma varies widely inpopulations residing in different geographicalareas and among different ethnic communi-ties.' 2 Indians in the UK have been reported tohave higher rates of asthma than white Cauca-sians,"5 although other reports have also indi-cated a lower"5 or similar910 prevalence. Thereis much evidence to indicate that environmen-tal factors are the major cause of variation inthe prevalence of asthma among differentpopulation groups. A westernised lifestyle,urban living, and higher material standards ofliving appear to be associated with a higherprevalence of asthma.21' However, the preciserole of specific environmental factors inexplaining ethnic differences in the prevalenceof asthma is unclear.

It has long been suspected that Malays andIndians in the multiracial population ofSingapore are more likely than Chinese to havesymptoms of asthma. In a questionnaire sur-vey of children and young people in 1972 ahistory of asthma was reported by 6 7% ofMalay primary and secondary students, 7 7%of Indian students, and 2 7% of Chinese stu-dents.'2 No corresponding data for older adultsare available. There are no known explanationsfor the observed ethnic differences in asthmamorbidity in Singapore. Singapore is a smallisland city state (633 square kilometres), situ-ated close to the Equator, with a uniformly hotand humid climate throughout the year. Thecountry enjoys a high standard of living and ishighly urbanised, but traditional ways are stillvery much the fabric of life for all except theyoung. A moderate difference in socioecono-mic levels exists among the races (with theChinese having higher standards of living), butthe differences in culture, lifestyle, and livinghabits are wide.

This study was undertaken to examine dif-ferences in the prevalence of adult asthmaamong the three major ethnic populations ofSingapore, and to determine whether thesecould be accounted for by differences in theprevalence of specific personal and environ-mental factors in each of the three ethnicgroups.

MethodsOver 85% of households (those within the lowto middle income brackets in Singapore) livein modern, low cost, high rise flats in publichousing estates. We chose to study residents ofpublic housing estates with this socioeconomicrestriction in mind for the study population.Public housing estates are uniform with regardto the socioeconomic distribution of residentsaccording to different housing types defined by

Division ofEpidemiology,Department ofCommunity,Occupational andFamily MedicineT P Ng

Division ofRespiratory Medicine,Department ofMedicineK P HuiWan C Tan

National University ofSingapore, LowerKent Ridge, Singapore0511

Reprint requests to:Associate Professor NgTze Pin.Received 23 December 1992Returned to authors3 March 1993Revised version received7 September 1993Accepted for publication6 January 1994

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the size of flats (one room, two rooms, threerooms, four rooms, five rooms, or executiveflats) and racial groups, as a result of thedeliberate government policy for social classintegration and racial coexistence. Five estates

were chosen from widely scattered parts of theisland, including areas close to heavy manufac-turing industries, near the city centre, and inthe suburbs. A stratified two stage clusterdisproportionate random sample of adultsaged between 20 and 74 years was selected. Afirst stage random sample of housing blocks ineach of the five public housing estates was

drawn and all eligible residents within each ofthe housing units (households) were asked to

participate. Disproportionate sampling was

employed to obtain equal quotas of subjectsbelonging to each of the three races and 11 fiveyear age groups in the study sample, in orderto ensure stable statistical estimates for theminority groups of Malays and Indians, whichform 141% and 71% of the population re-

spectively, and for older adults in the popula-tion.Among the households in 199 randomly

selected housing blocks 506 were not contact-able after at least three attempts (most of thesebeing due to relocation under estate renewalprogramme), 474 households refused, and1866 households responded. Of a total of 3940eligible persons residing in these respondinghouseholds between the ages of 20 and 74years, 2868 persons (72-8%) were interviewed.Data were collected by a team of trained

interviewers who administered a structuredquestionnaire in the languages usually under-stood and spoken by the respondents. Mostpeople in Singapore use English in addition totheir ethnic language, so in most cases theinterview could be conducted effectively inEnglish. In cases where very old respondentscould not understand English it was alwayspossible to enlist the help of their Englishspeaking children in the household or appro-

priate interviewers who spoke Chinese, Malay,or Tamil.Asthma was defined in a number of ways in

the survey: (1) episodic wheeze; (2) episodicwheeze and "attacks" of shortness of breath;(3) nocturnal "attacks" of wheeze or shortnessof breath; (4) any of the above symptoms witha physician diagnosis of asthma ("physiciandiagnosed asthma") in the absence of knowncardiac disease. The analyses showed that thesymptom of wheeze alone was more common

than either of the other sets of symptoms, butclosely corresponded with physician diagnosedasthma. We have therefore used physiciandiagnosed asthma as the end point for analysisand reporting. The subjects were askedwhether he or she had ever had these symp-toms (lifetime cumulative prevalence), andwhether he or she had had the symptoms in thepast year (one year period prevalence).The questionnaire included questions

related to a number of known personal andenvironmental risk factors for asthma. Foratopy, subjects were asked whether they hadever had symptoms of chronic rhinitis,eczema, or both, related to contact with aller-

gens. Other questions were asked on past andcurrent smoking habits, current and previousoccupations, and whether they currently keptrugs or carpets, cats or dogs, and birds.Socioeconomic status was defined according tothe sizes of flats occupied by the households,since residents in larger flats (four or morerooms) generally have higher mean monthlyhousehold incomes (S$3165) than those in oneto three room flats (S$2063).'3

Because of the disproportionate sampling byage, the prevalence of asthma for each of thethree ethnic groups was examined by extra-polating the sample estimates to reflect theactual age distribution in the general popula-tion in 1991 using the direct method for agestandardisation. Differences in age specificprevalence between races were evaluated usingthe Mantel-Haenzsel procedure for data strati-fied according to age. Multiple logistic regres-sion was used to calculate adjusted prevalenceodds ratios of asthma associated with Malaysand Indians with reference to Chinese whichtook into account age, housing estate, flat size,rhinitis/eczema, smoking (never smoker, ex-smoker, and current smoker), keeping of dogsor cats, keeping of birds, keeping of rugs orcarpets, and occupational risk groups. Basedon the results of a recent community basedcase-control study of asthma in Singapore'4several occupational risk groups were defined:a high risk group included those in service,agricultural and manufacturing occupations,while a low risk group included those in pro-fessional, technical, and executive occupa-tions, and the intermediate (reference) groupcomprised other occupations (sales, clerical,housewives, military, etc). An alternative defi-nition of specific high risk occupations such asbakers, cleaners, chemical workers, farmers,nursery workers, textile workers, garmentworkers, electronic workers, solderers, wood-workers, etc, was also used in the analyses withsimilar results, hence the previous definitionwas presented instead.The data were analysed using procedures

from the SAS package of statistical software'5and confidence interval analysis.'6

ResultsAs a result of the sampling procedure used, thethree ethnic subsamples in the sample popula-tion were about equally distributed by sex,age, and area of residence (table 1). Asexpected, a greater proportion of Chineseoccupied larger flats (four or more rooms) thandid Malays or Indians.The cumulative prevalence (standardised to

the age distribution of the general population)of physician diagnosed asthma was 4-7% (95%CI 3-6 to 5-8) in men and 4 3% (95% CI 3-1 to5 5) in women; period prevalences were 2-4%(95% CI 1-6 to 3-2) and 2-0% (95% CI 1-2 to2 7), respectively. The point estimates and95% confidence intervals of the prevalence ofasthma for the three ethnic groups are shownin table 2 and were significantly higher inMalays and Indians (cumulative prevalence6-6% in Indians, 6 0% in Malays, and 3 0% in

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Ethnic differences in prevalence of asthma

Table I Demographic characteristics of sample population by race

Total Chinese (%) Malays (%) Indians (%) p

No. 2868 1018 967 883

Sex:Men 1430 49-6 49-8 50-2Women 1438 50 4 50-2 49-8 NS

Age:20-39 years 1156 37-5 41-6 42-140-59 years 1048 37 2 34-8 37-660-74 years 664 25-2 23-6 20-3 NS

Housing estate:Bukit Merah 461 15 2 15-8 17-3Jurong East 678 23-4 246 22-6Geylang/Eunos 735 25-5 27-0 24 2Toa Payoh 427 16 7 12 7 15-2Yishun 567 19-0 20-0 20 6 NS

Apartment size:1-3 rooms 1779 54-1 66-8 65-94-5 rooms 1089 45 9 33-2 34-1 <0 001

Table 2 Sex and age standardised rates (%) ofphysician diagnosed asthma by ethnic group

Cumulative prevalence Period prevalence(95% CI) (95% CI)

Chinese 3-6 (2-21 to 5 06) 0 9 (0 33 to 1-57)Malay 6-0 (4 39 to 7-71) 3-3 (2-12 to 4 54)Indian 6-6 (4 77 to 8 44) 4-5 (2-98 to 6 03)

Rates were standardised to the sex and age distribution of the1991 census population (combining all three ethnic groups).

Chinese; period prevalence 4 5% in Indians,3 3% in Malays, and 0O9% in Chinese). Theseethnic differences were also consistentlyshown in the age-specific rates (data notshown), especially in the age groups between20 and 59 years which are less likely to beattributable to cardiac disease or other chronicobstructive pulmonary disease.We next examined whether the racial dif-

ferences in the prevalence of asthma were

explained wholly or in part by known pre-disposing factors in the environment. BothMalays and Indians were very much more

likely to keep cats or dogs than Chinese (table3). They were also more likely to keep birds,and rugs or carpets. Malays were more fre-quently ever smokers than Chinese or Indians,as well as being more likely to be engaged inservice, agricultural, and manufacturing occu-

pations. The prevalence of rhinitis or eczema,

however, was not higher in Indians or Malaysthan in Chinese; indeed the Chinese appearedto have a higher prevalence than Malays.The odds ratios of association of asthma

with race, when adjusted for these factors inmultiple logistic regression analyses, are

shown in table 4. These adjusted odds ratiosremained significantly high for Malays andIndians. Possible modifications between sex

and race were examined in the logistic models

Table 3 Percentage prevalence of environmental risk factors by race

Chinese Malays Indians p

Rhinitis/eczema 15-4 11-1 15-2 <0-01Smoking:Ex-smokers 8-0 7-2 6-8Current smokers 23-0 27-3 19-4 <0-001

Keeping of dogs or cats 8-8 15-4 11-2 <0-001Keeping of birds 4-2 9-4 6-8 <0-001Keeping of rugs or carpets 8-9 52-2 40-7 <0-001High risk occupations 21-7 27-2 23-3 <0-05

Table 4 Adjusted odds ratio of association of physiciandiagnosed asthma with ethnic group

Cumulative prevalence Period prevalence(95% CI) (95% CI)

Chinese 1-00 1 00Malay 1 83 (1-08 to 1-83) 4-41 (2-01 to 9 70)Indian 1-97 (1-23 to 3-15) 4 56 (2 27 to 9-17)

Odds ratios were adjusted for age, housing estate, flat size,smoking, rhinitis/eczema, ever keeping of dogs or cats, birds,rugs and carpets, and employment in high risk occupations.

but no statistically significant interactionswere observed.

In these multivariate analyses rhinitis oreczema was shown to be very strongly andsignificantly associated with all sets of asthmasymptoms including physician diagnosedasthma; the odds ratios for "ever physiciandiagnosed asthma" was 3-51 (95% CI 2-43 to5 09), and for "current physician diagnosedasthma" was 3-23 (95% CI 1-97 to 5 30). Theodds ratios of associations with past smokingwere significant for "current physician diag-nosed asthma" (2-28, 95% CI 1-16 to 4 48),and of borderline significance for "ever phys-ician diagnosed asthma" (1 74, 95°% CI 0-98 to3-10). Area of residence was also significantlyassociated with physician diagnosed asthmabeing higher in two areas (Jurong East andToa Payoh). The keeping of dogs or cats athome was significantly associated with evernocturnal symptoms (1-74, 95% CI 1-08 to2 95). Other factors were not shown to beindependently associated with asthma.

DiscussionThe difficulties of defining asthma in epi-demiological studies are well known. Despitethe efforts made to ensure valid responses toquestions about the presence of asthma, it ispossible that wheeze may nevertheless beunderstood differently among the three races.As is commonly observed in other studies,responses to wheeze appeared to be more vari-able and less specific, especially in women.However, the use of a combined set of asth-matic symptoms, supported by a physiciandiagnosis of asthma, would appear to lendgreat intrinsic validity. It would be desirable,nevertheless, to conduct further studies usingnon-specific bronchial provocation tests toconfirm the observed racial differences inasthma.The response rate of 72-8% was reasonably

high, but the possibility of sampling biasshould still be considered. There was somevariation in response rate by ethnic group:Chinese 68%, Malays 82%, and Indians 81%.However, we do not know how the responsesvaried among asthmatic and non-asthmaticsubjects within the population. If asthmaticsubjects were more inclined to agree to aninterview, it is possible to attribute theobserved ethnic differences in the prevalenceof asthma to differential response bias amongethnic groups. However, we do not think thisis likely. In many previous community surveyswe have repeatedly observed that non-Chinesesubjects are more likely to consent to interviewthan Chinese and this is likely to reflect a

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cultural and socioeconomic characteristicrather than a health related bias. In a relatedcommunity based polyclinic study'4 we havealso noted that Malays and Indians were, ifanything, less inclined than Chinese to seekmedical attention for their symptoms. Theresults should also be viewed together with apreviously reported observation of similarlyhigher rates of asthma in children and youngpeople among Malays and Indians.'2 Further-more, the present findings were well corrobor-ated by results from a parallel case-controlstudy of adult asthma'4 which also showed thatIndians and Malays do, indeed, experiencehigher rates of asthma and more severe diseasethan Chinese.The confidence intervals associated with the

adjusted odds ratio for race are not as wide asshould be the case if intraclass correlationarising from cluster sampling is taken intoaccount. Nevertheless, the directly stan-dardised estimates of asthma prevalenceshould carry even wider confidence intervalsthan with indirect adjustment, yet clear ethnicdifferences are still evident.The ethnic differences in the prevalence of

asthma were shown to be statistically signific-ant after multivariate adjustment for otherconfounding risk factors. It is possible, how-ever, that adjustment for these risk factorsmight be less than adequate. It is likely that theodds of association of asthma with certain riskfactors, such as the keeping of rugs and car-pets, or dogs and cats, were underestimated.Hence, although these factors were not shownto be significantly associated with asthma inmultivariate analyses, the possibility that someof these risk factors - for example, keeping ofrugs and carpets, pet animals, smoking, andoccupational exposures - may explain theobserved ethnic differences of asthma cannotbe ruled out. The results, based on a crosssectional study only, allow for limited depthsof examination of the underlying roles ofenvironmental factors, but the main findingsfirmly indicate a real difference in prevalencesof asthma and risk factors in these ethnicgroups.

Intercountry comparison of the prevalenceof asthma is made difficult by limited data inadult populations and differences in methodsemployed, but the overall prevalence of adultasthma observed in Singapore appears to belower than in many developed countries andhigher than in developing countries. Estimatesof the prevalence of self-reported currentasthma varies from 5 6% to 8% in Aus-tralia,'7 20 3 6% to 6.2% in the USA,2'22 and3-3% in the Maldives.2' The cumulative pre-valence of diagnosed asthma was as high as16-3% in Western Australia,20 but in Zim-babwe was only 1-6%.24 In Singapore, whilethe rates of asthma in Indians may well ap-proximate those in Caucasian populations inthe west, the rate in the Chinese is remarkablylow.

It is interesting to note that all three ethnicgroups live in close proximity to each otherrather than in ghettos, and occupy the sameblock of flats in each housing estate in the same

proportion as that found in the general popula-tion. It is therefore evident that climatic andother general environmental factors includingindustrial or urban air pollution cannot explainthese ethnic differences in rates of asthma. Thehigher proportion of Chinese who resided inlarger flats is consistent with a known moder-ate difference in socioeconomic status amongthe three races (average monthly householdincome for Chinese, Indians, and Malays areS$32 13, S$2859, S$2246, respectively in1990'3). The evidence for an association ofasthma with social class is variable,925 28 withthe weight of evidence favouring the view thatthose with lower socioeconomic status are lesslikely to have asthma. Hence, socioeconomicdifferences were even less likely to explain theobserved racial difference in the prevalence ofasthma.

Clinical atopic disease is well known to beassociated with asthma, as are other markers ofatopy. Although rhinitis and eczema wereshown to be the strongest risk factors forasthma in the present study, there were nosignificant differences in atopic disease amongthe ethnic groups to explain the observed eth-nic differences in asthma. Although dif-ferences in the prevalence of atopy may ac-count for regional differences in the prevalenceof asthma, such differences have not alwaysbeen shown when comparing areas of high andlow asthma prevalence.2>3'Environmental factors such as the keeping

of cats, dogs, or birds, and rugs or carpets arewell known to predispose to the developmentof common allergies. In this study only thekeeping of dogs or cats was shown to besignificantly associated with nocturnal symp-toms of asthma. Findings of nil or negativeassociations have been commonly reported,32 34however, probably because current exposuremay not be relevant and only a few subjectswith potential exposure to aeroallergensthrough contact with pets or the use of rugsand carpets develop asthma. Despite thedemonstrable lack of association of these fac-tors with asthma in the present study, it islikely that the higher frequencies of keepingrugs and carpets and pet animals is a contri-buting factor to the high rates of asthma inMalays and Indians.Smoking has been shown to be a positive

factor leading to the development of asthmasymptoms and exacerbations.3536 This was alsosuggested by the data in the present study, andwas especially evident in past smokers ratherthan current smokers (explainable by the factthat many with asthmatic symptoms hadstopped smoking, probably upon doctor's ad-vice). The prevalence of smoking is highest inMalays and lowest in Indians. Hence, whilesmoking may contribute to higher rates ofasthma in Malays, the higher prevalence ofasthma in Indians cannot be explained bysmoking at all. Occupational exposures are aknown factor causing or aggravating asthmaand, indeed, Malays are more likely to beengaged in service, agricultural, or manufac-turing jobs involving greater likelihood ofoccupational exposures. Hence, occupational

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exposures may also partly explain the higherrates of asthma in Malays.

In conclusion, higher rates of asthma wereobserved among Malays and Indians than Chi-nese in Singapore. These were not explainableby differences in socioeconomic status oratopy. Greater prevalence of environmentalfactors such as the keeping of carpets and rugsand of pet animals, smoking, and occupationalexposures were found in Malays and Indians,although these factors do not entirely explainthe observed ethnic differences in asthma.Other unmeasured environmental factors orgenetic influences are likely to account forresidual differences in the prevalence ofasthma.

This study was made possible by a research grant from theNational University of Singapore.

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