Social class in asthma and allergic rhinitis: a national cohort study over three decades

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1 Eur Respir J 25; 26: DOI: / CopyrightßERS Jourals Ltd 25 Social class i asthma ad allergic rhiitis: a atioal cohort study over three decades L. Bråbäck*, A. Hjer #," ad F. Rasmusse + ABSTRACT: The aim of this study was to assess whether the associatio with social class differed betwee allergic rhiitis ad asthma ad whether these associatios have chaged over time. The Swedish Military Service Coscriptio Register was liked to two other atioal registers for 1,247,38 male coscripts i successive cohorts bor betwee 1952 ad The percetage of asthma cases associated with allergic rhiitis was 15% i the oldest cohort ad 44% i the yougest cohort. Low socio-ecoomic status (SES) was associated with a icreased risk (assessed as odds ratio) of asthma without allergic rhiitis (1.14, 95% cofidece iterval (CI) ) but a slightly reduced risk of asthma with allergic rhiitis (.96, 95% CI.93 1.). The risk of allergic rhiitis was.84, 95% CI A positive iteractio betwee SES ad year of birth occurred i all three coditios. Low SES was related to a reduced risk of asthma with allergic rhiitis i the earliest cohort (.72, 95% CI.53.82) but a slightly icreased risk i the most recet cohort (1.7, 95% CI ). I coclusio, the role of social class has chaged over time. The steepest icrease i asthma ad allergic rhiitis occurred i coscripts with a low socio-ecoomic status. KEYWORDS: Asthma-allergic rhiitis, asthma epidemiology, birth cohort, socio-ecoomic status, time tred AFFILIATIONS *Dept of Public Health ad Research, Sudsvall Hospital, # Uit of Paediatrics, Dept of Wome s ad Childre s Health, Uppsala Uiversity, " Cetre for Epidemiology, Swedish Natioal Board of Health ad Welfare, ad + Child ad Adolescet Public Health Epidemiology Group, Dept of Public Health Scieces, Karoliska Istitutet, Stockholm, Swede. CORRESPONDENCE F. Rasmusse Child ad Adolescet Public Health Epidemiology Group Dept of Public Health Scieces Karoliska Istitutet SE Stockholm Swede Fax: fi.rasmusse@phs.ki.se The icreased prevalece of hay fever i the UK durig the 19th cetury was liked to idustrialisatio ad started i the affluet classes [1]. The associatio betwee social advatage ad hay fever remaied i the 2th cetury [2] ad was also demostrated i studies based o objective tests [3 5]. I particular, socioecoomic status (SES) early i life appeared to be importat. Low SES i early childhood provided protectio from atopic sesitisatio at the age of 33 yrs i a British birth cohort from 1958 [5]. High social class ad educatioal level was a risk factor for atopy i Easter Germay [4], although atopic diseases were more likely i the wester, more affluet, part of Germay [6]. Low SES has bee related to a icrease i the severity of asthma but the associatio betwee social class ad the occurrece of asthma is less clear [7, 8]. Coflictig evidece regardig the relatioship betwee social class ad asthma could be related to geographical or temporal variatios. Furthermore, icosistet fidigs may arise from studies that do ot distiguish betwee atopic ad oatopic asthma, sice they are separate etities with at least partly differet backgrouds [9]. The rise i childhood asthma has comprised both atopic ad oatopic asthma. It has bee argued that the role of atopy has bee overemphasised, as,5% of all asthma is atopic [1]. Substatial chages i the stadard of livig have take place i Swede over the last 4 5 yrs. The curret authors have previously demostrated that the icrease i asthma amog youg males i Swede accelerated i cohorts bor after 196 [11]. I the curret survey, the authors have ivestigated the prevalece of allergic rhiitis ad asthma (with ad without allergic rhiitis) i Swedish coscripts by year of birth ad social class. It has bee evaluated whether the associatio with social class differed betwee allergic rhiitis ad asthma with ad without allergic rhiitis ad whether these associatios have chaged over three decades. MATERIALS AND METHODS Swede has a log traditio of atioal registers with high-quality data relatig to socio-ecoomic ad health idicators for the etire Swedish populatio. The key to these registers is the uique persoal idetificatio umber for each Swedish citize. I this study, the curret authors used the Swedish Military Service Coscriptio Register (MSCR; ), the Register of the Received: February Accepted after revisio: July 7 25 Europea Respiratory Joural Prit ISSN Olie ISSN VOLUME 26 NUMBER 6 EUROPEAN RESPIRATORY JOURNAL

2 L. BRÅBÄCK ET AL. SOCIAL CLASS IN ASTHMA AND ALLERGIC RHINITIS Total Populatio (RTP; ) ad the Populatio ad Housig Cesuses (PHC; 197, 1975, 198, 1985, 199). Statistics Swede maitais the Multi-Geeratio Register with records of the uique persoal idetificatio (ID) umbers of the mothers ad fathers of almost all the idividuals bor after 1932, who were alive ad residet i Swede i The parets of the study subjects were idetified by record likage to this register. Study populatio All Swedish-bor males bor betwee 1952 ad 1977 were idetified at age 17 yrs, usig their uique persoal ID umber, i the RTP. All the idividuals recorded as livig i Swede are icluded i these registers. Of the youg males bor i ad who were thus idetified, the 9.9% (1,239,75) who were recorded i the MSCR as havig participated i the military service coscriptio examiatio at aged 17 2 yrs were icluded i the study populatio. Of the cohort bor i 196, oly 14.6% (7,333) participated i a military service coscriptio examiatio due to admiistrative chages to the etire coscriptio system i Swede. These subjects were added to the study populatio. The Swedish military service coscriptio examiatio is required by law. Foreig citizeship or a severe chroic medical coditio or hadicap documeted i a medical certificate are the oly reasos that are accepted for oparticipatio accordig to Swedish law. Variables i the study Allergic disorders The examiatio of the coscripts cosisted of a health questioaire, a persoal iterview by a medical doctor ad a comprehesive physical ad psychological examiatio that icludes a physical exercise test. The health questioaire is set to the coscripts prior to the examiatio ad cotais specific questios about itchy or watery eyes, cogested ose/ allergic rhiitis, wheezig, asthma ad eczema. The examiig doctor makes a registratio of diagoses o a practical cliical basis, with the emphasis o coditios that may affect the youg male s performace i the military service. The diagostic codes were based o the Iteratioal Classificatio of Diseases (ICD-8 ad ICD-9) i : 493 (asthma) ad 477 (allergic rhiitis). The diagoses were set by the doctor after havig see the aswers to the questioaire ad ivestigated the coscript. The diagostic procedure has chaged very little over the years, with the exceptio of the health questioaire, which has bee chaged several times. The diagostic category of asthma was divided ito two parts: asthma without allergic rhiitis (as a proxy to oallergic asthma) ad asthma with allergic rhiitis (as a proxy to allergic asthma). Socio-ecoomic status SES was defied accordig to the categorisatio of the head of the household i the PHC earest to the military coscriptio examiatio made by Statistics Swede [12]. The categories were based o occupatio but also took accout of the educatioal level of occupatio, the type of productio ad the positio at work of the head of the household. Two differet classificatios were used durig the study period, oe i the PHCs from 197 ad 1975 ad aother i 198, 1985 ad 199. These classificatios oly overlapped i the broad categories of white collar/blue collar, which forced the use of this limited dichotomous variable (high/low). Socio-demographic cofouders A umber of available variables that have bee show to be importat determiats of allergic disorders i previous studies of Swedish coscripts [11 14] were added to the aalysis as possible cofouders. Iformatio from the PHC earest i time to the military coscriptio ad the geographical locatio of the home i the RTP for coscripts aged 17 yrs were used to create a dichotomous rural/urba variable. I the PHC, urba is defied as a home located i a settlemet with o2 ihabitats. Paretal occupatio from the PHC earest i time to the military coscriptio was used to create a dichotomous farmer variable. Couty of residece was idetified i the RTP at aged 17 yrs ad classified ito six categories, oe for souther Swede ad five for orther Swede. Materal age at the birth of the coscript was calculated usig iformatio relatig to the year of birth of the mother ad the coscript from the RTP. Variables of family size ad overcrowdig (defied as more tha two people per room excludig the kitche ad oe more room) were created usig data from the PHC earest to the coscriptio examiatio. A variable of beig the first-bor male i the family, as a proxy to first-bor child i the absece of sisters, was created by likage to the multi-geeratio register of the coscripts i the study. Body mass idex Weight ad height measuremets at the military coscriptio examiatio were used to calculate body mass idex (BMI). Four BMI categories were created accordig to WHO criteria [15]: o3. (obese), (overweight), (referece) ad f18.49 (low). Statistical aalyses Asthma with allergic rhiitis, asthma without allergic rhiitis ad allergic rhiitis were used as depedet variables i logistic regressio models. These models icluded a dichotomous low/high SES variable, dichotomous cofouders ad a four-category BMI variable as defied above. Missig was icluded i the models as a third category for cofouders ad SES. Year of birth was etered as a cotiuous variable i all the models. Logistic regressio models that cotaied all the variables described above were used to calculate the iteractio of time (year of birth) with other idepedet variables. The test-based method was used to calculate 95% cofidece itervals (CI). Ethical cosideratios The study was approved by the Ethics Committee at the Karoliska Istitutet i Stockholm. RESULTS The study comprised 1,247,38 male coscripts. I all, 32.5% were classified as low SES, 4.2% as high SES, while 27.4% could ot be classified. Additioal socio-demographic characteristics have bee preseted i a previous paper [11]. c EUROPEAN RESPIRATORY JOURNAL VOLUME 26 NUMBER 6 165

3 SOCIAL CLASS IN ASTHMA AND ALLERGIC RHINITIS L. BRÅBÄCK ET AL. The prevalece of asthma ad allergic rhiitis icreased cotiuously with year of birth durig the etire study period. I coscripts bor i , the prevalece rates for asthma without allergic rhiitis, asthma with allergic rhiitis ad allergic rhiitis were 1.7%,.3% ad 4.9%, respectively. The correspodig prevalece rates i coscripts bor i were 3.1%, 2.4% ad 16.4% respectively. As a result, 15% of all asthma cases had allergic rhiitis i the earliest birth cohort compared with 44% i the most recet cohort. Low SES was associated with a icreased risk of asthma without allergic rhiitis (odds ratio (OR) 1.14; 95% CI ), but a reduced risk of asthma with allergic rhiitis, (OR.96; 95% CI.93 1.), ad allergic rhiitis (OR.84; 95% CI.82.85), after multivariate aalyses of the etire sample. However, the steepest icrease i asthma ad allergic rhiitis occurred i coscripts with a low SES (fig. 1) ad the ORs for asthma ad allergic rhiitis chaged over time (table 1). The exclusio of overcrowdig from the logistic model had margial effects o the ORs for social class i cohorts bor before 1962 ad o effect at all i cohorts bor after The multivariate aalyses cofirmed a positive iteractio betwee low SES ad year of birth i all three coditios (p,.1; data ot show). The iteractio effect was similar for the three decades i the study for all three coditios. DISCUSSION The likage of atioal registers i Swede at a idividual level has eabled the curret authors to carry out this epidemiological study of.1.2 millio coscripts i successive birth cohorts bor betwee 1952 ad A two-fold icrease i asthma without allergic rhiitis ad a three- to four-fold icrease i allergic rhiitis ad asthma associated with allergic rhiitis was foud over a period of less tha three decades. A icrease i asthma ad allergic rhiitis was demostrated both i high ad i low social class, but the steepest icrease occurred i coscripts with low SES. Low social class was associated with a icreased risk of asthma without allergic rhiitis i all birth cohorts, but it oly cotributed to a icreased risk of asthma with allergic rhiitis i the most recet birth cohorts. A strog iverse relatioship betwee low SES ad allergic rhiitis was revealed i coscripts bor i the 195s. The associatio betwee low social class ad allergic rhiitis decreased over time ad the associatio was fairly weak i coscripts bor after 197. The decliig associatio betwee allergic rhiitis ad social class i recet Swedish birth cohorts suggests that lifestyles promotig allergic diseases have bee adopted i all social classes. No differece will be see betwee cases ad cotrols i a society i which all the idividuals are exposed [16]. The curret authors caot wholly exclude the possibility that differetial misclassificatio with a icreasig awareess of allergic diseases i lower social classes may cotribute to the chages i time treds. However, the fidigs are cosistet with those relatig to British birth cohorts from 1958 ad 197, showig a more proouced associatio betwee hay fever ad social class i the early cohort [2]. The associatio betwee social class ad asthma has bee coflictig. A America study showed a positive associatio betwee icreasig educatio level ad asthma with hay fever a) Asthma without allergic rhiitis % b) Asthma with allergic rhiitis % c) Allergic rhiitis % but a egative associatio betwee educatio level ad asthma without hay fever [17]. Similarly, a recet British study revealed that social class had differetial effects o atopic ad oatopic asthma [9]. I cotrast, poverty at both a idividual ad a area level was associated with a icreased risk of asthma regardless of sesitisatio i the large Europea Commuity Respiratory Health Survey comprisig youg adults from 15 coutries [18]. The curret Swedish survey suggests that the role of SES i asthma has chaged over time ad such chages may explai the coflictig fidigs i other Year of birth FIGURE 1. Asthma ad allergic rhiitis i Swedish male coscripts by year of birth ad social class (-----: high socio-ecoomic status; : low socio-ecoomic status). a) Asthma without allergic rhiitis; b) asthma with allergic rhiitis; c) allergic rhiitis. 166 VOLUME 26 NUMBER 6 EUROPEAN RESPIRATORY JOURNAL

4 L. BRÅBÄCK ET AL. SOCIAL CLASS IN ASTHMA AND ALLERGIC RHINITIS TABLE 1 The associatio betwee social class, asthma ad allergic rhiitis Asthma without allergic rhiitis Missig data 1.17 ( ) 1.9 ( ) 1.13 ( ) Low SES 1.5 ( ) 1.17 ( ) 1.2 ( ) Asthma with allergic rhiitis Missig data.86 (.74.99).9 (.83.97) 1.5 ( ) Low SES.72 (.53.82).92 (.86.99) 1.7 ( ) Allergic rhiitis Missig data.84 (.83.96).89 (.87.91).91 (.88.93) Low SES.79 (.77.81).83 (.82.86).92 (.9.94) Data preseted as adjusted odds ratio (95% cofidece iterval) after multivariate aalyses of differet birth cohorts bor i , ad ad adjustig for body mass idex, couty of residece, rural livig, overcrowdig ad beig the first-bor male. SES: socio-ecoomic status. studies. The differeces betwee asthma with allergic rhiitis ad asthma without allergic rhiitis i relatio to low SES ted to disappear i recet cohorts. A low SES implied a reduced risk of allergic asthma i the oldest birth cohorts but a icreased risk i the yougest cohort. Moreover, the steepest icrease i asthma with allergic rhiitis ad asthma without allergic rhiitis has occurred i idividuals from less advataged social classes. This is i agreemet with the high prevalece of allergic asthma [19] ad sesitivity to ihalat allerges [2] i poor ier-city areas i the USA. A disproportioate icrease i asthma i lower social classes has also bee observed i British schoolchildre bor after 1983 [21]. A ogoig Swedish birth cohort study recetly demostrated a icreased risk of asthma ad allergic sesitisatio i childre with low SES aged 4 yrs [22]. Housig stadard has improved i Swede sice the 195s, particularly i low social classes. Domestic crowdig is closely associated with social class but adjustmet for overcrowdig had oly margial effects o the ORs for social class i the multivariate aalyses. However, poor housig coditios also iclude, for example, psychosocial stress ad icreased exposure to dampess ad mould, which may cotribute to a icreased risk of asthma i idividuals with low SES. Differeces i food habits ofte explai socio-ecoomic differeces i health. Itake of fruit ad usaturated fat has bee associated with a icreased risk of atopy ad asthma [23]. Cosumptio of fruit is lower ad cosumptio of saturated fat higher i idividuals with low SES [24, 25]. Obesity is liked both to asthma ad to low SES [14] but the icreased risk of asthma i coscripts with low SES i the curret study persisted after adjustmet for BMI. A register-based study is limited by the array of variables available for statistical aalyses. I this study, iformatio o atopic heredity ad exposure to tobacco smoke, for example, would have bee most valuable. Gees ad eviromet iteract ad smokig may cotribute to the icreased risk of asthma i coscripts with a low SES [26]. Smokig is more likely i oaffluet groups but smokig has bee decliig i Swede sice 198, both i males ad i females ad i all social classes [27]. However, reports from other coutries i Wester Europe idicate that the differeces i smokig habits betwee the social classes have icreased over time. Smokig i Norway was more commo i males with a high icome ad less commo i males with a low icome i the 195s. A switch-over occurred aroud 1975 ad smokig was more commo i low icome males i 199 but at a lower level i compariso with the 195s [28]. The proportio of the Swedish populatio that is categorised as low SES has decreased over time, as maual jobs have bee replaced by white-collar jobs o the Swedish labour market. This may have caused a steeper social gradiet i recet years, which may explai some or all of the steeper time tred for low SES compared with high SES for oatopic asthma. However, this is ot the case for the atopic disorders, where the social gradiet is reversed. The ORs for social class i the curret study are fairly low. There is less icome iequality i Swede ad other Scadiavia coutries tha there is i the UK, for example [29]. Moreover, chages i the classificatio of SES i the Swedish Populatio ad Housig Cesuses have restricted the classificatio of social class to blue- ad white-collar workers i the curret study. Eve if the sesitivity ad specificity of exposure ad outcome variables i this register-based study may be lower tha those i smaller studies based o data sets created ad hoc, the misclassificatio bias is odifferetial. It is therefore likely that the associatios betwee social class ad disease outcome are uderestimated i the preset study. The clear-cut associatios betwee social class ad asthma with ad without allergic rhiitis see i table 1 may thus be uderestimates of the true associatios. The curret study has further limitatios. I the statistical aalyses, the curret authors have distiguished betwee asthma with allergic rhiitis ad asthma without allergic rhiitis. Asthma without allergic rhiitis does ot exclude atopy, but asthma with ad without allergic rhiitis could be used as a reasoably good proxy for allergic ad oallergic c EUROPEAN RESPIRATORY JOURNAL VOLUME 26 NUMBER 6 167

5 SOCIAL CLASS IN ASTHMA AND ALLERGIC RHINITIS L. BRÅBÄCK ET AL. asthma. The diagoses of asthma or allergic rhiitis were always made by a doctor ad a previous validatio suggested that the accuracy of the diagosis at the coscript examiatio was fairly good. Approximately 8% of all cases of asthma ad allergic rhiits were detected at the coscript examiatios [12]. Misclassificatio is likely to be odifferetial ad reduce the associatios. Over the years the questios related to allergic disorders i the health questioaire became somewhat more specific. However, it does ot seem probable that this affected the prevalece rates i ay major way sice the diagosis was based primarily o follow-up questios made by physicias durig the health examiatio. To summarise, Swede is a westerised coutry with a high stadard of livig. A substatial icrease i the percetage of asthma cases associated with allergic rhiitis has occurred over a period of three decades. 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