Environmental factors

Abstract

Environmental factors have a key role in the genesis of asthma. Epidemiological studies are starting to provide information1Woolcock AJ Asthma. 2nd ed. Textbook of respiratory medicine. W B Saunders, Philadelphia1994: 1288-1330Google Scholar, 2Chaulet T Asthma and chronic bronchitis in Africa: evidence from epidemiologic studies.Chest. 1989; 96 (3 suppl): 3345-3395Google Scholar on the relevant environmental determinants1Woolcock AJ Asthma. 2nd ed. Textbook of respiratory medicine. W B Saunders, Philadelphia1994: 1288-1330Google Scholar, 2Chaulet T Asthma and chronic bronchitis in Africa: evidence from epidemiologic studies.Chest. 1989; 96 (3 suppl): 3345-3395Google Scholar, 3Becklake MR International Union Against Tuberculosis and Lung Disease (IUATLD): initiatives in non-tuberculous lung disease.Tubercule Lung Dis. 1995; 76: 493-504Summary Full Text PDF PubMed Scopus (15) Google Scholar, 4Strachan DP Anderson HR Limb ES O'Neill A Wells N A national survey of asthma prevalence, severity and treatment in Great Britain.Arch Dis Child. 1994; 70: 174-178Crossref PubMed Scopus (193) Google Scholar, 5Sears MR The epidemiology of asthma.in: O'Byrne PM Asthma as an inflammatory disease. Marcel Dekker, New York1990: 15-48Google Scholar, 6von Mutius E Martinez FD Fritzsch C Nicolai T Roell G Thiemann HH Prevalence of asthma and atopy in two areas West and East Germany.Am J Respir Crit Care Med. 1994; 149: 358-364Crossref PubMed Scopus (835) Google Scholar, 7Odhiambo J Ng'ang'a L Mungai M et al.Rural and urban respiratory health surveys in Kenya school children: participation rates and prevalence of markers of asthma.Am J Respir Crit Care Med. 1994; 149 (pt 2): A385Google Scholar, 8Yobo EODA Custovic A Taggart SCO Asafo-Agyei AP Woodcock A Exercise induced bronchospasm in Ghana: differences in prevalence between urban and rural schoolchildren.Thorax. 1997; 52: 161-165Crossref PubMed Scopus (127) Google Scholar, 9Yermaneberhan H Bekele Z Venn A Lewis S Parry E Britton J Prevalence of wheeze and asthma and relation to atopy in urban and rural Ethiopia.Lancet. 1997; 350: 85-90Summary Full Text Full Text PDF PubMed Scopus (314) Google Scholar and are for practical purposes the only way of identifying these determinants and the potential causes of asthma, and of evaluating the impact of preventive strategies, whether for those at high risk or at the population level. However, epidemiological studies are often limited by the need to use markers of asthma suitable for field work, rather than the criteria necessary for clinical diagnosis. Asthma has usually been defined in terms of symptoms determined by a questionnaire, supplemented in some studies by a measure of airway responsiveness to a non-specific challenge such as histamine, methacholine, cold air, or, in the case of children, exercise.1Woolcock AJ Asthma. 2nd ed. Textbook of respiratory medicine. W B Saunders, Philadelphia1994: 1288-1330Google Scholar Skin tests as a measure of atopy are also increasingly included in such surveys.1Woolcock AJ Asthma. 2nd ed. Textbook of respiratory medicine. W B Saunders, Philadelphia1994: 1288-1330Google Scholar Elucidation of the mechanisms underlying the development of asthma (physiological, cellular, molecular, and genetic) remains in the domain of the laboratory sciences, but must be combined with epidemiological findings to fully understand the importance of the environment.Determinants of asthmaEvidence for environmental factors as important determinants of asthma comes from several sources. Geographical differences in prevalence rates within countries, which have been documented by the same researchers with comparable methodology (population sampling, measurement tools, calculation of study variables, and analysis)3Becklake MR International Union Against Tuberculosis and Lung Disease (IUATLD): initiatives in non-tuberculous lung disease.Tubercule Lung Dis. 1995; 76: 493-504Summary Full Text PDF PubMed Scopus (15) Google Scholar, 4Strachan DP Anderson HR Limb ES O'Neill A Wells N A national survey of asthma prevalence, severity and treatment in Great Britain.Arch Dis Child. 1994; 70: 174-178Crossref PubMed Scopus (193) Google Scholar, 6von Mutius E Martinez FD Fritzsch C Nicolai T Roell G Thiemann HH Prevalence of asthma and atopy in two areas West and East Germany.Am J Respir Crit Care Med. 1994; 149: 358-364Crossref PubMed Scopus (835) Google Scholar, 7Odhiambo J Ng'ang'a L Mungai M et al.Rural and urban respiratory health surveys in Kenya school children: participation rates and prevalence of markers of asthma.Am J Respir Crit Care Med. 1994; 149 (pt 2): A385Google Scholar, 8Yobo EODA Custovic A Taggart SCO Asafo-Agyei AP Woodcock A Exercise induced bronchospasm in Ghana: differences in prevalence between urban and rural schoolchildren.Thorax. 1997; 52: 161-165Crossref PubMed Scopus (127) Google Scholar, 9Yermaneberhan H Bekele Z Venn A Lewis S Parry E Britton J Prevalence of wheeze and asthma and relation to atopy in urban and rural Ethiopia.Lancet. 1997; 350: 85-90Summary Full Text Full Text PDF PubMed Scopus (314) Google Scholar, 10European Community Respiratory Health Survey Variations in the prevalence of respiratory symptoms, self-reported asthma attacks, and use of asthma medication in the European Community Respiratory Health Survey (ECRHS).Eur Respir J. 1996; 9: 687-695Crossref PubMed Scopus (943) Google Scholar are unlikely to be due solely to differences in diagnostic practices and/or criteria. International studies in adults10European Community Respiratory Health Survey Variations in the prevalence of respiratory symptoms, self-reported asthma attacks, and use of asthma medication in the European Community Respiratory Health Survey (ECRHS).Eur Respir J. 1996; 9: 687-695Crossref PubMed Scopus (943) Google Scholar and in children11Asher MI Keil U Anderson HR et al.International study of asthma and allergies in childhood (ISAAC): rationale and methods.Eur Respir J. 1995; 8: 483-491Crossref PubMed Scopus (2752) Google Scholar in which meticulous attention has been given to methodological detail support the clinical impression that differences in disease occurrence between countries are real. In addition there is evidence of an increase in prevalence, particularly in children, in industrialised1Woolcock AJ Asthma. 2nd ed. Textbook of respiratory medicine. W B Saunders, Philadelphia1994: 1288-1330Google Scholar, 5Sears MR The epidemiology of asthma.in: O'Byrne PM Asthma as an inflammatory disease. Marcel Dekker, New York1990: 15-48Google Scholar and in industrialising countries.1Woolcock AJ Asthma. 2nd ed. Textbook of respiratory medicine. W B Saunders, Philadelphia1994: 1288-1330Google Scholar, 3Becklake MR International Union Against Tuberculosis and Lung Disease (IUATLD): initiatives in non-tuberculous lung disease.Tubercule Lung Dis. 1995; 76: 493-504Summary Full Text PDF PubMed Scopus (15) Google Scholar, 5Sears MR The epidemiology of asthma.in: O'Byrne PM Asthma as an inflammatory disease. Marcel Dekker, New York1990: 15-48Google Scholar For instance, in Great Britain, one in seven children is now thought to have asthma,4 whereas sub-saharan African countries, where the disease was nonexistent in the 1960s, now exhibit rates similar to those in many industrialised countries.3Becklake MR International Union Against Tuberculosis and Lung Disease (IUATLD): initiatives in non-tuberculous lung disease.Tubercule Lung Dis. 1995; 76: 493-504Summary Full Text PDF PubMed Scopus (15) Google Scholar, 7Odhiambo J Ng'ang'a L Mungai M et al.Rural and urban respiratory health surveys in Kenya school children: participation rates and prevalence of markers of asthma.Am J Respir Crit Care Med. 1994; 149 (pt 2): A385Google Scholar Finally, the increase in prevalence, particularly of childhood asthma, over the past few decades, has raised the question of whether “a more toxic environment or a more susceptible population”12Seaton A Godden DJ Brown K Increase in asthma: a more toxic environment or a more susceptible population?.Thorax. 1994; 49: 171-174Crossref PubMed Scopus (413) Google Scholar, 13Björksten B Environmental issues in childhood asthma.Eur Respir Rev. 1997; 7: 11-14Google Scholar is responsible. Seaton has argued that the increased prevalence of asthma associated with a westernised lifestyle,6von Mutius E Martinez FD Fritzsch C Nicolai T Roell G Thiemann HH Prevalence of asthma and atopy in two areas West and East Germany.Am J Respir Crit Care Med. 1994; 149: 358-364Crossref PubMed Scopus (835) Google Scholar, 13Björksten B Environmental issues in childhood asthma.Eur Respir Rev. 1997; 7: 11-14Google Scholar, 14Wichmann HE Possible explanation for the different trends of asthma and allergy in east and west Germany.Clin Exp Allergy. 1996; 26: 621-623Crossref PubMed Scopus (71) Google Scholar, 15Platts-Mills TAE Carter MC Asthma and indoor exposure to allergens.N Engl J Med. 1997; 336: 1382-1384Crossref PubMed Scopus (111) Google Scholar or changing from rural to urban living1Woolcock AJ Asthma. 2nd ed. Textbook of respiratory medicine. W B Saunders, Philadelphia1994: 1288-1330Google Scholar, 3Becklake MR International Union Against Tuberculosis and Lung Disease (IUATLD): initiatives in non-tuberculous lung disease.Tubercule Lung Dis. 1995; 76: 493-504Summary Full Text PDF PubMed Scopus (15) Google Scholar, 5Sears MR The epidemiology of asthma.in: O'Byrne PM Asthma as an inflammatory disease. Marcel Dekker, New York1990: 15-48Google Scholar, 7Odhiambo J Ng'ang'a L Mungai M et al.Rural and urban respiratory health surveys in Kenya school children: participation rates and prevalence of markers of asthma.Am J Respir Crit Care Med. 1994; 149 (pt 2): A385Google Scholar, 8Yobo EODA Custovic A Taggart SCO Asafo-Agyei AP Woodcock A Exercise induced bronchospasm in Ghana: differences in prevalence between urban and rural schoolchildren.Thorax. 1997; 52: 161-165Crossref PubMed Scopus (127) Google Scholar, 9Yermaneberhan H Bekele Z Venn A Lewis S Parry E Britton J Prevalence of wheeze and asthma and relation to atopy in urban and rural Ethiopia.Lancet. 1997; 350: 85-90Summary Full Text Full Text PDF PubMed Scopus (314) Google Scholar is more likely to be due to a change in individual and population susceptibility than to an increase in exposure to a sensitising agent such as allergen.The determinants (see panel) of asthma can be considered under the two general categories commonly used in epidemiology, namely environmental and host (see table).1Woolcock AJ Asthma. 2nd ed. Textbook of respiratory medicine. W B Saunders, Philadelphia1994: 1288-1330Google Scholar, 3Becklake MR International Union Against Tuberculosis and Lung Disease (IUATLD): initiatives in non-tuberculous lung disease.Tubercule Lung Dis. 1995; 76: 493-504Summary Full Text PDF PubMed Scopus (15) Google Scholar, 16Miettlinen OS Theoretical epidemiology.in: John Wiley and Sons, New York1985: 317-344Google Scholar Some determinants are modifiable, and have been the target for preventive measures; others such as age, race, and sex are not but are important because they modify the risk of developing asthma in response to environmental exposures in consistent ways and therefore affect the approach to diagnosis and management. For instance, the ratio of male to female rates for asthma varies across the human lifespan.42Kauffmann F Becklake MR Maladies obstructives pulmonaires: un paradigme de la complexité des différences de santé entre hommes et femmes.in: Saurel-Cubizolles MJ Blondell B La santé des femmes. Flammarion, Medecine et Sciences, Paris1996: 209-233Google Scholar Prevalence rates of asthma are higher in boys than in girls whereas from adolescence and throughout the reproductive period of a woman's life, the rates in women exceed those of men but revert after the menopause, though this may be modified by hormone replacement therapy.42Kauffmann F Becklake MR Maladies obstructives pulmonaires: un paradigme de la complexité des différences de santé entre hommes et femmes.in: Saurel-Cubizolles MJ Blondell B La santé des femmes. Flammarion, Medecine et Sciences, Paris1996: 209-233Google Scholar These differences are attributed to the effect of sex hormones associated with menstruation, pregnancy, lactation, and the menopause on airway function in women.PanelDefinition and characteristics of a determinantAn individual characteristic (constitutional, environmental, or behavioural) on which the study outcome (asthma) dependsCan be causal or not16Miettlinen OS Theoretical epidemiology.in: John Wiley and Sons, New York1985: 317-344Google ScholarCan increase or decrease riskCan be established or putativeIf also a risk factor, can be primary (ie, affects incidence of asthma) or secondary (triggers symptoms, exacerbates disease, or increases its severity)TableDeterminants of asthmaCategoryDeterminantsPrimary or secondaryHostGenetic factors1Woolcock AJ Asthma. 2nd ed. Textbook of respiratory medicine. W B Saunders, Philadelphia1994: 1288-1330Google Scholar, 6von Mutius E Martinez FD Fritzsch C Nicolai T Roell G Thiemann HH Prevalence of asthma and atopy in two areas West and East Germany.Am J Respir Crit Care Med. 1994; 149: 358-364Crossref PubMed Scopus (835) Google Scholar, 17Sandford A Weir T Pare P The genetics of asthma.Am J Respir Crit Care Med. 1996; 153: 1749-1765Crossref PubMed Scopus (244) Google ScholarPrimaryFamily history of allergies1, in particular maternal history18PrimaryAtopy1,5,13,18PrimaryRace and/or ethmc otigin1,3,5,20PrimaryEnvironmentalCertain occupational exposures—eg, sensitising chemicals such as isocyanates21PrimaryCommunity air pollution by allergen1,5,22Primary (eg, soy bean)Sustained exposure to indoor allergens, post and pre natal13,15,23,25Primary and secondaryViral infections13,20Secondary, possibly primaryEnvironmental exposure to tobacco smoke in childhood, pre and postnatal26,27Secondary, possibly primaryChanging lifsstyles—eg, to westernised lifestyle,13,14 urban <i>vs</i> rural,1,3,5,9,28,30 migration,1,5 changing homes31Probably secondary but may be primaryCommunity air polution related to vehicle exhaust30,32Certain diets,1,33 breastfeeding practices (absence or short duration),13,5,7,23 and absence of certain infections particularly in the first year of life34,36Probably secondary but may be primaryCertain home characteristics-eg, dampness,1,5,23,27 gas cooking,23,38 carpeting, 13-15 electric home heating23Probably secondarySocioeconomic disadvantage (poverty)20.39Probably secondaryTable includes risk factors implicated in the genesis of asthma and factors that appear to be protective. Note that established determinants and risk factors are in bold type. Primary: will increase the incidence (new cases): if the determinant is susceptible to intervention and if the intervention is successful, the incidence of asthma is reduced. Secondary: triggers symptoms and/or exacerbates the disease and increases its severity. Clinical management depends on interventions on these factors.1 Note that references cited are necessarily selective and readers are referred to several excellent texts1,5,40,41 and revfew articles3,17 Open table in a new tab Several important primary determinants are host factors, which are mainly genetic and are reflected in family history1Woolcock AJ Asthma. 2nd ed. Textbook of respiratory medicine. W B Saunders, Philadelphia1994: 1288-1330Google Scholar, 5Sears MR The epidemiology of asthma.in: O'Byrne PM Asthma as an inflammatory disease. Marcel Dekker, New York1990: 15-48Google Scholar (particularly maternal history) of allergic conditions,18Cookson WOCM Young RP Sandford AJ et al.Maternal inheritance of atopic IgE responsiveness on chromosome 11q.Lancet. 1992; 340: 381-384Summary PubMed Scopus (318) Google Scholar atopy,1Woolcock AJ Asthma. 2nd ed. Textbook of respiratory medicine. W B Saunders, Philadelphia1994: 1288-1330Google Scholar and, more controversially, racel1Woolcock AJ Asthma. 2nd ed. Textbook of respiratory medicine. W B Saunders, Philadelphia1994: 1288-1330Google Scholar, 3Becklake MR International Union Against Tuberculosis and Lung Disease (IUATLD): initiatives in non-tuberculous lung disease.Tubercule Lung Dis. 1995; 76: 493-504Summary Full Text PDF PubMed Scopus (15) Google Scholar, 20Weiss KB Wagener DK Asthma surveillance in the United States: a review of current trends and knowledge gaps.Chest. 1990; 98 (suppl): 179S-184SPubMed Google Scholar and/or ethnic grouping.5Sears MR The epidemiology of asthma.in: O'Byrne PM Asthma as an inflammatory disease. Marcel Dekker, New York1990: 15-48Google ScholarHost determinantsAtopy is considered by some to be the only established risk factor for asthma,19Burney P Why study the epidemiology of asthma?.Thorax. 1988; 43: 425-428Crossref PubMed Scopus (15) Google Scholar and is certainly one of the most important. Most asthma is associated with atopy,1Woolcock AJ Asthma. 2nd ed. Textbook of respiratory medicine. W B Saunders, Philadelphia1994: 1288-1330Google Scholar, 5Sears MR The epidemiology of asthma.in: O'Byrne PM Asthma as an inflammatory disease. Marcel Dekker, New York1990: 15-48Google Scholar, 9Yermaneberhan H Bekele Z Venn A Lewis S Parry E Britton J Prevalence of wheeze and asthma and relation to atopy in urban and rural Ethiopia.Lancet. 1997; 350: 85-90Summary Full Text Full Text PDF PubMed Scopus (314) Google Scholar, 13Björksten B Environmental issues in childhood asthma.Eur Respir Rev. 1997; 7: 11-14Google Scholar and in susceptible individuals exposure levels that induce atopy, asthma, and asthma attacks are different.5Sears MR The epidemiology of asthma.in: O'Byrne PM Asthma as an inflammatory disease. Marcel Dekker, New York1990: 15-48Google Scholar, 37Platts-Mills TAE De Wech AL Dust mite allergens and asthma: a world wide problem.J Allergy Clin Immunol. 1989; 83: 416-427Summary Full Text PDF PubMed Scopus (555) Google Scholar In epidemiological studies, which rely on exposure-response relations to test causal associations, these features of the biology of asthma are not easy to take into account in the statistical modelling techniques used to examine diseases of multifactorial aetiology such as asthma.Environmental determinantsAmong the environmental determinants, occupational exposure is generally accepted as a primary determinant of asthma, for example to sensitising chemicals such as the isocyanates.21Chan-Yeung M Malo JL Occupational asthma.N Engl J Med. 1995; 333: 107-112Crossref PubMed Scopus (338) Google Scholar Community air pollution by allergen has also been responsible for sensitisation (ie, is a primary determinant) as shown by the soy bean Barcelona ~pidemic.1Woolcock AJ Asthma. 2nd ed. Textbook of respiratory medicine. W B Saunders, Philadelphia1994: 1288-1330Google Scholar, 5Sears MR The epidemiology of asthma.in: O'Byrne PM Asthma as an inflammatory disease. Marcel Dekker, New York1990: 15-48Google Scholar, 22Anto JM Sunyer J Reed CE et al.Preventing asthma epidemics due to soybeans by dust-control measures.N Engl J Med. 1991; 329: 1760-1763Crossref Scopus (99) Google Scholar Sustained exposure to indoor allergens postnatally13Björksten B Environmental issues in childhood asthma.Eur Respir Rev. 1997; 7: 11-14Google Scholar, 15Platts-Mills TAE Carter MC Asthma and indoor exposure to allergens.N Engl J Med. 1997; 336: 1382-1384Crossref PubMed Scopus (111) Google Scholar is thought to be a primary determinant for atopy37Platts-Mills TAE De Wech AL Dust mite allergens and asthma: a world wide problem.J Allergy Clin Immunol. 1989; 83: 416-427Summary Full Text PDF PubMed Scopus (555) Google Scholar and a secondary determinant for asthma, responsible for ongoing inflammatior? and asthma severity.1Woolcock AJ Asthma. 2nd ed. Textbook of respiratory medicine. W B Saunders, Philadelphia1994: 1288-1330Google Scholar, 26Weiss ST The origins of childhood asthma.Monaldi Arch Chest Dis. 1994; 49: 154-158PubMed Google Scholar Prenatal exposure to allergens may also be important.24Chan-Yeung M Ferguson AC Manfreda J et al.Does exposure to house dust mite allergens occur in utero?.Am J Respir Crit Care Med. 1997; 155: A73Google Scholar Other possible environmental determinants of asthma in childhood include exposure to respiratory irritants such as environmental tobacco smoke, both before and after birth23Infante-Rivard C Childhood asthma and indoor environmental risk factors.Am J Epidemiol. 1993; 137: 834-844PubMed Google Scholar, 26Weiss ST The origins of childhood asthma.Monaldi Arch Chest Dis. 1994; 49: 154-158PubMed Google Scholar, 27Stick SM Burton PR Gurrin L Sly PD LeSouëf PN Effects of maternal smoking during pregnancy and a family history of asthma on respiratory function in newborn infants.Lancet. 1996; 348: 1060-1064Summary Full Text Full Text PDF PubMed Scopus (375) Google Scholar and some viral infections.13Björksten B Environmental issues in childhood asthma.Eur Respir Rev. 1997; 7: 11-14Google Scholar, 26Weiss ST The origins of childhood asthma.Monaldi Arch Chest Dis. 1994; 49: 154-158PubMed Google Scholar Asthma exacerbations have been linked to urban pollutant concentrations,43Romieu I Meneses F Ruiz S Sierra JJ White MC Etzel RA Effects of air pollution on the respiratory health of asthmatic children living in Mexico City.Am J Respir Crit Care Med. 1996; 154: 300-307Crossref PubMed Scopus (262) Google Scholar while exposure to motor vehicle exhaust in children in Africa30Ng'ang'a LN The epidemiology of childhood asthma in Kenya.in: PhD thesis. McGill University, Montreal1996Google Scholar and Europ32Brunekreef B Dockery DW Krzyzanowski M Epidemiologic studies on short term effects of low levels of major ambient air pollution components.Environ Health Perspect. 1995; 103: 3-13Crossref PubMed Scopus (242) Google Scholar has been linked to increased prevalence of symptoms and other markers of asthma such as non-specific airway responsiveness.30Ng'ang'a LN The epidemiology of childhood asthma in Kenya.in: PhD thesis. McGill University, Montreal1996Google Scholar Paradoxically, certain infections, in addition to being risk factors, appear in some circumstances to be protective.34Shaheen SO Aaby P Hall AJ et al.Measles and atopy in Guinea-Bissau.Lancet. 1996; 347: 1792-1796Summary PubMed Scopus (477) Google Scholar, 35Shirakawa T Enomoto T Shimazu SI Hopkin JM The inverse association between tuberculin responses and atopic disorder.Science. 1997; 275: 77-79Crossref PubMed Scopus (1167) Google Scholar, 36von Mutius E Martinez FD Fritzsch C Nicolai T Reitmeir P Thiemann HH Skin test reactivity and number of siblings.BMJ. 1994; 308: 692-695Crossref PubMed Scopus (396) Google ScholarChanging environments and changing lifestylesSeveral of the environmental factors listed in the table are associated with changing lifestyles, and are probably surrogates for various environmental determinants of asthma. For instance, socioeconomic disadvantage (poverty) has been associated with increased rates of asthma in children in the USA living in inner citie20Weiss KB Wagener DK Asthma surveillance in the United States: a review of current trends and knowledge gaps.Chest. 1990; 98 (suppl): 179S-184SPubMed Google Scholar and in Montreal school children.39Ernst P Demissie K Joseph L Locher U Becklake MR Socioeconomic status and indicators of asthma in children.Am J Respir Crit Care Med. 1995; 152: 570-575Crossref PubMed Scopus (71) Google Scholar This association may be a reflection of poor housing conditions, dampness1Woolcock AJ Asthma. 2nd ed. Textbook of respiratory medicine. W B Saunders, Philadelphia1994: 1288-1330Google Scholar, 5Sears MR The epidemiology of asthma.in: O'Byrne PM Asthma as an inflammatory disease. Marcel Dekker, New York1990: 15-48Google Scholar, 23Infante-Rivard C Childhood asthma and indoor environmental risk factors.Am J Epidemiol. 1993; 137: 834-844PubMed Google Scholar, 37Platts-Mills TAE De Wech AL Dust mite allergens and asthma: a world wide problem.J Allergy Clin Immunol. 1989; 83: 416-427Summary Full Text PDF PubMed Scopus (555) Google Scholar (which encourages the growth of moulds and house-dust mite37Platts-Mills TAE De Wech AL Dust mite allergens and asthma: a world wide problem.J Allergy Clin Immunol. 1989; 83: 416-427Summary Full Text PDF PubMed Scopus (555) Google Scholar), use of certain fuels for home heating,23Infante-Rivard C Childhood asthma and indoor environmental risk factors.Am J Epidemiol. 1993; 137: 834-844PubMed Google Scholar or cooking with unvented gas appliances23Infante-Rivard C Childhood asthma and indoor environmental risk factors.Am J Epidemiol. 1993; 137: 834-844PubMed Google Scholar, 38Brauer M Kennedy SM Gas stoves and respiratory health.Lancet. 1996; 347: 412Summary Full Text PDF PubMed Scopus (5) Google Scholar wood.44von Murius E Illi S Nicolai T Martinez FD Relation of indoor heating with asthma, allergic sensitisation, and bronchial responsiveness: survey of children in South Bavaria.BMJ. 1996; 312: 1448-1450Crossref PubMed Scopus (99) Google Scholar Children who have lived in more than one home also appear to be at greater risk of developing asthma.31Austin JB Russell G Wheezing, cough, atopy and indoor environment in the Scottish highlands.Arch Dis Child. 1997; 76: 22-26Crossref PubMed Scopus (68) Google Scholar The higher prevalences of atopy and of allergic conditions including asthma in children and adults of former west than east Germany6von Mutius E Martinez FD Fritzsch C Nicolai T Roell G Thiemann HH Prevalence of asthma and atopy in two areas West and East Germany.Am J Respir Crit Care Med. 1994; 149: 358-364Crossref PubMed Scopus (835) Google Scholar, 14Wichmann HE Possible explanation for the different trends of asthma and allergy in east and west Germany.Clin Exp Allergy. 1996; 26: 621-623Crossref PubMed Scopus (71) Google Scholar have been attributed to changes in lifestyle which followed the political separation of a country with a homogeneous ethnic population and previously similar housing and living conditions.14Wichmann HE Possible explanation for the different trends of asthma and allergy in east and west Germany.Clin Exp Allergy. 1996; 26: 621-623Crossref PubMed Scopus (71) Google Scholar Explanations suggested are higher concentrations of allergens in west German homes due to better home insulation and more frequent use of wall-to-wall carpeting (a source of mites and moulds), while less frequent use of daycare facilities may have protected former west German children from exposure to certain childhood infections which, if experienced early (particularly in the first year of life),45Cookson WOCM Moffatt MF Asthma: an epidemic in the absence of infection?.Science. 1997; 275: 41-42Crossref PubMed Scopus (274) Google Scholar may be protective.23Infante-Rivard C Childhood asthma and indoor environmental risk factors.Am J Epidemiol. 1993; 137: 834-844PubMed Google Scholar, 34Shaheen SO Aaby P Hall AJ et al.Measles and atopy in Guinea-Bissau.Lancet. 1996; 347: 1792-1796Summary PubMed Scopus (477) Google Scholar, 35Shirakawa T Enomoto T Shimazu SI Hopkin JM The inverse association between tuberculin responses and atopic disorder.Science. 1997; 275: 77-79Crossref PubMed Scopus (1167) Google Scholar, 36von Mutius E Martinez FD Fritzsch C Nicolai T Reitmeir P Thiemann HH Skin test reactivity and number of siblings.BMJ. 1994; 308: 692-695Crossref PubMed Scopus (396) Google ScholarMigration from industrialising to industrialised countries has been identified as a risk factor for Asian children moving to the UK and for Pacific islanders moving to Australia and New Zealand.1Woolcock AJ Asthma. 2nd ed. Textbook of respiratory medicine. W B Saunders, Philadelphia1994: 1288-1330Google Scholar, 5Sears MR The epidemiology of asthma.in: O'Byrne PM Asthma as an inflammatory disease. Marcel Dekker, New York1990: 15-48Google Scholar The same phenomenon has been observed in relation to within country migration from rural to urban areasL1Woolcock AJ Asthma. 2nd ed. Textbook of respiratory medicine. W B Saunders, Philadelphia1994: 1288-1330Google Scholar, 3Becklake MR International Union Against Tuberculosis and Lung Disease (IUATLD): initiatives in non-tuberculous lung disease.Tubercule Lung Dis. 1995; 76: 493-504Summary Full Text PDF PubMed Scopus (15) Google Scholar, 5Sears MR The epidemiology of asthma.in: O'Byrne PM Asthma as an inflammatory disease. Marcel Dekker, New York1990: 15-48Google Scholar in most,3Becklake MR International Union Against Tuberculosis and Lung Disease (IUATLD): initiatives in non-tuberculous lung disease.Tubercule Lung Dis. 1995; 76: 493-504Summary Full Text PDF PubMed Scopus (15) Google Scholar, 9Yermaneberhan H Bekele Z Venn A Lewis S Parry E Britton J Prevalence of wheeze and asthma and relation to atopy in urban and rural Ethiopia.Lancet. 1997; 350: 85-90Summary Full Text Full Text PDF PubMed Scopus (314) Google Scholar, 28Van Niekerk CH Weinberg EC Shore SC Heese HV Van Schalkwyk KR Prevalence of asthma: a study of urban and rural Xhosa children.Clin Allergy. 1979; 9: 319-324Crossref PubMed Scopus (200) Google Scholar, 29Keeley DJ Neill P Gallivan S Comparison of the prevalence of reversible airways obstruction in rural and urban Zimbabwean children.Thorax. 1991; 46: 549-553Crossref PubMed Scopus (165) Google Scholar, 30Ng'ang'a LN The epidemiology of childhood asthma in Kenya.in: PhD thesis. McGill University, Montreal1996Google Scholar but not all8Yobo EODA Custovic A Taggart SCO Asafo-Agyei AP Woodcock A Exercise induced broncho