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Economic evaluation of the benefits of reducing acute cardiorespiratory morbidity associated with air… Stieb, David M; De Civita, Paul; Johnson, F Reed; Manary, Matthew P; Anis, Aslam H; Beveridge, Robert C; Judek, Stan Dec 18, 2002

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ralEnvironmental Health: A Global ssBioMed CentAccess Science SourceOpen AcceResearchEconomic evaluation of the benefits of reducing acute cardiorespiratory morbidity associated with air pollutionDavid M Stieb*1, Paul De Civita1, F Reed Johnson2, Matthew P Manary3, Aslam H Anis4, Robert C Beveridge5 and Stan Judek1Address: 1Healthy Environments and Consumer Safety Branch, Health Canada, Ottawa, ON, Canada, 2Research Triangle Institute, Research Triangle Park, NC, USA, 3Triangle Economic Research, Durham, NC, USA (at time of study completion), 4Department of Health Care & Epidemiology, University of British Columbia, Vancouver, BC, Canada and 5Department of Medicine, Queen's University, Brockville, ON, CanadaEmail: David M Stieb* - dave_stieb@hc-sc.gc.ca; Paul De Civita - paul_decivita@hc-sc.gc.ca; F Reed Johnson - frjohnson@rti.org; Matthew P Manary - matthewmanary@yahoo.com; Aslam H Anis - anis@hivnet.ubc.ca; Robert C Beveridge - drbob_beveridge@bigfoot.com; Stan Judek - stan_judek@hc-sc.gc.ca* Corresponding author    AbstractBackground: Few assessments of the costs and benefits of reducing acute cardiorespiratorymorbidity related to air pollution have employed a comprehensive, explicit approach to capturingthe full societal value of reduced morbidity.Methods: We used empirical data on the duration and severity of episodes of cardiorespiratorydisease as inputs to complementary models of cost of treatment, lost productivity, and willingnessto pay to avoid acute cardiorespiratory morbidity outcomes linked to air pollution inepidemiological studies. A Monte Carlo estimation procedure was utilized to propagate uncertaintyin key inputs and model parameters.Results: Valuation estimates ranged from $13 (1997, Canadian) (95% confidence interval, $0–28)for avoidance of an acute respiratory symptom day to $5,200 ($4,000–$6,400) for avoidance of acardiac hospital admission. Cost of treatment accounted for the majority of the overall value ofcardiac and respiratory hospital admissions as well as cardiac emergency department visits, whilelost productivity generally represented a small proportion of overall value. Valuation estimates fordays of restricted activity, asthma symptoms and acute respiratory symptoms were sensitive toalternative assumptions about level of activity restriction. As an example of the application of thesevalues, we estimated that the observed decrease in particulate sulfate concentrations in Torontobetween 1984 and 1999 resulted in annual benefits of $1.4 million (95% confidence interval $0.91–1.8 million) in relation to reduced emergency department visits and hospital admissions forcardiorespiratory disease.Conclusion: Our approach to estimating the value of avoiding a range of acute morbidity effectsof air pollution addresses a number of limitations of the current literature, and is applicable tofuture assessments of the benefits of improving air quality.Published: 18 December 2002Environmental Health: A Global Access Science Source 2002, 1:7Received: 11 September 2002Accepted: 18 December 2002This article is available from: http://www.ehjournal.net/content/1/1/7© 2002 Stieb et al; licensee BioMed Central Ltd. This is an Open Access article: verbatim copying and redistribution of this article are permitted in all media for any purpose, provided this notice is preserved along with the article's original URL.Page 1 of 13(page number not for citation purposes)BackgroundNumerous assessments have been conducted around theworld of the costs and/or benefits of interventions to im-prove air quality [1–7]. Benefits assessments typicallyEnvironmental Health: A Global Access Science Source 2002, 1 http://www.ehjournal.net/content/1/1/7employ the damage function approach, consisting of esti-mating changes in air pollution emissions and resultantchanges in ambient air quality; calculating the number ofavoided adverse health and environmental outcomes us-ing concentration-response functions; and applying eco-nomic value measures to avoided outcomes [8], oftenextrapolating valuation estimates from the original con-text in which data were collected [9]. The majority of ben-efits generally derive from improved human health,including both reduced mortality and morbidity. Benefitsof reduced mortality and chronic morbidity tend to dom-inate these assessments, accounting for over 90 percent ofbenefits in most instances [1,7]. Nonetheless, acute mor-bidity is still viewed as an important component of thebroad public health burden associated with air pollution[10], and benefits assessments will undoubtedly continueto include acute morbidity effects. Thus, future assess-ments will need to address deficiencies in past work deal-ing with acute morbidity, including failure to consistentlyemploy a comprehensive, explicit approach to capturingall components of the societal value of reduced morbidityand differing levels of severity of symptoms and activityrestrictions.In earlier analyses [3,5], consideration of morbidity effectsof air pollution was restricted to a limited range of symp-toms and activity restrictions as observed in clinical andepidemiological studies. These assessments, as well asmore recent ones [2], relied heavily on contingent valua-tion studies [11–13] in which individual willingness topay (WTP) to avoid a variety of symptoms of varying du-ration was evaluated. In more recent assessments, therehas been a greater reliance on cost of illness (COI) data,comprising medical costs and lost production experi-enced by society as a result of a health effect. However,these measures represent at best a lower bound on the to-tal societal value of avoiding these health outcomes [14],since they reflect only one element of this value. In someassessments, no adjustments have been made to correctfor this bias [6,7], while in others, empirical evidencefrom studies which simultaneously measured COI and in-dividual WTP have been used to derive adjustment factors[1,4,15–17]. Such adjustments recognize deficiencies inavailable estimates, but applying them uniformly to a va-riety of cardiorespiratory conditions could introduce errorin benefits assessments. An additional issue is that mostprevious assessments have relied heavily upon valuationestimates derived from U.S. data, the applicability ofwhich outside the U.S. can be questioned.In the present study, our objective was to estimate thebenefits of avoiding a variety of acute cardiorespiratorymorbidity outcomes which have been linked epidemio-sistent, comprehensive approach in earlier studies, partic-ular features of our approach include application to alloutcomes of: a consistent conceptual model of the natureof acute episodes of cardiorespiratory disease, includingdiffering levels of severity of symptoms and activity re-strictions; a comprehensive valuation framework encom-passing all aspects of the value of avoided morbidity anda recognition of the perspective (societal versus individu-al) from which primary data were collected; a commonsource of inputs based on empirical data on the durationand severity of episodes of cardiorespiratory disease; andan estimation procedure which propagates uncertainty inkey inputs.MethodsConceptual model of acute cardiorespiratory morbidityWe identified several types of acute cardiorespiratory mor-bidity which have been linked with air pollution in epide-miological studies: respiratory and cardiac hospitaladmissions and emergency department visits, restrictedactivity days, asthma symptom days, and acute respiratorysymptom days [18–21]. Based on this literature, we iden-tified asthma, chronic obstructive pulmonary disease(COPD), respiratory infections, congestive heart failure,cardiac dysrhythmias, and myocardial infarction/angina,as well as mild non-specific respiratory symptoms, as theprincipal conditions of interest [18–22].As a general framework, we viewed these conditions as oc-curring in the form of acute episodes superimposed uponthe underlying baseline health state. The extent to whichhealth status departed from its premorbid level could beexpected to vary during the course of the episode, so thatthe episode of illness could be characterized as the sum ofseveral periods, each of which is associated with a givendecrement from baseline health status. Depending on thenature of the morbidity outcome, these periods might in-clude periods spent in or out of hospital, and in variousstates of restriction of activity.Valuation frameworkReduced morbidity due to cleaner ambient air representsreal economic value to individuals and society, measuredmost appropriately in a welfare economic framework byWTP to improve health or willingness to accept compen-sation (WTA) to worsen it [23]. Measures such as quality-or disability-adjusted life years (QALYs or DALYs) repre-sent an alternative measure which generically reflects pref-erences among health states involving loss of quality and/or quantity of life [24]. However, experience with applica-tion to acute effects of air pollution is limited [25], andunlike dollar valuation of health outcomes, these meas-ures do not permit direct comparisons to costs. WTP (andPage 2 of 13(page number not for citation purposes)logically to air pollution, using complementary data onCOI and individual WTP. In contrast to the lack of a con-WTA) are generally regarded as comprising several com-ponents: expenditures related to medical treatment ofEnvironmental Health: A Global Access Science Source 2002, 1 http://www.ehjournal.net/content/1/1/7adverse health effects, including costs of treatment as wellas related out of pocket expenditures (for example, for as-sistance in completing usual unpaid chores that the indi-vidual is unable to undertake as a result of ill health); thedollar value of interruption in work duties, which canrange from decreased productivity to absence from theworkplace; expenditures to avert or mitigate future occur-rences of the effect (eg. purchasing an air conditioningunit as a means of reducing the chances of an asthma at-tack associated with outdoor air pollution); and the dollarvalue of pain and suffering, inconvenience, restrictionsand reduced enjoyment of leisure activities, anxiety aboutthe future, and concern and inconvenience to familymembers and others [14]. The perspective of the analysis(societal versus individual) is clearly an important consid-eration, in that individual WTP will not reflect medical ex-penditures and lost productivity where there is third partypayment of medical expenditures or paid sick leave[8,14,26].A comprehensive measure of the total value, from a soci-etal perspective, of avoiding the acute cardio-respiratorymorbidity effects of air pollution can therefore be ex-pressed as follows:VT = VAE + VPS + VCOT + VLP  (1)where,VAE = value of reduced averting expenditures;VPS = value of reduced pain and suffering;VCOT = cost of treatment including reduced expenditureson mitigation of the illness; andVLP = value of lost productivity.Sources of empirical dataWe used empirical results from two recent studies in orderto generate estimates of VT. Both studies have been de-scribed in detail elsewhere [27–29], but are summarizedbriefly below.In the first study, joint estimates were constructed of indi-vidual WTP to reduce acute episodes of respiratory andcardiovascular ill health based on graded-pairs andchoice-based stated preference survey responses among399 randomly selected subjects in Toronto in 1997[28,30]. The survey instrument employed health-state de-scriptions based on the Quality of Well Being (QWB) in-dex [31,32], with attributes comprising symptoms, motorfunction, and ability to fulfill roles and expectations. Theabove. Four levels of activity restriction were considered(Table 1). The payment vehicle employed in this surveywas described as illness related costs not covered by thegovernment health system or a company insurance plan.Because pre-testing revealed that respondents with andwithout paid sick leave interpreted costs differently andthis led to ambiguity in interpretation of their WTP, re-spondents were instructed to assume that they had paidsick leave. By combining results from graded-pairs andchoice formats, the resulting estimates from this study aremore robust than estimates based on a single format. Inaddition, relative to other studies in the WTP literature inthis area, estimates from this study were based on compre-hensive multi-attribute health state descriptions whichensured that key attributes were clearly and consistentlypresented to respondents, for the wide range of symptomsand severity levels covered in the study. Estimates fromthis study were not systematically smaller or larger thanother literature and varied logically with duration and se-verity of outcomes. However, given the payment vehicleand the instructions regarding sick leave, we determinedthat individual WTP estimated in this study would not re-flect VLP or that portion of VCOT covered by public healthcare system or third party insurance. We obtained thesedata from the following study.In the second study, data were collected prospectively onindividuals visiting emergency departments for cardiores-piratory conditions, as part of an epidemiological study ofair pollution in Saint John, Canada between 1992 and1996 [29,33]. A random sample of those agreeing to fol-low-up was contacted two weeks after their visit and askedto complete a structured telephone interview, in which in-formation was collected on the duration of the disease ep-isode which prompted their visit, disposition of their visit(whether they were admitted or discharged), and the oc-currence of various outcomes including restricted activitydays, days of missed work for the patient, parent, or othercare giver, health care utilization and illness-related ex-penses, from the onset of their earliest symptoms until thefollow-up interview. Data were obtained for 1,772 indi-viduals. Inpatient utilization data were also abstractedfrom the clinical records of 393 randomly selected pa-tients representing the same groups of cardiorespiratoryconditions, and cost of illness estimates were generatedwhich included fully allocated costs related to hospitalutilization and emergency department visits, as well ascosts of physician visits, medication use, equipment, andout-of-pocket expenses [27]. We analyzed these data usingstepwise ordinary least squares regression. Selection of thefinal multi-variate model was based on minimization ofAkaike's Information Criterion, the deviance penalizedfor the number of parameters being estimated [34]. Com-Page 3 of 13(page number not for citation purposes)scope of symptoms was selected to correspond to the car-diorespiratory conditions of policy interest as describedpared to other literature in this area, this study has the ad-vantage of having been designed specifically toEnvironmental Health: A Global Access Science Source 2002, 1 http://www.ehjournal.net/content/1/1/7prospectively quantify the broad quality of life impactsand COI associated with episodes of cardiorespiratory dis-ease, thus ensuring that the required data elements werecollected in a consistent, coordinated fashion. Cost analy-sis was carried out in a manner consistent with currentmethodological standards.Estimation procedureBased on these two studies, we calculated VT according toEquation (1) as follows:VPS + VAE = value from stated preference survey = VSP(SiA-jDk) [28,30]where Si, Aj, and Dk are the symptom complex, level of ac-tivity restriction and duration of the illness episode re-spectively (See [28] for details of the stated preferencevaluation estimation procedure).VCOT = f (diagnosis, duration, admission to critical care/noncritical care) (See results section below for modelspecification);VLP = DLP × WDwhere DLP = duration of lost productivity in days; and WDis the average daily wage for Canada in 1997 of $119.60[35].While recent recommendations note the friction costmethod as an alternative to the conventional human cap-ital approach as potentially more accurately reflecting theability to replace disabled workers, thus minimizing soci-etal productivity losses [26,36], we assumed that this wasnot applicable in the current context, where illness epi-sodes are of short duration. We did not attempt to valueproductivity losses not resulting in lost time (i.e. reducedwork capacity), but we did capture time lost by parentsand other care givers.In estimating VSP, we mapped symptoms and activity re-strictions to the various health outcomes identifed in epi-demiolgical and clinical studies as shown in Table 2.Some outcomes were mapped to time periods both in andout of hospital, in keeping with the expected natural his-tory of an acute episode of cardiorespiratory morbidity.Hospital admissions, for example, were considered to oc-cur following a period over which symptoms and activityrestrictions began and worsened to the point that hospitaladmission was required, and in turn, to be followed by aperiod of recovery out of hospital before returning to thebaseline health state. Similarly, a proportion of emergen-cy department visits was expected to result in eventual ad-mission to hospital, with accompanying periods ofsymptom onset and recovery. While, a priori, both asthmasymptom days and restricted activity days would includedays in hospital, on the basis of population-based data,Table 1: Description of symptoms and activity restrictionsType of symptom/activity restriction DescriptionUpper Respiratory Stuffy or runny nose and sore throat.Eye Irritation Eye irritation.Asthma/COPDa Coughing, wheezing, shortness of breath.Respiratory Infection Coughing or wheezing with fever, chills, aching all over.Dysrhythmia Fluttering in chest and feeling light-headedCongestive Heart Failure Shortness of breath, and swelling in ankles and feet.Myocardial Infarction/Angina Pain in chest or arm.Mild Limitationsb Can go to work, go to school, do housework, and participate in social or recreational activities, but have some physicial limitations (trouble bending, stooping, or doing vigorous activities because of this health condition).Can go to work, go to school, do housework, but have some physical limitations (trouble bending, stoop-ing, or doing vigorous activities), and cannot participate in social or recreational activities because of this health condition.Housebound Cannot leave house, go to work, go to school, do housework, participate in social or recreational activities, and have some physical limitations (trouble bending, stooping, or doing vigorous activities) because of this health condition, but can care for self.Help caring for Self Cannot leave house, go to work, go to school, do housework, participate in social or recreational activities, and need help caring for self (feeding, bathing, dressing, toilet).In Hospital In hospital and need help caring for self (feeding, bathing, dressing, toilet)achronic obstructive pulmonary disease bcollapsed into a single level corresponding to mild restrictions, for the purposes of the final model used to generate WTP estimates.Page 4 of 13(page number not for citation purposes)we estimated that the latter would account for a very smallEnvironmental Health: A Global Access Science Source 2002, 1 http://www.ehjournal.net/content/1/1/7proportion of these less severe outcomes [1,22,37]. Wetherefore did not map these outcomes to the in-hospitallevel of activity restriction in generating our estimates.Mapping of symptoms and levels of activity restrictions toa given outcome was derived from empirical data fromour own emergency department study [29], and from apopulation-based survey in Strasbourg, France on theprevalence of minor respiratory symptoms [38]. Adjust-ments were made, where applicable, to shift the distribu-tion towards the mild end of the spectrum to reflect thoseindividuals who do not visit the emergency departmentand whose symptoms are presumably less severe onaverage.The definitions of a number of health outcomes as exam-ined in the air pollution epidemiology literature overlapwith one another (e.g. hospital admissions and emergen-cy department visits, restricted activity days and acute res-piratory symptoms), and this is reflected in our mappingof these outcomes as shown in Table 2. This needs to beconsidered in order to avoid double counting of benefitsamong these health outcomes. Procedures have been sug-gested to deal with this, for instance by subtracting occur-rences of restricted activity days, from those related toacute respiratory symptom days, prior to applying valua-tion estimates [1].All inputs to our estimates pertaining to the duration ofical care level (Table 3), were derived from Stieb et al. [29].Based on data from the U.S. National Health InterviewSurvey, it was determined that work loss days comprise 21percent of restricted activity days (standard error = 1.07%)[39] and we applied this factor in estimating lost produc-tivity costs associated with restricted activity days. This isconsistent with our mapping to the three levels of activityrestriction, where 20% fell into the housebound or needhelp category. We assumed that 10 percent of asthmasymptom days and acute respiratory symptom days werework loss days, based on the evidence described earlier.Valuation estimates for cardiac and respiratory hospitaladmissions and emergency department visits were calcu-lated as the weighted average among specific cardiac andrespiratory conditions, with the weights based on the pro-portion of hospital admissions and emergency depart-ment visits accounted for by these conditions as reportedin Burnett et al. [22] and Stieb et al. [33].We obtained Monte Carlo estimates for each morbidityoutcome by iteratively calculating its components (n =1000), sampling from the observed distributions of mod-el parameters, accounting for correlation among parame-ters, and from the observed distributions of input values.We report a central estimate which reflects model resultsevaluated at the mean of input values, and a 95% confi-dence interval bounded by the 2.5th and 97.5th percentilesof the output distribution. Where demographic informa-Table 2: Mapping of symptoms and activity restrictions to health outcomes examined in epidemiological studiesARSa RAD ASD REDV RHA CEDV CHASymptom Weightings (proportions)Upper Respiratory 0.70 0.40Eye irritation 0.30 0.15Asthma/COPDb 0.15 1.00 0.44 0.60Respiratory Infection 0.15 0.56 0.40Dysrhythmia 0.05 0.23 0.13Congestive Heart Failure 0.05 0.27 0.28Myocardial Infarction/Angina 0.05 0.50 0.59Activity Level Weightings (proportions)Mild limitations 0.90 0.80 0.90 0.74 0.50 0.41 0.34Housebound 0.10 0.15 0.10 0.14 0.09 0.08 0.06Need help caring for self 0.05 0.05 0.03 0.03 0.02In hospital 0.07 0.37 0.48 0.57aARS = Acute Respiratory Symptom Day ASD = Asthma Symptom Day RAD = Restricted Activity Day REDV = Respiratory Emergency Depart-ment Visit RHA = Respiratory Hospital Admission CEDV = Cardiac Emergency Department Visit CHA = Cardiac Hospital Admission bchronic obstructive pulmonary diseasePage 5 of 13(page number not for citation purposes)time in and out of hospital and of lost work days, and theprobability of requiring inpatient care at the ward or crit-tion (age, education, income) and population health sta-tus were employed as independent variables in calculatingEnvironmental Health: A Global Access Science Source 2002, 1 http://www.ehjournal.net/content/1/1/7stated-preference values, we conducted a sensitivity analy-sis by employing both the values of these variables in theoriginal survey sample, as well as values based on the1996 Canadian census and the 1996 National PopulationHealth Survey [40]. While specific subgroups of those ex-periencing acute episodes of cardiorespiratory diseasemay be more specifically susceptible to air pollution as atrigger, and the severity of their disease episodes may dif-fer from others, it is not possible to identify these groupswith confidence. In order to deal with uncertainty gener-ated by this issue, we also conducted a sensitivity analysisby varying the weight given to the least versus most severelevels of activity restriction and symptoms. When compar-ing our results with those from earlier studies, we convert-ed values from other countries to the same year inCanadian dollars using purchasing power parity [41].They were adjusted to 1997 Canadian dollars using theconsumer price index for Canada for either all goods andservices (WTP based estimates) or for health care (cost oftreatment based estimates) [42]. While caution should beexercised in transferring our estimates to other countries,a similar procedure would be employed to do this, by ap-plying the 1997 purchasing power parity between Canadaand the target country, and adjusting to the currency yearof interest using the country specific consumer price indexfor all goods and services.ResultsCost of treatment modelResults from our analysis of cost of treatment data are pre-sented in Table 4. The largest coefficients were associatedwith a diagnosis of congestive heart failure, and admis-sion to critical care or non-critical care units. As reflectedin the interaction terms, critical-care (CC) costs for respi-ratory infections (RI) and dysrhythmias (DYS) wereconsiderably lower (RI*CC and DYS*CC), as were noncritical-care (NCC) costs for asthma (AS*NCC). However,non critical-care costs were higher for those with a diagno-sis of myocardial infarction/angina (MIA*NCC). Dura-tion was associated with a coefficient of approximately$30 per day, but interaction terms essentially eliminatedthis effect for some diagnoses.Comprehensive valuation estimatesResults of our calculations of overall valuation for eachendpoint are presented in Table 5. Cost of treatment ac-counted for the majority of the overall value in the case ofrespiratory and cardiac hospital admissions, as well as car-diac emergency department visits. In the case of respirato-ry emergency department visits, cost of treatmentaccounted for approximately forty-five percent of theoverall value. Lost productivity costs represented a smallproportion of the overall value, although the proportionwas greater for those endpoints which do not include acost of treatment component. For both hospital admis-Table 3: Inputs on duration and disposition for valuation calculationsCondition Admitted to: Number of days:Critical Care Non-critical Care In Hospital Out of Hospital Lost WorkPercent Standard ErrorPercent Standard ErrorMean Standard ErrorMean Standard ErrorMean Standard ErrorIndividuals admitted to hospitalAsthma 23.1% 4.8% 76.9% 4.8% 4.4 0.3 9.3 1.1 2.8 0.5Chronic Obstructive Pulmonary Disease0.0% 0.0% 100.0% 0.0% 7.9 0.7 9.4 2.6 0.8 0.6Respiratory Infection 11.3% 3.4% 88.7% 3.4% 5.1 0.4 8.7 1.4 3.3 0.6Congestive Heart Failure 12.9% 6.0% 87.1% 6.0% 7.2 0.7 8.9 2.4 1.2 0.6Dysrhythmia 27.8% 10.6% 72.2% 10.6% 5.6 0.8 6.5 2.3 3.5 1.5Myocardial infarction/angina 65.0% 3.8% 35.0% 3.8% 6.0 0.3 3.2 0.8 2.4 0.4Individuals visiting emergency departmentAsthma 2.6% 0.6% 8.5% 1.1% 0.6 0.1 12.9 0.5 1.2 0.1Chronic Obstructive Pulmo-nary Disease0.0% 0.0% 38.5% 5.5% 3.9 0.6 14.0 1.7 0.7 0.3Respiratory Infection 1.5% 0.5% 12.1% 1.3% 0.9 0.1 14.3 0.5 1.5 0.1Congestive Heart Failure 8.9% 4.2% 60.0% 7.3% 4.8 0.7 8.5 1.8 0.8 0.4Dysrhythmia 9.6% 4.1% 25.0% 6.0% 1.9 0.5 5.3 1.0 1.7 0.6Myocardial infarction/angina 61.8% 3.8% 33.3% 3.7% 5.8 0.3 3.1 0.8 2.3 0.4Page 6 of 13(page number not for citation purposes)sions and emergency department visits, the overall valuewas larger for cardiac compared to respiratory conditions.Environmental Health: A Global Access Science Source 2002, 1 http://www.ehjournal.net/content/1/1/7This is primarily attributable to the larger cost of treat-ment for cardiac versus respiratory conditions. The ratiobetween total value and COI (i.e. cost of treatment pluslost productivity), ranged from 1.3 to 1.9 for those end-points with a cost of treatment component, while the ratiotended to be larger for those endpoints without a cost oftreatment component.Results were not sensitive to choice of input values for de-mographic and health status characteristics (original sur-vey sample characteristics versus nationally representativevalues for Canada). Sensitivity to the weighting given tothe levels of activity restriction differed among the end-points. The total value of avoiding respiratory and cardiachospital admissions and emergency department visits var-ied by less than 10% from base estimates when we variedthe weighting given to least versus most restrictive healthstates based on empirical data as described earlier. Rela-tive to the base values, the total value of avoiding restrict-ed activity days and asthma symptom days wasrespectively 54 and 57% lower and 65 and 104% higherwhen weighting of the different health states was varied.In all of the sensitivity analyses for restricted activity daysand asthma symptom days, point estimates fell within the95% confidence intervals on the base case estimates.However, the 95% confidence interval on the valuationestimate for asthma symptom days with less severe activi-ty restrictions overlapped 0. For acute respiratory symp-tom days, valuation estimates for less and greater activityestimate. In all cases the 95% confidence interval over-lapped 0.DiscussionOur estimates of the value of avoiding a range of acute ef-fects of air pollution on cardiorespiratory morbidity aregenerally consistent with expectation in terms of the mag-nitude of values relative to severity and duration. Thehigher valuation of avoiding cardiac compared to respira-tory disease episodes is driven mainly by the much higherproportion of cardiac conditions requiring admission tohospital, critical care, and invasive procedures [29]. Valu-ation for emergency department visits for both types ofconditions includes cost of treatment related to hospitaladmission for those patients who were ultimately admit-ted. Because cardiac patients were much more likely to beadmitted, this is reflected in a much larger cost of treat-ment value for cardiac compared to respiratory emergencydepartment visits. While this would result in doublecounting of benefits where both hospital admissions andemergency visits are assessed and monetized in a benefitsanalysis, this approach was considered appropriate be-cause concentration-response relationships for air pollu-tion and emergency department visits are based on allthose who visit the emergency department, includingthose who are ultimately admitted to hospital. Whereboth types of outcomes are being considered in benefitsanalysis, double counting can be avoided by using con-centration-response relationships for emergency depart-Table 4: Parameter estimates for cost of treatment modelVariable NameVariable Description Parameter Estimate ($) Standard Error ($)VCOT = α + βASAS + βCHFCHF + βDD + βCCCC + βNCCNCC + βAS*DAS*D + βRI*DRI*D + βMIA*DMIA*D + βRI*CCRI*CC + βDYS*CCDYS*CCIntercept Intercept term 348.58 90.71AS Dummy variable for diagnosis of asthma (0,1) 440.33 125.97CHF Dummy variable for diagnosis of congestive heart failure (0,1) 1680.09 406.92D Total duration of disease episode (days) 31.70 7.85CC Dummy variable for admission to hospital in critical care unit (0,1) 4530.36 176.20NCC Dummy variable for admission to hospital in non-critical care unit (0,1) 1977.94 163.67AS*D Interaction term (see variable definitions above) -27.71 9.17RI*D Interaction term where RI is a dummy variable for diagnosis of respiratory infec-tion (0,1)-29.02 8.07MIA*D Interaction term where MIA is a dummy variable for diagnosis of myocardial inf-arction/angina (0,1)-30.54 14.50RI*CC Interaction term (see variable definitions above) -1544.58 538.12DYS*CC Interaction term where DYS is a dummy variable for diagnosis of dysrhythmia (0,1)-2402.73 906.97AS*NCC Interaction term (see variable definitions above) -431.01 272.89MIA*NCC Interaction term (see variable definitions above) 1180.79 301.74CHF*NCC Interaction term (see variable definitions above) -2158.82 539.88Page 7 of 13(page number not for citation purposes)restrictions were respectively $12 and $28, both of whichwere within the 95% confidence interval on the base casement visits to capture both types of effects, either by usingrelationships taken directly from epidemiological studiesEnvironmental Health: A Global Access Science Source 2002, 1 http://www.ehjournal.net/content/1/1/7of air pollution and emergency department visits, or scal-ing up concentration response relationships from studiesof hospital admissions, based on the expected ratio be-tween hospital admissions and emergency departmentvisits [1]. (See illustrative example below.)Comparison with existing estimatesWe present a comparison of our estimates with those fromearlier studies [1–7,43,44] in figures 1,2,3 and Table 6(see additional file 1). We have categorized estimatesfrom earlier studies based on the authors' description ofthe health state, and the primary sources cited. It isdifficult to make precise comparisons, however, becausemany other studies do not specify the level of activity lim-itation associated with the condition being valued, andour estimates pertaining to emergency department visitsdiffer from earlier estimates in that they also capture costsfor those patients who were ultimately admitted.Nonetheless, in general, our estimates are comparable inmagnitude to those from earlier studies in this area. A pri-ori, we might have expected that our more comprehensiveapproach would have resulted in somewhat higher esti-mates compared to earlier studies. We observed that valu-ation estimates for the less severe outcomes were sensitiveto specification of severity level, such that specifying ahigher level of severity produced estimates at the higherend of those reported in earlier studies. It is also possiblethat in the context of the less severe outcomes which areassociated with relatively low valuation, our approach,though more comprehensive, adds only small incrementswhich do not result in a substantial change in magnitudeOur estimates for hospital admissions are less than inmost earlier studies. With respect to the cost of treatmentcomponent for this outcome, compared to the Anis et al.study [27], two studies based on U.S. data reported highercosts for pneumonia [45] and congestive heart failure[46], while another from Canada revealed similar costs forasthma hospital admissions [[47], personal communica-tion, Dr. W. Ungar, November 2002]. Estimates from theU.K. were also similar to those derived here [6]. The esti-mates advanced by Chestnut et al. [1], though developedfor Canada, are also based partially on U.S. data [48].These findings may reflect true systematic differences incosts of treatment in the U.S. compared to Canada and theU.K.. It has been documented that intensity of care forsome conditions is greater in the United States than Can-ada [49], and that administrative/overhead costs aresignificantly higher in the U.S. than in Canada in relationto the multiple versus single payer systems in the twocountries [50,51]. Given that the ratios we observed fortotal value to COI were generally similar to (or indeed atthe lower end of) those reported previously [1,4,15–17],this indicates that the component of valuation related toavoided pain and suffering and averting expenditures isalso lower than earlier estimates. This may suggest thatWTP to avoid health outcomes associated with significanthealth care costs may be lower in the context of a publiclyfunded universal health care system, possibly because oflower perceived risks of catastrophic financial conse-quences. Thus it appears that in the case of hospital ad-missions, the impact of our more comprehensiveapproach to valuation is overshadowed by lower costs oftreatment in Canada compared to the U.S., which hasTable 5: Valuation estimatesa and ratio of total value to cost of illness, by endpoint and componentEndpoint Cost of Treatment (CoT)Lost Productivity (LP)Pain, Suffering and Averting ExpendituresTotal Total Value/(CoT + LP)In Hospital Out of HospitalPoint Estimate95% CIRespiratory Hospital Admission$2,800 $300 $670 $410 $4,200 ($3,400, $5,000)1.3Cardiac Hospital Admission $3,800 $270 $760 $340 $5,200 ($4,000, $6,400)1.3Respiratory Emergency Department Visit$930 $160 $430 $520 $2,000 ($1,700, $2,500)1.9Cardiac Emergency Department Visit$3,200 $210 $680 $330 $4,400 ($3,300, $5,600)1.3Restricted Activity Day $25 $23 $48 ($13, $82) 1.9Asthma Symptom Day $12 $16 $28 ($11, $71) 2.3Acute Respiratory Symptom Day$12 $1 $13 ($0, $28) 1.1aestimates rounded to two significant figuresPage 8 of 13(page number not for citation purposes)of the overall estimate been the predominant source of earlier such estimates.Environmental Health: A Global Access Science Source 2002, 1 http://www.ehjournal.net/content/1/1/7Limitations of our approachWhile we believe our results represent an advance overearlier estimates, there are several limitations which couldbe addressed in future work in this area. The model weemployed for stated preference values was non-linear withrespect to duration of the illness episode, implying dimin-ishing marginal utility for reduced morbidity with respectto duration [28]. However, we assumed simple additivityof the individual blocks of time constituting each segmentof activity restriction of varying severity in the context ofan illness episode. This was necessitated by the finding inthe context of the original stated preference survey, thatpresenting individuals with illness episode scenarios in-volving an evolution of level of activity restriction wasoverly burdensome for respondents. Our assumption ofsimple additivity is likely to lead to overestimates of totalvalue relative to the observation in the original study ofdiminishing marginal utility with respect to duration [28].The extent of bias requires further empirical evaluation.Additional development work would be needed to deter-spondents in a straightforward yet accurate way, and toelicit valuation information appropriately.Our analysis relied on data from two studies carried out insingle centres (Saint John and Toronto). While we foundthat our results were not sensitive to demographic andhealth status differences between the original stated pref-erence survey sample and Canada as a whole, repeatingthe survey in multiple centres would nevertheless be desir-able, as would collecting illness episode, cost oftreatment, and stated preference information from thesame sample of respondents.Response rate is a potential concern for both the cost ofillness study and the stated preference survey. Responserates to the emergency department follow-up survey weregreater than 80% for individuals with asthma and respira-tory infections, but only 50% and 67% for those withCOPD and cardiac conditions respectively. Those withcardiac conditions who completed follow-up interviewsFigure 1Comparison of valuation estimates for hospital admissions and emergency department visits with earlier studies. Legend: Labels denote reference number. Open symbols denote estimates from the present study. See Table 6  (additional file 1) for additional details on individual estimates.Page 9 of 13(page number not for citation purposes)mine how best to communicate this information to re- were younger (and possibly healthier) than those who didnot [29]. Cost of treatment estimates may therefore beEnvironmental Health: A Global Access Science Source 2002, 1 http://www.ehjournal.net/content/1/1/7conservative relative to a fully representative populationof patients of all degrees of severity. Lower and upperbounds on the response rate for the stated preferencesurvey were 22 and 63 percent, assuming that respectivelyall or none of the telephone numbers which could not becontacted or verified as households were included in thedenominator [30]. Of prospective subjects who complet-ed the screening telephone survey, 45 percent agreed totake the in-person survey, while 72 percent of those whoscheduled an appointment actually completed the in-per-son survey [30]. Thus, a substantial portion of the sampledid not complete the in-person survey. While a lowresponse rate would raise concerns about the representa-tiveness of the sample, no major differences were ob-served with respect to demographic characteristics orhealth status characteristics which were found to influ-ence WTP in this study.An additional potential limitation of our results is thepossibility of double counting of out of pocket expendi-ingness to pay study. Ungar et al. [47] found that out ofpocket expenses accounted for only 1 per cent of annualper patient treatment costs for asthma, which suggests thatthis is probably not an important issue.An illustrative exampleWe illustrate the application of our approach by examin-ing the benefits associated with reduced cardiorespiratoryemergency department visits and hospital admissions at-tributable to the decline in particulate sulfate concentra-tions observed in Toronto from the mid 1980s to the late1990s. Average particulate sulfate concentrations in theToronto Census Metropolitan Area (CMA) declined from5.0 µg/m3 (1984–1986 average) to 3.8 µg/m3 (1997–1999 average) during this period. We estimated the bene-fits in 2000 when the estimated population was 4.771million, using results from an epidemiological study ex-amining the association of air pollution with hospital ad-missions for respiratory and cardiac disease in southernOntario [22]. We used this study rather than more recentFigure 2Comparison of valuation estimates for restricted activity days and asthma symptom days with earlier studies. Legend: Labels denote reference number. Open symbols denote estimates from the present study. See Table 6 (additional file 1) for additional details on individual estimates.Page 10 of 13(page number not for citation purposes)tures, which could potentially have been captured by boththe cost of treatment study and the stated preference will-work because of recently identified problems with the sta-tistical analysis software which has become the standardEnvironmental Health: A Global Access Science Source 2002, 1 http://www.ehjournal.net/content/1/1/7over the past 5 to 6 years in the analysis of time-series data[52], the full impact of which is still being evaluated. Therates of admission reported in the study translate into25,000 cardiac and 28,000 respiratory hospital admis-sions in the Toronto CMA in 2000. Applying the propor-tion of cardiac and respiratory emergency departmentvisits resulting in admission to hospital from the SaintJohn study (76% and 20%), [29,33] the numbers ofhospital admissions correspond to 33,000 cardiac and140,000 respiratory emergency department visits in 2000.Based on this study, we estimated that a 1.2 µg/m3 de-crease in sulfate concentration would respectively result ina 0.30 % decrease in cardiac hospital admissions/emer-gency department visits (95% CI 0.16–0.45) and a 0.32%decrease in respiratory hospital admissions/emergencydepartment visits (95% CI 0.21–0.43), accounting for thesimultaneous effects of ozone and temperature. The pro-portional reduction is the same for admissions and emer-gency visits because in this example the concentrationresponse function is simply scaled based on the relativeWe estimated the benefits of the observed changes in sul-fate concentration, specifying both the concentration re-sponse relationship and the valuation estimates generatedabove as normal distributions based on the point estimateand standard error, using a Monte Carlo simulation with1,000 iterations implemented using Analytica software[53]. It was estimated that the benefits of reducedemergency department visits for respiratory and cardiacdisease were respectively $910,000 and $440,000, whilethe benefits of reduced respiratory and cardiac hospitaladmissions were $380,000 and $390,000. However, be-cause essentially all non-elective hospital admissions oc-cur through the emergency department, and the valuationestimates generated above for emergency department vis-its reflect the probability and valuation of subsequent ad-mission to hospital, the total benefits are simply the sumof those related to emergency department visits or $1.4million (95% CI 0.91–1.8 million). We emphasize thatthis estimate does not reflect the much larger benefits at-tributable to reduced mortality and chronic morbidity.Figure 3Comparison of valuation estimates for acute respiratory symptom days with earlier studies. Legend: Labels denote reference number. Open symbols denote estimates from the present study. See Table 6 (additional file 1) for additional details on individ-ual estimates.Page 11 of 13(page number not for citation purposes)frequency of the two types of outcome.Environmental Health: A Global Access Science Source 2002, 1 http://www.ehjournal.net/content/1/1/7ConclusionsWe believe our overall approach addresses a number ofthe limitations of the current literature and is widely ap-plicable to future assessments of the benefits of improvedair quality. However, because we have noted potentiallysignificant differences between the U.S. and Canada invaluation of certain outcomes, the revised morbidity val-uation estimates presented here are most suitable for usein assessing the benefits of air pollution mitigation initia-tives in Canada. We recommend employing sensitivityanalyses with respect to severity of activity limitation, forthose health outcomes where we observed significant sen-sitivity to this factor.AbbreviationsCOI – cost of illnessCOPD – chronic obstructive pulmonary diseaseCOT – cost of treatmentLP – lost productivityWTP – willingness to payWTA – willingness to accept compensationCompeting interestsNone declared.Authors' ContributionsAll authors contributed to drafting of the manuscript andapproved the final version. DMS conceived of the studydesign and contributed to the statistical analysis. PDconceived of the study design. FRJ participated in studydesign and supervised the statistical analysis. MPM pro-grammed the statistical analysis. AHA participated instudy design and the statistical analysis. RCB participatedin study design. SJ contributed to the statistical analysis.Additional materialAcknowledgmentsThe authors are grateful to Curt Rabon for programming assistance and to References1. 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