Health impact assessment of decreases in PM10 and ozone concentrations in the Mexico City Metropolitan Area. A basis for a new air quality management program
Evaluación de impacto en salud ante reducciones de PM10 y ozono en la Zona Metropolitana del Valle de México. Base para un nuevo programa de calidad del aire
Horacio Riojas-Rodríguez, MSc, PhD,(1) Urinda Álamo-Hernández, MSc,(1)José Luis Texcalac-Sangrador, MSc,(1) Isabelle Romieu, MD, MPH, ScD.(1)
(1) Instituto Nacional de Salud Pública. Cuernavaca, Morelos, México.
Objective. To conduct a health impact assessment (HIA) to quantify health benefits for several PM and O3 air pollution reduction scenarios in the Mexico City Metropolitan Area (MCMA). Results from this HIA will contribute to the scientific support of the MCMA air quality management plan (PROAIRE) for the period 2011-2020.
Materials and methods. The HIA methodology consisted of four steps: 1) selection of the air pollution reduction scenarios, 2) identification of the at-risk population and health outcomes for the 2005 baseline scenario, 3) selection of concentration-response functions and 4) estimation of health impacts.
Results. Reductions of PM10 levels to 20 μg/m3 and O3 levels to 0.050ppm (98 µg/m3) would prevent 2300 and 400 annual deaths respectively. The greatest health impact was seen in the over-65 age group and in mortality due to cardiopulmonary and cardiovascular disease.
Conclusion. Improved air quality in the MCMA could provide significant health benefits through focusing interventions by exposure zones.
Key words: air pollution; health impact assessment; Mexico City.
Objetivo. Realizar una evaluación de impacto en salud (EIS) que documente los beneficios en salud ante diversos escenarios de reducción de PM10 y O3 en el aire de la Zona Metropolitana del Valle de México (ZMVM). Los resultados contribuyen al sustento científico del plan de gestión de calidad del aire (PROAIRE 2011-2020).
Material y métodos. La metodología de EIS comprende cuatro pasos: 1) selección de los escenarios de reducción, 2) identificación de la población en riesgo y de los eventos en salud para el año basal 2005, 3) selección de las funciones de concentración-respuesta y 4) estimación del impacto en la salud.
Resultados. Reducciones de PM10 a 20μg/m3 y de O3 a 0.050ppm (98 µg/m3) evitarían, respectivamente, cerca de 2 300 y 400 muertes por año. El mayor impacto se observa en el grupo de más de 65 años y en la mortalidad por causas cardiopulmonares y cardiovasculares.
Conclusiones. Mejorar la calidad del aire en la ZMVM podría reflejar importantes beneficios para la salud focalizados por zonas o áreas de exposición.
Palabras clave: contaminación del aire; evaluación del impacto en la salud; Ciudad de México.
A large amount of scientific literature documents the different health effects due to exposure to atmospheric pollutants.1-3 The results of research conducted worldwide serves to support local governments and international organizations in implementing public policies protective of human health.4
Ozone (O3) and particulate matter (PM10 and PM2.5) have received special attention because of the health effects they pose to exposed populations5,6 and because they are the two criteria pollutants whose concentrations have remained elevated over time in the Mexico City Metropolitan Area (MCMA). Acute and chronic exposures to these pollutants are associated with increased mortality caused by cardiovascular and respiratory disease and increased morbidity from cardiovascular and respiratory disease,1,6-8 certain types of cancer,9,10 and reproductive, developmental and neurological effects.2,11 Exposure to air pollutants during pregnancy and early stages of life is associated with premature birth, delayed intrauterine growth, low birth weight, early death syndrome and infant mortality.12,13 The groups most susceptible to the health effects of air pollution are children, adults over 65 years of age, persons with chronic diseases such as asthma, chronic obstructive pulmonary disease, and cardiac ischemia, and pregnant women because of prenatal exposure of the fetus.3,14
A variety of studies have documented the relationship between decreased exposure to air pollution and decreases in population mortality and morbidity as well as increases in life expectancy.15,16 Findings from epidemiological studies have supported the implementation of health protective policies among populations exposed to air pollution.4,17
Over the past 20 years, the health impact assessment (HIA) has served as a methodological tool used by decision-makers in diverse countries, to quantify the impact of interventions on air pollution and human health.18-20 It has been used in several studies throughout the world, particularly in Europe and the United States,19-23 and its use has enabled the estimation of economic and health impacts of diverse air quality management measures.8,24-27
Mexico has a history of evaluating health impacts from exposure to air pollutants, a practice that has been essential for planning interventions in Mexico City and the Metropolitan Area (MCMA).28-32 Interventions are planned through an air quality management plan, the PROAIRE (Programa para mejorar la calidad del aire de la Zona Metropolitana del Valle de México), which is developed every 10 years. The plan puts forth air quality guidelines and public policies for the protection of public health in MCMA. For the 2011-2020 PROAIRE, local authorities took a health-based approach, with a focus on the potential health benefits of reductions in PM10 and O3 concentrations.
This work documents the results of MCMA of HIA from three different reduction scenarios for PM10 and O3 concentrations; and was conducted according to the research methods recommended by the World Health Organization (WHO) and the United States Environmental Protection Agency (US-EPA), using the best calculations available to date, according to the literature reviewed.
We selected this methodology because it has been developed and validated in similar scenarios. This methodology is the standard approach for linking epidemiolgical evidence with new air quality standards in Europe and the United States.49
Materials and methods
The four steps of an HIA include: 1) selection of the air pollution reduction scenarios, 2) identification of the at-risk population and health outcomes for the baseline scenario, 3) selection of concentrationresponse functions (CRF) and 4) estimation of health impacts.18,19,21
The study area was defined by crossing two spatial layers of information (figure 1) using a Geographic Information System (GIS). The first layer represented PM10 and O3 monitoring stations administered by the Atmospheric Monitoring System (SIMAT, by its acronym in Spanish) of the Mexico City Ministry of Environment (SMA-GDF, by its acronym in Spanish). The second layer was the boundary of Mexico City. A grid of 500m2 resolution was overlaid on the two layers in GIS. As some monitoring stations were located beyond the boundaries of Mexico City and in the neighboring State of Mexico, the final study area included 33 municipalities in the State of Mexico in their entirety.
Population and cartographic information was obtained from the National Statistics and Geography Institute (INEGI, by its acronym in Spanish) using data from the 2005 population census. The spatial information was processed and analyzed using ArcGIS Desktop software, release 9.3 and its Spatial Analyst module (Environmental Systems Research Institute, 2009).
Air pollution scenarios
The reference year used for pollutant concentrations was 2005. Annual average PM10 and O3 values were calculated for each SIMAT monitoring station. Annual average PM10 was calculated using 24-hour daily means. Annual average ozone was calculated using the highest 8-hour moving average for each day. The data were provided by the National Ecology Institute; this data had been previously generated and validated for the "ESCALA Study of Air Pollution and Health effects in Latin America".33-34 ESCALA is a multicity project carried out in Brazil, Chile and Mexico. The study performed time-series analysis to obtain estimates for total, cause-specific and age-specific mortality associated with changes in air pollution concentrations.34
Health effects were evaluated for three hypothetical PM10 and O3 pollution reduction scenarios. For annual average PM10, the following concentrations were used: 1) the WHO recommendation of 20µg/m3, 2) the European Union recommendation of 40µg/m3, and 3) the Mexican Federal standard of 50µg/m3. For annual average O3, the following concentrations were used: 1) 0.05 ppm, 2) 0.06 ppm and 3) a 5% reduction from the 2005 annual average.
Estimating population exposure to pollutants in the study area
PM10 and O3 dispersion was calculated based on the grid delimiting the study area and using inverse distance weighting (IDW) as a geostatistical calculation method.35 Concentration gradients in the study area were classified using the ArcGIS manual classification method. The study area was classified into 5 zones for PM10 and 4 zones for O3 (figure 1).
Because the spatial distribution of air pollution does not align with the spatial distribution of the population, we used a population-weighted annual average concentration to estimate population exposure. This weighting approach aims to quantify population exposure by linking the concentration of a pollutant in a particular geographic area with the population residing in that same area.36,37 To estimate the population-weighted annual average concentration, the GIS layer with concentration zones was overlaid with a layer containing municipal-level population data. The population-weighted annual average concentration for the study area was computed with the following formula:
Cp= Population-weighted concentration for the study area
Ci= Concentration in municipality i
Ni= Number of persons in municipality i
N= Total population in the study area
The value obtained for the indicator represents an estimate of the average exposure of the population in each municipality.
At-risk population and health outcomes for the baseline scenario
This HIA uses health outcomes which have been consistently reported as having an association with PM10 and O3 exposure and for which the Mexican Health System reports statistics. Table I presents the population groups included in the study, the health outcomes selected for HIA estimations, and the corresponding International Classification of Diseases (ICD) 9 and 10 codes.
The year 2005 was chosen for the baseline scenario since it was the year for which the most recent, highest quality population and health data were available. Data for population, total mortality and cause-specific mortality were provided by Inegi. Data for hospital admissions were obtained from the National Health Information System (SINAIS, by its Spanish acronym).
Selection of concentration-response functions (CRF)
The CRFs used in this study correspond to the relative risks (RR), or a pooled RR, found in epidemiological studies that associate air pollution with health effects. We used CRFs from studies conducted in the MCMA, including the ESCALA project, and recent international epidemiological studies and meta-analyses, which provide reliable estimates (table I).31,33,34,38-47
Estimation of health impacts
The benefits of the air pollution reduction scenarios are expressed in attributable, or preventable, cases per year for each health outcome evaluated (total mortality, mortality from respiratory disease, etc.). These are estimated using the attributable population fraction, which is a function of the CRFs and the fraction of exposed persons in the study population.21 In this context, attributable population fraction is defined as the reduction in incidence that would be observed if the population exposure were decreased to the levels proposed in the reduction scenarios. This approach assumes that air pollution is only a part of the causal model for each health outcome.
The formula for the attributable population fraction (AFpop) of the total population is:
where pp represents the fraction of the population exposed to the environmental factor under consideration and RR is the concentration-response function for the change in exposure being evaluated. When the entire population has been exposed, as in the majority of air pollution studies, the pp is equal to 1 and the above formula is simplified to the attributable fraction of those who have been exposed (AFexp):
Multiplying the AFexp by the total number of cases observed in the baseline scenario and using the different concentration scenarios, we can estimate the number of preventable cases as follows:
• ∆conc is the change in PM10 or O3 concentration in each scenario,
• CRFcorrected is the CRF expressed as relative risk, per unit change in pollutant concentration,
• Number of cases in MCMA is the number of cases observed in 2005 for the outcome evaluated.
All of the stages described above present a series of assumptions and uncertainties associated with the following aspects: a) selection and frequency of health effects, b) the CRFs, c) the selection of air pollution indicators and d) the exposure distribution in the population. Therefore, the results obtained are only an approximation of what could be expected if air quality was improved. The discussion below describes the expected influence of each uncertainty factor. In addition, a sensitivity analysis was performed to evaluate uncertainty with regard to the CRFs.
Generally, a sensitivity analysis estimates results using different thresholds for the estimated point, or suggests different assumptions and examines the variation in the results. For this study, a sensitivity analysis was performed according to the method proposed by Kunzli and Perez,21 in which the effect of variability in CRFs (upper and lower 95% confidence interval limits) was evaluated. At least two CRF sources were used for the estimates whenever possible.
The project was evaluated and approved by research and ethics committees at the National Institute of Public Health (INSP, by its Spanish initials).
The study area obtained by crossing information in the GIS included Mexico City and 33 municipalities representing 68.9% of the population of the State of Mexico. The total population for the study area in the year 2005 was 18 419 138 inhabitants.
The highest concentrations of PM10 were found in the northern part of the study area, while the highest O3 concentrations were found in the southeast. Figure 1 summarizes the annual average concentration for the study area. The municipalities of Jaltenco and Tlahuac had the highest annual average concentrations of PM10 and O3, 70.1 µg/m3 and 0.069 ppm, respectively. Based on these results, the following estimates were performed.
Evaluation of the impact on total and cause-specific mortality (short-term)
The estimates for preventable cases, or deaths, vary with the CRF used. Table II presents total preventable deaths per study zone for each pollution reduction scenario and CRF used. The results indicate that, using the CRF from the ESCALA study and a PM10 concentration of 20 µg/m3, it would be possible to prevent approximately 2300 deaths per year in the entire study area; that is, 13 deaths per 100 000 inhabitants. If a reduction to 40µg/m3 was achieved roughly 1 040 deaths would be prevented per year, or 6 deaths per 100 000 inhabitants. If the Mexican standard of 50 µg/m3 was met, approximately 400 deaths would be prevented per year, or 2 per 100 000 inhabitants. Disaggregating by zone the most number of deaths would be prevented by reducing PM10 concentrations in the center (Z2PM10) and northwest zones (Z3PM10).
For O3, using the CFR from the ESCALA study, the most reliable CRF available, a reduction in annual average concentration to 0.05 ppm would prevent approximately 400 deaths per year; that is, 2 deaths per 100 000 inhabitants. A reduction to 0.06 ppm would prevent approximately 110 deaths per year, and a 5% decrease in concentration would prevent approximately 90 deaths per year. The greatest impact would be expected in the center zone (Z2O3).
The results indicate that the greatest benefit in mortality would be the group over 65 years of age. Using the baseline population of 982 876 for this age group and CRFs from the ESCALA project, approximately 1400 (95%CI:1 029-2 184) and 300 (95%CI:195-393) deaths would be avoided annually for the most extreme PM10 and O3 reduction scenarios. Using the same CRFs, achieving the Mexican standard for PM10 and decreasing O3 concentrations by 5% would achieve a reduction of 212 (95%CI 158-340) and 68 (95%CI:45- 091) annual deaths respectively.
Using the baseline population of 288 694 for children under 1 year of age in the study area, 265 (95%CI:55-468) deaths would be prevented by reducing PM10 levels to 20 µg/m3 and 47 (95%CI:10-84) deaths would be avoided by complying with the Mexican PM10 standard of 50 µg/m3.
Table III presents the estimates for mortality due to specific causes in the entire study area using the best available CRFs. The greatest benefit is seen for cardio-pulmonary and cardiovascular mortality.
Long-term estimates indicate that achieving the 20, 40 and 50 µg/m3 PM10 scenarios would prevent roughly 10 500 (95%CI:6563-14282), 4,800 (95%CI:2984-6704) and 1 800 (95%CI:1836-1125) deaths, respectively, over a period of 15 years in the population over 30 years of age, using a baseline population of 8 105 538 persons of that age in the study area.
Evaluation of the impact on hospitalizations due to specific causes
In the Mexico City population, approximately 1 650 (95%CI:1415-1902), 660 (95%CI:563-760) and 150 (95%CI:129-175) hospitalizations due to respiratory disease could be prevented per year, for the PM10 reduction scenarios of 20 µg/m3, 40 µg/m3, 50 µg/m3 respectively. For the same scenarios, 800 (95%CI:570-1 063), 320 (95%CI:225-421) and 70 (95%CI:51-97) hospitalizations due to cardiovascular disease could be prevented. The greatest decrease in hospitalizations would occur in the group over 65 years of age. In this age group, under the most extreme reduction scenario for PM10, 360 (95%CI:292-427) hospitalizations due to respiratory disease and 700 (95%CI:548-864) hospitalizations due to cerebrovascular disease would be prevented.
Meanwhile, the results indicate that reducing the annual maximum moving average for O3 to 0.05 ppm would prevent approximately 2000 annual (95%CI:249-3608) hospitalizations due to respiratory disease and 430 (95%CI:185-612) due to cardiovascular disease. Reducing the annual average O3 concentration by 5% below baseline level would prevent approximately 440 (95%CI:54-809) annual hospitalizations due to respiratory disease and 90 (95%CI:40-133) hospitalizations due to cardiovascular disease.
The sensitivity analysis evaluates uncertainty in CRFs, specifically the effect of CRF variability on calculations and the effect of using CRFs other than those from the ESCALA project, which are considered to be more reliable for the study area. Figure 2 presents the percentage change in the central estimates for the different effects evaluated. The uncertainty margin associated with the 95% CI is between +/- 14% and +/- 95%, with an average of 51%. The greatest uncertainty corresponds to the estimate of preventable deaths from respiratory disease by reducing O3 concentrations, while for total preventable deaths and deaths among persons over 65 years of age the uncertainty margin is less than 45%.
The sensitivity analysis using alternative CRFs suggests that for reductions in PM10 concentrations, the central estimate could be considerably higher for preventable deaths among children under one year of age and deaths due to respiratory and cardiovascular disease; and for reductions in O3 concentrations, estimates for total preventable deaths and deaths among persons over 65 years of age could also be considerably higher. This finding indicates a possible underestimation of the results ranging from 33 to 105%. On the other hand, the negative percentages found with regard to estimates of total preventable deaths (-19%) and preventable deaths for persons over 65 years of age (-45%) point to a possible overestimation of these results.
According to the estimates based on CRFs from the ESCALA project, we can see that if PM10 levels were to comply with the current limits established by Mexican standards, total mortality in the study area could be reduced by 0.5% (400 deaths, 2 per 100 000). With the strictest measures for reducing PM10 concentrations to comply with current California EPA standards, greater benefits could be obtained and total annual mortality in the study area could be reduced by 3% (2 300 deaths, 13 per 100 000). Based on this scenario, the estimates for impact on long-term mortality (15 years) indicate that 10500 deaths could be avoided among persons over 30 years of age.
Although the benefit is less when reducing O3 levels, estimates indicate that it would be possible to prevent approximately 400 deaths if the annual 8-hour moving average for O3 were reduced to 0.05 ppm.
A comparative analysis of the results in each of the concentration zones indicates that the impact of reducing particulates is greater in center (Z2PM10) and northwest (Z3PM10) zones and the impact of reducing O3 occurs primarily in center (Z2O3) and north (Z1O3) zones. It is important to remember that the specific calculations for the zones take into account the mortality rate, the exposed population and concentrations of PM10 and O3 weighted for the population. Therefore, a greater number of cases are found in zones with a larger exposed population, higher mortality rate and higher PM10 or O3 concentrations.
With regard to this last point, the annual average exposure to particulate matter varies among the 5 zones, with a difference of 20 micrograms per cubic meter between the zones with the greatest (southwest) and the lowest (northeast) concentrations. These differences occur primarily because a large amount of emissions are generated by mobile sources in the central MCMA region and by activity on unpaved roads in the periphery, while industrial emissions are generated in the northern Mexico City municipalities.48 The construction material and iron industries generate roughly 400 tons of PM10 per year in the municipality of Tlalnepantla alone. In Ecatepec, the most significant emissions are generated by metal smelting and forging, and in Acolman by electricity generation. All of these municipalities are located in the northern part of the MCMA. In addition, in the central and northern districts of Mexico City, PM10 emissions can be as much as 80 tons per year, primarily from heavy vehicular activity, along with industrial activity in some cases. O3, on the other hand, tends to be concentrated in the southern and southeastern zones. This is mainly because the dominant air stream transports ozone precursors and ozone, concentrating them in these zones.48
The results show that improving air quality in the MCMA area would provide considerable health benefits. Nevertheless, interpretation of these results should take into account the assumptions and uncertainties associated with the data and methods used. The validity of the results depends mainly on the quality of population and health data, as well as PM10 and O3 concentrations and the CRFs used. In our case, with the exception of morbidity data, which may be underestimated, the information comes from reliable information and solid data sources.
Most of the assumptions and uncertainties may have resulted in an underestimation of benefits. This underestimation could be due restricting the analysis to O3 and PM10 without considering effects of and interactions with other pollutants, such as PM2.5, NO2, SO2, COVs, polycyclic aromatic compounds and CO. In Mexico, the standard for O3 establishes an annual limit of 0.08 ppm, which should not be exceeded more than 5 times per year as an 8-hour moving average. Nevertheless, using only an annual average for ozone could have underestimated the impacts on health since the average does not reflect that the limit of 0.08 ppm was exceeded on more than 59% of the days of the year in 2005.48 The annual average for ozone was chosen for this study, rather than the established standard, so as to not overestimate health impact, especially for zones that are further away from air monitoring stations.
It is also likely that benefits were underestimated by not evaluating effects such as: changes in pulmonary function; lung cancer; school absenteeism; days with restricted activities; visits to emergency rooms; effects on morbidity from certain illnesses such as chronic bronchitis, asthma or lung cancer; and effects on the reproductive system such as complications in pregnancy, low birth weight, premature birth and delayed cognitive development.
Furthermore, because the long-term CRFs used were not from studies on Mexican populations, only calculations for total mortality were performed for persons over 30 years of age, underestimating the impact due to chronic exposure. For instance, one indicator not included is change in life expectancy. Previous studies show a decrease in this indicator associated with chronic exposure to air pollutants. The Harvard Study for 51 cities in the United States during the 1980s and 1990s reported that a gradual decrease in air pollution levels results in an increase in life expectancy of the population of roughly five months.16 An APHEIS study showed that reducing chronic exposure to particulates less than 2.5 microns (PM2.5) and concentrations less than 15 µg/m3 would increase life expectancy by one month to as much as over two years. That analysis was conducted with a methodology similar to the one used here. The APHEIS analysis could be reproduced for the MCMA when the necessary data is available.
The CRF is one of the most important components for calculating health impacts. The sensitivity analysis indicates that, depending on the health outcome under evaluation, a 95% CI for CRFs results in a margin of error of 14 to 95%. Another uncertainty associated with the CRFs is their applicability to different populations and locations. Therefore, for most of the calculations, preference was given to those that included CRFs generated in the MCMA, and especially those estimated in the ESCALA project, since it included data from 1997 to 2005. Further, the ESCALA project incorporated the most recently developed statistical methods which yield the most accurate and reliable calculations for time-series analyses. Using these CRFs reduces the uncertainty since epidemiological and statistical methods have improved in recent years, allowing for the extrapolation of results and the use of indicators in other populations, as is the case of Mexico City. The sensitivity analysis also evaluated the use of alternate CRFs, finding that in most scenarios the central estimates were underestimated. This finding indicates that the benefit would be expected to be greater when other CRFs are used for the estimate. More complex sensitivity analyses using probability models to include other uncertainties were not developed since they require additional data that is not available for the study area.
Finally, an economic evaluation of the health effects presented may be complementary to this study. For mortality, we have avoided assigning costs to expected mortality because we consider determining the rate and number of events to be sufficient in terms of benefits. For morbidity, the study could be complemented by assigning a value to each hospitalization event in order to obtain and estimate of the preventable costs in the health sector.
In conclusion, air pollution continues to have an impact on public health in the MCMA and the results suggest that reducing the current levels of pollutants such as ozone and PM10 would prevent hundreds of deaths and hospitalizations. The spatial distribution of concentrations in the metropolitan area enables identifying and implementing focused interventions in the geographic area, specifically by concentration zones. Even with the limitations and uncertainties in the calculations performed, the HIA is a useful tool for the design of public policies. It is recommended that existing standards be reviewed and adjusted in the context of current scientific evidence and international guidelines. It is also recommended that HIAs be performed immediately for other pollutants such as PM2.5 and for other cities in the country which may have air monitoring networks.
This study had the financial support of the Mexico City Government (Instituto de Ciencia y Tecnología del Distrito Federal). The authors thank Irma Soyachi, Alan Castillo Ferraez and Ana Elizabeth Sandoval Loredo for their assistance in the preparation of this document.
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Received on: March 19, 2014
Accepted on: September 1, 2014
Dr. Isabelle Romieu.
Instituto Nacional de Salud Pública. Av. Universidad 655,
col. Santa María Ahuacatitlán. 62100 Cuernavaca, Morelos, México.
Declaration of conflict of interests. The authors declare that they have no conflict of interests.