12
Pergamon 0264-2751(96)00019--4 Cities, Vol. 13, No. 5, pp. 315-328, 1996 Copyright ~) 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0264-2751/96 $15.00 + 0.00 Uncontrolled air pollution in Mexico City Gustavo Garza E1 Colegio de M~xico, AC, Camino al A]usco No 20, Codigo Postal 01000, Mexico, DF (e-mail: ggarza@colmex, mx) This document analyzes the environmental problems and the information available on air pollution levels in Mexico City, their sources, their impact on the health of the population and the policies aimed at reducing them. The author warns that ecological deterioration will occur in the valleys adjacent to the Valley of Mexico, just as it has there, unless strict policies are established to prevent it. The analysis begins in 1986, when pollution levels began to be measured systematically, and ends in 1994, although some forecasts concerning urban expansion up to the year 2010 are presented. Copyright (~) 1996 Elsevier Science Ltd The megalopolis of Mexico City Mexico City is located in a basin covering 9560 km z at an altitude of 2240 m above sea level; it is situated between 19 and 20° latitude north and 98 and 99° longitude west. Centuries ago, the crystalline waters from its mountains formed three lakes. On an island in the Texcoco lake, the Aztecs founded the great city of Tenochtitlfin in the year 1324, where the Spanish Conqueror Hernfin Cort6s began building the "Renowned Imperial City of Mexico" on the ruins of Tenochtitlfin in 1521. In 1994, Mexico City will be 473 yr old, although it did not become a metropolitan centre until the mid-20th century. In 1900, it was a medium-sized city with 344 000 inhabitants. By 1930, its population had grown to 1 million, and by 1950 to 2.9 million. Prior to that time, the city had not spread beyond the limits of the Federal District, but in 1950 it began to grow in the north towards the municipalities in the State of Mexico, one of the 31 states that form the Mexican Republic. Technically speaking, this marked the emergence of the Metropolitan Area of Mexico City. For the sake of simplicity, use of the terms "Mexico City" or the "capital of the country" in this paper always refer to the metropolitan area. Between 1950 and 1980, Mexico City grew rapidly and reached a population of 12.6 million inhabitants. In the 1980s, its growth rate dropped in response to the deep economic crisis that affected it when its share in national industrial production fell: from 43.3% in 1980 to 32.1% in 1988. Nevertheless, by 1990 the capital's population had increased to 15 million inhabitants, located in the Federal District and 27 municipalities in the State of Mexico. The transformation from a city of 344 000 inhabi- tants in 1900 to a vast concentration of 15 million in 1990 gave rise to serious environmental problems, aggravated by the geographical and climatic condi- tions of the Valley of Mexico basin: less oxygen because of its altitude; high solar impact that causes great atmospheric reactivity; and mountain systems and wind patterns that intensify thermal inversions and most frequently carry pollutants to the western and southern parts of the city. In these final years of the 20th century, the Metropolitan Area of Mexico City and the Metropo- litan Area of the city of Toluca have overlapped, forming a multi-centred conglomerate which, in the terminology of regional science, is called a megalo- polis. Forecasts indicate that in the next two decades the megalopolis of Mexico City will incorporate the metropolitan areas of Cuernavaca, Puebla and Quer6taro, forming a multi-centred urban region. Will sustainable development of this conglomerate be feasible? Air pollution trends from 1986 to 1994 The seriousness of air pollution in Mexico City became evident in the early 1970s when the first research warning of this problem was published (Bravo, 1973; Escobar, 1973; Vizcaino, 1975; M~ir- quez Mayaud6n, 1978; SAHOP, 1978). In the 1980s, population growth and economic concentration accelerated air degradation in the Valley of Mexico, which was once poetically referred to as "the region with the most transparent air". Efforts to measure pollutants began in the 1960s, when 14 sulphur 315

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Page 1: Uncontrolled air pollution in Mexico City

Pergamon 0264-2751(96)00019--4 Cities, Vol. 13, No. 5, pp. 315-328, 1996

Copyright ~) 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved

0264-2751/96 $15.00 + 0.00

Uncontrolled air pollution in Mexico City

Gustavo Garza E1 Colegio de M~xico, AC, Camino al A]usco No 20, Codigo Postal 01000, Mexico, DF (e-mail: ggarza@colmex, mx)

This document analyzes the environmental problems and the information available on air pollution levels in Mexico City, their sources, their impact on the health of the population and the policies aimed at reducing them. The author warns that ecological deterioration will occur in the valleys adjacent to the Valley of Mexico, just as it has there, unless strict policies are established to prevent it. The analysis begins in 1986, when pollution levels began to be measured systematically, and ends in 1994, although some forecasts concerning urban expansion up to the year 2010 are presented. Copyright (~) 1996 Elsevier Science Ltd

The mega lopo l i s o f M e x i c o City

Mexico City is located in a basin covering 9560 km z at an altitude of 2240 m above sea level; it is situated between 19 and 20 ° latitude north and 98 and 99 ° longitude west. Centuries ago, the crystalline waters from its mountains formed three lakes. On an island in the Texcoco lake, the Aztecs founded the great city of Tenochtitlfin in the year 1324, where the Spanish Conqueror Hernfin Cort6s began building the "Renowned Imperial City of Mexico" on the ruins of Tenochtitlfin in 1521.

In 1994, Mexico City will be 473 yr old, although it did not become a metropolitan centre until the mid-20th century. In 1900, it was a medium-sized city with 344 000 inhabitants. By 1930, its population had grown to 1 million, and by 1950 to 2.9 million. Prior to that time, the city had not spread beyond the limits of the Federal District, but in 1950 it began to grow in the north towards the municipalities in the State of Mexico, one of the 31 states that form the Mexican Republic. Technically speaking, this marked the emergence of the Metropolitan Area of Mexico City. For the sake of simplicity, use of the terms "Mexico City" or the "capital of the country" in this paper always refer to the metropolitan area.

Between 1950 and 1980, Mexico City grew rapidly and reached a population of 12.6 million inhabitants. In the 1980s, its growth rate dropped in response to the deep economic crisis that affected it when its share in national industrial production fell: from 43.3% in 1980 to 32.1% in 1988. Nevertheless, by 1990 the capital's population had increased to 15 million inhabitants, located in the Federal District and 27 municipalities in the State of Mexico.

The transformation from a city of 344 000 inhabi- tants in 1900 to a vast concentration of 15 million in 1990 gave rise to serious environmental problems, aggravated by the geographical and climatic condi- tions of the Valley of Mexico basin: less oxygen because of its altitude; high solar impact that causes great atmospheric reactivity; and mountain systems and wind patterns that intensify thermal inversions and most frequently carry pollutants to the western and southern parts of the city.

In these final years of the 20th century, the Metropolitan Area of Mexico City and the Metropo- litan Area of the city of Toluca have overlapped, forming a multi-centred conglomerate which, in the terminology of regional science, is called a megalo- polis. Forecasts indicate that in the next two decades the megalopolis of Mexico City will incorporate the metropolitan areas of Cuernavaca, Puebla and Quer6taro, forming a multi-centred urban region. Will sustainable development of this conglomerate be feasible?

Air pol lut ion trends f r o m 1986 to 1994

The seriousness of air pollution in Mexico City became evident in the early 1970s when the first research warning of this problem was published (Bravo, 1973; Escobar, 1973; Vizcaino, 1975; M~ir- quez Mayaud6n, 1978; SAHOP, 1978). In the 1980s, population growth and economic concentration accelerated air degradation in the Valley of Mexico, which was once poetically referred to as "the region with the most transparent air". Efforts to measure pollutants began in the 1960s, when 14 sulphur

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Table I Mexico City: air quality standards

Pollutant Standard"

Carbon monoxide (CO) 11 ppm in 8 h b Sulphur dioxide (SO2) 0.13 ppm in 24 h Nitrogen dioxide (NO2) 0.21 ppm in 1 h Ozone (O3) 0.11 ppm in 1 h Particles less than 10 Ixm (PM-10) 150 lag/m 3 in 24 h Total suspended

particulates (TSP) 275 ~g/m 3 in 24 h Lead (Pb) 1.5 ~tg/m 3 quarterly average

Source: DDF, Programa Integral contra la Contaminaci6n Atmosf6rica, M6xico, D.F. (s.f.: 29) a The abbreviation ppm refers to parts per million, and gg/m 3 to micrograms per cubic metre. b The source indicates 13 ppm in 8 h, but it was reduced to 11 ppm in 1993.

monitoring stations were installed. Although they were increased to 22 stations in the 1970s, they were only used sporadically. The measuring of other pollutants and more stringent government efforts to combat them did not begin until the second half of the 1980s and will be discussed further on. This section is limited to systematically describing air pollution trends in the city from 1986 to 1994 and to evaluating their seriousness. Section 5 will deal with the results of policies to combat them.

In 1982, the Ministry of Health and Public Wel- fare established air quality criteria for the principal air pollutants, based on the standards suggested by the World Health Organization (see Table 1). The main air pollutants in Mexico City are similar to those in the vast majority of major metropolises: ozone (O3), total suspended particulates (TSP), nitrogen oxides (NOx), sulphur oxides (SOx), carbon monoxide (CO), lead (Pb) and other toxics. To measure these pollutants, there is now an automatic air-monitoring network with 32 stations and a manu- al network with 19. The automatic stations measure the levels of ozone, carbon monoxide, sulphur diox- ide, nitrogen dioxide and nitrogen oxide, in addition to meteorological parameters (temperature, humid- ity, wind, and so forth). The manual stations calcu- late levels of suspended particulate matter and lead (Comisi6n Metropolitana para la Prevensi6n y Con- trol de la Contaminaci6n Ambiental en el Valle de M6xico-Metropolitan Commission for the Preven- tion and Control of Environmental Pollution in the Valley of Mexico, 1994a, pp 8-9). 1

To facilitate general understanding of the level of each pollutant and its relation to the established standard, the authorities designed the Metropolitan Air Quality Index (IMECA), in which the maximum allowable amount for each pollutant was assigned a value of 100. In the case of ozone, for example, 100

1Since no one has at tempted to give the Commission an acronym, it will be called the Metropolitan Commission hereinafter for the sake of simplicity. In the Reference list, it is referred to as Comisi6n Metropolitana.

IMECAS are equivalent to 0.11 ppm of ozone/h, 200 IMECAS to 0.22 ppm, and so on, successivelyfl

Ozone formation Ozone is produced by the photochemical breakdown of nitrogen dioxide (NO2) by UV rays. The presence of hydrocarbons makes the reaction more complex, producing an ozone mixture, organic components with oxygen in their molecular structure and nit- rogen oxides. Owing to the latitude and altitude of Mexico City, UV rays are more intense and stimu- late ozone formation.

The standard above which ozone is considered harmful to human health, established by the Mex- ican Government, is 0.11 ppm/h, while it is 0.12 ppm in the US, and the standard recommended by the World Health Organization is 0.10 ppm.

In relation to the Mexican standard, ozone pollu- tion levels in Mexico City rose dramatically between 1986 and June 1994. In the five geographical zones into which the city is divided, the annual percentage of days when ozone pollution rose above the stan- dard in 1986 ranged from 6.8% in the northeast to 54.8% in the southwest, with an average of 19.7% (see Table 2). In 1994, the northeast zone continued to be the least polluted, although 81.1% of the days up to 13 June surpassed the standard, while the figure was 89.6% in the southwest zone. For 1994, the average percentage of days with pollution above the standard was 88.5% for the five zones (see Table 2). At the present time, practically all the days of the year surpass the existing ozone standard by 200%, dangerously approaching 300 IMECAS, a situation which calls for the application of "Emergency Plan II".

In the winter months from December to February, thermal inversions in the atmosphere of the Valley of Mexico are more frequent and accentuate pollu- tion problems by hindering the dispersion of pollu- tants. In January and February, average daily ozone concentrations in the city not only exceed the stan- dard of 0.11 ppm by far, but continue to climb even higher. As indicated in Fig. 1, the average ozone concentration in January and February 1989 was 0.17 ppm, that is, 55% greater than the standard, and it increased to 0.21 ppm in 1993, 90% above the standard.

Since 1989, major governmental measures have been taken to reduce air pollution in the capital of Mexico, but in the case of ozone they have been completely ineffective. Taking into account the fu- ture growth of the city and its transformation into a megalopolitan conglomerate, the problem of air pollution could escalate completely out of control and jeopardize the very survival of the city.

2The procedure for converting ppm or microgrammes/m 3 of pollutants into 1MECAS, or vice versa, is presented in Metropoli- tan Commission (1993a, pp VI-II) .

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0 . 2 5

0 . 2

0 . 1 5

O.1

0 . 0 5 / O Z O N E

IITROGEN DIOXIOE

-IUR DIOXIDE o

1 9 8 9 1 9 9 0 1 9 9 1 1 9 9 2 1 9 9 3

Figure 1 Mexico City: levels of ozone sulphur dioxide and nitrogen dioxide in Jan-Feb 1989-93 (ppm) Source: Ingenierfa y Gesti6n Ambiental (1993)

Suspended particulate matter Suspended particulate matter consists mainly of aerosols, organic and metallic vapours, combustion elements and dust. The smallest particles, less than 10 ~tm in diameter (PM-10) are the most harmful to health, since they penetrate the lungs. The principal PM-10 particles are those produced by diesel com- bustion, organic aerosols, and sulphates and nitrates formed by photochemical reactions.

Suspended particulate matter is measured only sporadically (between one and six times a month), and PM-10 measurements did not begin until 1991 (DDF, 1991, pp 2-10; Ingenieria y Gesti6n Ambien- tal, 1993, p 7). In 1988, levels of total suspended particulates (TSP) exceeded the established stan- dard of 275 Ixg/m 3 by 43.8% on the days when samples were taken, although they varied greatly by geographical zone: in the southeast, 81.9% of the days were above the standard; in the southwest, only 3.0% (see Table 2). Measures taken to address this problem, which will be examined further on, showed positive results in 1993, when the overall average of days with readings above the standard dropped to 26.2%. In the southeast zone, the figure fell to 68.0% of the days, and in the southwest to zero. From January to May 1994, these percentages re- mained in the same range, which means that TSP levels were above the recommended maximum level of 275 pg/m 3 on one out of every 4 days. In the industrial zones located in the northeast of the city, however, levels from 1100 to 1300 ~tg/m 3 have been

recorded, that is, between 300 and 370% above the standard (DDF, 1991, pp 2-11). In synthesis, although a downward trend in these pollutants can be observed up to 1993, they are still high and could become worse as the city continues to expand to- wards the semi-arid municipalities in the State of Mexico.

Nitrogen dioxide Nitrogen dioxide (NO2) is produced mainly by the nitrogen oxides resulting from the combustion of petroleum products. The levels of this pollutant should be closely monitored, since it is one of the precursor components of ozone.

In 1986, nitrogen dioxide levels in Mexico City's air only exceeded the established standard of 0.21 ppm on an annual average of 0.9% of the days, ranging from 0.3% in the centre zone to 1.6% in the northwest and southeast (see Table 2). Even before more significant measures against pollution began in 1989, this pollutant presented no acute problem for the population, although its level remained steady (see Fig. 1). By 1993, the average percentage of days with pollution above the standard was 1.7%, twice the percentage in 1986, but still low. Significant disparity between geographical zones can, however, be noted. The figure for the northwest was 6.0%, while it was only 0.3% for the northeast and south- east.

The winter months are when the standard is exceeded, and the situation improves significantly

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Table 2 Mexico City: number of days exceeding the standards for ozone, total suspended particulates and nitrogen dioxide by zones, 1986-94 a

Northwest Northeast Centre Southwest Southeast Average Year Days % Days % Days % Days % Days % Days %

Ozone 1986 33 9.0 25 6.8 31 8.5 200 54.8 70 19.2 72 19.7 1987 91 24.9 10 2.7 179 49.0 244 66.8 136 37.3 132 36.1 1988 172 47.1 74 20.3 236 64.6 313 85.8 127 34.8 184 50.5 1989 165 45.2 100 27.4 143 39.2 286 78.3 168 46.0 172 47.2 1990 182 49.9 75 20.5 242 66.3 299 81.9 221 60.5 204 55.8 1991 298 81.6 221 60.5 307 84.1 329 90.1 303 83.0 292 79.9 1992 283 77.5 214 58.6 304 83.3 300 82.2 273 74.8 275 75.3 1993 264 72.3 206 56.4 291 79.3 306 83.8 275 75.3 268 73.4 1994 b 152 92.7 133 81.1 150 91.5 147 89.6 144 87.8 145 88.5

Total suspended particulates 1988 131 35.9 186 50.9 172 47.1 11 3.0 299 81.9 160 43.8 1989 124 34.0 120 32.9 120 32.9 4 1.1 318 87.1 137 37.6 1990 120 32.9 99 27.1 91 24.9 0 0.0 256 70.1 113 31.0 1991 124 34.0 120 32.9 95 26.0 0 0.0 256 70.1 119 32.6 1992 26 7.1 26 7.1 18 4.9 0 0.0 153 41.9 45 12.2 1993 110 30.1 99 27.1 37 10.1 0 0.0 233 63.8 96 26.2 1994 c 30 20.0 42 28.0 24 16.0 0 0.0 102 68.0 40 26.4

Nitrogen dioxide 1986 6 1.6 2 0.5 1 0.3 2 0.5 6 1.6 3 0.9 1987 21 5.8 0 0.0 15 4.1 5 1.4 2 0.5 9 2.4 1988 19 5.2 0 0.0 13 3.6 2 0.5 0 0.0 7 1.9 1989 9 2.5 0 0.0 17 4.7 2 0.5 2 0.5 6 1.6 1990 3 0.8 0 0.0 20 5.5 11 3.0 0 0.0 7 1.9 1991 3 0.8 0 0.5 8 2.2 2 0.5 6 1.6 4 1.1 1992 0 0.0 0 0.0 6 1.6 0 0.0 2 0.5 2 0.4 1993 22 6.0 1 0.3 6 1.6 2 0.5 1 0.3 6 1.7 1994 c 13 3.6 0 0.0 10 2.7 0 0.0 0 0.0 5 1.3

Source: The information on ozone and nitrogen dioxide from 1986 to 1992 is from the Ministry of Social Development , National Institute of Ecology, Head Office of Environmenta l Standards and Administrat ive Office for Environmental Quality, Mexico, D.F. The information on TSP and nitrogen dioxide for 1993 and 1994 is from the Metropoli tan Commiss ion for the Prevention and Control of Environmenta l Pollution in the Valley of Mexico.

This information may be underest imated, since up to 30% of daily data have been lost through failures in the stations. Each day without data is considered a day on which the s tandard was not exceeded (information provided by Adrian Fern~indez). h Includes the 164 day until 13 June 1994. c Information from January to May 1994.

during the rainy season from June to September. In December 1993, for example, three of the five zones had maximum levels above the standard (see Table 3).

It may be concluded that from 1986 to mid-1994 there was a slight increase in the number of days with nitrogen dioxide levels that exceeded the stan- dard. Although the absolute levels remain steady, they apparently pose no threat to the health of the population.

Sulphur oxides Sulphur oxides (SOx) are emitted when fuels con- taining sulphur, such as diesel oil and diesel, are used. The main component is sulphur dioxide (SO2), a colourless gas which reacts with a variety of air particles and drops of water to form sulphates, aerosols and acid rain.

Sulphur dioxide levels in the air of Mexico City have remained below the standard of 0.13 ppm/day and tend to be stable (see Fig. 1). In 1986, however,

6 days in the northeast zone and 1 day in the southeast had levels above the standard, so the SO2 standard was exceeded on an annual average of 0.4% of the days. In 1993 and 1994, the standard for

Table 3 Mexico City: maximum levels of nitrogen dioxide, sulphur dioxide and carbon monoxide by zones in December 1993

Nitrogen dioxide Sulphur dioxide Carbon monoxide Zone IMECA ppm a IMECA ppm b IMECA ppm c

Standard 100 0.21 100 0.13 100 11.0 Northwest 119 0.29 37 0.05 54 7.0 Northeast 86 0.18 49 0.06 46 6.0 Centre 111 0.26 38 0.05 57 7.4 Southwest 116 0.28 33 0.04 40 5.2 Southeast 91 0.19 28 0.04 44 5.7

Source: Comisi6n Metropoli tana para la Prevenci6n y Control de la Contaminaci6n Ambienta l en el Valle de M6xico, December 1993b: VI-2 and 3. a Parts per million in 1 h.

Parts per million in 8 h. c Parts per million in 24 h.

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Table 4 Mexico City: number of days exceeding the standards for sulphur dioxide, carbon monoxide and lead by zones, 1986-94

Northwest Northeast Centre Southwest Southeast Average Years Days % Days % Days % Days % Days % Days %

Sulphur dioxide 1986 0 0.0 6 1.6 0 0.0 0 0.0 1 0.3 2 0.4 1987 0 0.0 0 0.0 0 0.0 0 0.0 0 0.0 0 0.0 1988 0 0.0 15 4.1 0 0.0 0 0.0 0 0.0 3 0.8 1989 0 0.0 8 2.2 12 3.3 0 0.0 0 0.0 4 1.1 1990 0 0.0 6 1.6 0 0.0 0 0.0 0 0.0 1 0.3 1991 0 0.0 13 3.6 7 1.9 0 0.0 0 0.0 4 1.1 1992 1 0.3 8 2.2 3 0.8 0 0.0 0 0.0 3 0.8 1993 0 0.0 0 0.0 0 0.0 0 0.0 0 0.0 0 0.0 1994 a 0 0.0 0 0.0 0 0.0 0 0.0 0 0.0 0 0.0

Carbon monoxide 1986 2 0.5 0 0.0 0 0.0 0 0.0 0 0.0 0 0.1 1987 1 0.3 0 0.0 1 0.3 7 1.9 3 0.8 2 0.7 1988 0 0.0 0 0.0 3 0.8 0 0.0 6 1.6 1 0.5 1989 0 0.0 4 1.1 4 1.1 0 0.0 2 0.5 1 0.5 1990 0 0.0 14 3.8 22 6.0 0 0.0 4 1.1 8 2.2 1991 0 0.0 1 0.3 11 3.0 1 0.3 2 0.5 3 0.8 1992 14 3.8 0 0.0 0 0.0 0 0.0 11 3.0 5 1.4 1993 0 0.0 0 0.0 0 0.0 0 0.0 0 0.0 0 0.0 1994 b 0 0.0 0 0.0 0 0.0 0 0.0 0 0.0 0 0.0

Lead 1989 62 17.0 365 100.0 62 17.0 29 7.9 62 17.0 116 31.8 1990 0 0.0 365 I00.0 29 7.9 0 0.0 0 0.0 79 21.6 1991 0 0.0 245 67.1 29 7.9 0 0.0 0 0.0 55 15.1 1992 0 0.0 91 24.9 0 0.0 0 0.0 0 0.0 18 4.9 1993 0 0.0 0 0.0 0 0.0 0 0.0 0 0.0 0 0.0 1994 h 0 0.0 0 0.0 0 0.0 0 0.0 0 0.0 0 0.0

Source: From 1986 to 1992, figures on sulphur dioxide and carbon monoxide are from the Ministry of Social Deve lopment , National Institute of Ecology, Head Office of Environmenta l Standards, Administrat ive Office for Environmenta l Control. For 1993-94, as well as information on lead, from the Comisi6n Metropoli tana para la Prevenci6n y Control de la Contaminaci6n Ambienta l en el Valle de M6xico, 1993b: VI-2 and 3. a Information up to June 1994. b Information up to May 1994.

this pollutant was not exceeded on any day in any zone, which shows that the specific measures to reduce it have been effective (see Table 4).

In December 1993, the five geographical zones of Mexico City had sulphur dioxide levels at least 50% below the standard, although there were slight dif- ferences among them, since it was 0.06 ppm in the northeast and 0.04 ppm in the southwest and south- east (see Table 3).

Carbon monoxide

Carbon monoxide (CO) is a colourless, odourless and tasteless gas somewhat lighter than air, pro- duced mainly by the incomplete combustion of gaso- line in motor vehicles. From 1982 to 1992, the standard established for this pollutant was 13 ppm/8 h, but reduced to 11 ppm as of 1993.

Since 1986, CO, just as SO2 and NO2, has ex- ceeded the above-mentioned standard on very few days. In 1986, the northwest region was the only zone that had a few days above the standard, and the general average was merely 0.1% of the days. CO reached its maximum level in 1990 with 2.2% of the days, but it has dropped to zero in recent years (see Table 4). In December 1993, none of the five zones

of the city exceeded the established standard (see Table 3).

Since carbon monoxide is emitted near the ground and in streets full of buildings, it disperses very slowly and produces concentrations in the atmos- phere several times smaller that those at street level. Consequently, the monitoring stations do not mea- sure the extent to which people are exposed (DDF, 1991, 2-14). In this regard, it has been shown the CO levels within vehicles are 4.2 times greater than those measured by the monitoring stations (Fernfin- dez, 1993). This means that CO is a health risk factor for people in the nation's capital, especially for those who work on the street and use public thorough- fares.

Lead

The presence of lead (Pb) particles in the environ- ment is caused basically by leaded gasoline fuel, although it is also produced by paint manufacturing and foundry processes. The standard established as the maximum allowable amount is a quarterly aver- age of 1.5 Ixg/m 3.

In 1988, the standard for lead was exceeded at nine monitoring stations. Levels have dropped since

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Table 5 Mexico City: inventory of air pollution emissions, 1989 (pollutants in thousands of metric tonnes per year)

Sulphur dioxide Nitrogen oxide Hydrocarbons Carbon monoxide TSP Total Sector Total % Total % Total % Total % Total % Total %

Energy 73 028 35.5 9 846 5.6 31 843 5.5 53 205 1.8 4 699 1.1 172 621 4.0 Refineries 14 781 7.2 3 233 1.8 31 730 5.5 52 645 1.8 1 154 0.3 103 543 2.4 Electric

power plants 58 247 28.3 6 613 3.8 113 0.0 560 0.0 3 545 0.8 69 078 1.6

Industries and services 87 792 42.7 32 881 18.5 40 102 7.0 16 282 0.6 12 711 2.8 189 768 4.4 Industries 65 732 32.0 28 883 16.3 39 981 7.0 15 816 0.6 10 242 2.3 160 654 3.7 Businesses 22 060 10.7 3 998 2.2 121 0.0 466 0.0 2 469 0.5 29 114 0.7

Transport 44 774 21.7 133 691 75.4 300 380 52.6 2 853 778 96.7 9 549 2.1 3 342 172 76.7 Private cars 3 557 1.7 41 976 23.7 141 059 24.7 1 328 133 45.0 4 398 1.0 1 519 123 34.9 Taxi cabs 806 0.4 9 518 5.4 31 986 5.6 301 162 10.2 997 0.2 344 469 7.9 Collective

taxi cabs 856 0.4 10 059 5.7 42 748 7.5 404 471 13.7 1 062 0.2 459 196 10.6 Diesel buses a 5 224 2.5 8 058 4.5 2 439 0.4 6 260 0.2 240 0.1 22 221 0.5 Diesel buses b 13 062 6.3 18 262 10.3 5 298 0.9 12 612 0.4 601 0.1 49 835 1.1 Gasoline-

fuelled trucks 955 0.5 16 994 9.6 67 864 11.9 779 585 26.4 1 186 0.3 866 584 19.9

Diesel- fuelled trucks 20 063 9.8 26 126 14.7 7 293 1.3 16 515 0.6 923 0.2 70 920 1.6

Others 251 0.1 2 698 1.5 1 693 0.3 5 040 0.2 142 0.0 9 824 0.2

Environmental deterioration 131 0.1 931 0.5 199 776 34.9 27 362 0.9 423 640 94.0 651 840 14.9 Eroded areas 0 0.0 0 0.0 0 0.0 0 0.0 419 439 93.1 419 439 9.6 Fires 131 0.1 931 0.5 199 776 34.9 27 362 0.9 4 201 0.9 232 401 5.3

Total 205 725 100.0 177 349 100.0 572 101 100.0 2 950 627 100.0 450 599 100.0 4 356 401 100.0

Sources: Programa Integral contra la Contaminaci6n Atmosf6rica de la Zona Metropolitana de la Ciudad de M6xico (1990, p 32). In the Federal District.

b In the State of Mexico.

1989, as m a y be seen in Tab le 4. F r o m 1989 to 1992, lead concen t ra t ions fell by approx . 90% and , dur ing 1992, it a m o u n t e d to one - th i rd of the 1.5 ~tg/m 3 l imit (Comis i6n M e t r o p o l i t a n a , 1994b, p 11). S imi lar ly , a l though the s t a n d a r d was e x c e e d e d on an ave rage o f 31.8% of the days in 1989, the f igure fell to ze ro in 1993 and 1994 (see Tab le 4). The dras t ic r educ t ion in l ead levels in the air has bas ica l ly been the resul t of the gaso l ine subs t i tu t ion pol icy , as will be seen in the fo l lowing sect ion.

E n v i r o n m e n t a l p r o b l e m s a s a w h o l e

B e t w e e n 1986 and 1994, effor ts were successful in reduc ing the air po l lu t ion level of l ead (Pb) , and the levels of ca rbon m o n o x i d e (CO) and su lphur d iox ide (SO2) r e m a i n e d s table at levels be low the s t anda rd . In con t ras t , high and growing levels of ozone (03 ) a re ev iden t , and the a m o u n t of n i t rogen d iox ide (NO2) and to ta l s u s p e n d e d pa r t i cu la tes (TSP) con- t inues to be s ignif icant , so the e n v i r o n m e n t a l s i tua- t ion of the ci ty is far f rom sat is fac tory .

The re la t ive i m p r o v e m e n t ach ieved was the resul t of s igni f icant g o v e r n m e n t a l effor ts beg inn ing in 1989, as will be seen in Sect ion 5 of this p a p e r , but such effor ts a re far f rom solving the p r o b l e m s as a whole . In view of this s i tua t ion , we can on ly ask

what will h a p p e n in the long run if Mexico Ci ty con t inues growing and b e c o m e s a m e g a l o p o l i t a n c o n g l o m e r a t e i nco rpo ra t i ng the m e t r o p o l i t a n a reas of To luca , C u e r n a v a c a , Pueb l a and Q u e r 6 t a r o .

Decisive pollution factors: stationary and mobile sources The p p m and m i c r o g r a m m e s / m 3 in which po l lu tan t s a re m e a s u r e d add up to mi l l ions of tons of toxic p roduc t s each year , owing to the large vo lume in which they are emi t t ed . In Mexico City, it is esti- m a t e d tha t 4.4 mi l l ion tons of air po l lu t an t s were e mi t t e d in 1989 to di f fer ing degrees by vehic les (mobi l e sources) and e c o n o m i c act ivi t ies ( s t a t iona ry sources) . The growth in emiss ions f rom both sources d e p e n d s on the p o p u l a t i o n and economic dynamics of the capi ta l of Mexico , whose e x t r e m e l y high concen t r a t i on is now the mos t decis ive fac tor contr i - bu t ing to the m a g n i t u d e o f air po l lu t ion .

The mos t r ecen t i nven to ry of emiss ions , con- duc ted in 1989, s t ressed tha t mob i l e sources in the t r anspo r t sec to r a re r e spons ib le for 76.7% of to ta l po l lu t ion , e n v i r o n m e n t a l d e g r a d a t i o n for 14.9% and indus t ry , services and ene rgy for the r ema in ing 8 .4% (see T a b l e 5 and Fig. 2). In t e rna l c ombus t i on vehi-

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Table 7 Mexico City: percentage distribution of total emissions by stationary sources

Economic sectors %

Industry 30.4 Thermoelectric plants 26.7 Commerce and services 11.2 Coating and lining 9.0 Mineral processing 8.8 Chemical industry 5.1 Metal processing 3.3 Dry cleaning 3.1 Printing 1.1 Industrial degreasing 0.8 Paper and wood 0.4 Total 100.0

Source: Comisi6n Metropolitana para la Prevenci6n y Control de la Contaminaci6n Ambiental en el Valle de M6xico (1992), preliminary version, p 17.

plants with 26.7% and, in third place, commercial and service activities, including dry cleaning, with 14.3% (see Table 7).

In Mexico City, industry, electric power genera- tion and services consumed more than 3.4 million barrels of special diesel, 6.8 million barrels of fuel oil and 480 000 barrels of industrial diesel oil in 1991. The special diesel distributed in the Valley of Mex- ico since May 1986 has 34% less sulphur than that distributed in the rest of the country. In December 1991, the goverment-operated oil company Pet- roleos Mexicanos (PEMEX) began supplying indust- rial diesel oil instead of fuel oil in the Valley of Mexico. Industrial diesel oil contains 33% less sul- phur, 60% less nitrogen and eliminates 99% of vanadium, nickel and various other elements. Furth- ermore, 150 million more cubic feet of gas were consumed in 1992 than in 1991, owing to the change from diesel to gas in the industries. Since gas does not contain sulphur, this measure has reduced the emission of sulphur dioxide into the air. The mea- sures mentioned above must be supplemented by changes in the industrial companies, since inspec- tions indicated that many companies had not yet installed emission control equipment and that when equipment had been installed, it was obsolete or lacked suitable maintenance (Departamento del Distrito Federal, 1991). In the future, air pollution caused by industrial plants and commercial and service activities in Mexico City will depend on economic growth perspectives and the degree to which provisions on the installation of anti-polluting equipment are fulfilled and official Mexican stan- dards are observed.

Mexico City's share in the nation's industrial ouput grew from 32.1% of the gross domestic pro- duct (GDP) in 1940 to 43.3% in 1980. During the acute economic crisis of the 1980s, however, the nation's capital was the city most affected. Its indust- rial production fell in absolute terms between 1980 and 1988 and its share of the national total dropped

Uncontrolled air pollution in Mexico City: G Garza

to 32.1%. Certain economic recovery began to occur in 1989 when the national GDP grew to 3.3% ; it also grew 4.4% in 1990, 3.6% in 1991 and 2.8% in 1992, but its drop to 0.4% in 1993 resulted in fear of a new recession. The North American Free Trade Agree- ment (NAFTA) between Canada, Mexico and the US in January 1994 augured recovery, but the first 6 months of its operation showed no signs of reactivat- ing the national economy. Be that as it may, the economic future of Mexico will unquestionably de- pend to a great extent on the way NAFTA develops. Although it is assumed that successful development of the agreement will stimulate cities in northern Mexico and check the growth of Mexico's capital, the growth of Mexico City will continue to consoli- date the megalopolis, growing significantly in size and accentuating the environmental deterioration of its ecosystem.

A i r p o l l u t i o n a n d t h e h e a l t h o f t h e p o p u l a t i o n

Although Mexico City is one of the most polluted cities in the world, the human health risks involved have not been determined with any precision. In principle, when people are exposed to air pollutants above the established standards, there may be effects harmful to their health. The ozone levels in Mexico City are clearly above the standards, but there is insufficient research to determine the degree of risk or whether immunological mechanisms may be developed to tolerate such aggression without detectable clinical problems (Castillejos, 1992). In general, it is difficult to determine the effects of pollutants on health because of the difficulty in demonstrating the relationship between the adverse health symptoms and the pollutant within a system of synergic relationships that cannot be easily de- scribed or measured.

Additionally, different concentrations of the va- rious types of pollutants are frequently found in Mexico City, which makes the study of their rela- tionship with health more complex. While the in- habitants in the southwest zone are continuously exposed to high levels of ozone, those who live in the northeast are exposed to a poorly defined mixture of hydrocarbons, ozone, sulphur dioxide and high par- ticulate concentrations. The inhabitants of the city's centre, in turn, are sometimes afflicted by high levels of ozone and, other times, by nitrogen oxides and particulate matter.

In recent years, research on the health effects of daily exposure to such toxic substances has taken on great importance, particularly descriptions of their toxicological potential. It is known, however, that organisms may have a variety of pathological reac- tions, such as stunted growth, altered reproductive patterns, behavioural changes and even death (Moreno S~nchez, 1991). In Mexico, most resear-

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chefs who study the impact of air pollutants on health have focused their attention on the effect of lead on child development and the respiratory effects of ozone on asthmatic children. It was found, for example, that the children in three primary schools in Mexico City presented differences in morbidity rates for respiratory diseases, with the highest rate among those in the southwest, where the ozone levels are highest (Castillejos, 1992). Other studies conducted among school children of different socio-economic strata in the same zone showed that there was a relationship between high ozone levels and reduced respiratory function. Nevertheless, such studies do not enable us to precisely determine the adverse consequences on the health of the entire population over time as they carry out their daily activities.

There are also studies on the risks of exposure to carbon monoxide in the various means of transport used by the population of Mexico City, including a very recent study (Ferndndez, 1993) which shows that the readings obtained at the monitoring stations do not represent the levels to which people are exposed in vehicles and on the streets. Consequently the spatial distribution and environmental dynamics of carbon monoxide need to be analysed in much greater depth.

In synthesis, research on the effects of air pollu- tion on the health of its inhabitants is at a very incipient stage in Mexico City, and the related epidemiological information is also far from suffi- cient (Santos-Burgoa and Rojas Bracho, 1992). Fu- ture studies will have to expand knowledge on the nature of polluting sources and the real possibility of reducing their emissions, exposure levels, toxicolo- gical factors associated with each pollutant and, in general, the characteristics of environmental dyna- mics linked to the city's expansion.

The insufficiency of policies to fight pollution One of the first programmes to reduce air pollution in Mexico, called the "February 14 Decree" , was established in 1986. The main purpose of the prog- ramme was to reduce the high emissions of air pollutants by the poorly maintained motor vehicles in the public transport system. It called for measures to renovate the system by acquiring new buses with better motors, to supply gasoline with a lower lead content and diesel with less sulphur, and to continue developing the electric transport system of trolley- buses, the Metro underground transport system and the light rail train. The actual impact of this initiative was limited, since it was carried out slowly and had no direct effect on the volume and flows of private automobiles. The appearance of the programme, however, marked the beginning of a number of strict

measures to fight environmental deterioration in the capital of Mexico.

The following year, a more ambitious national environmental improvement programme was put into operation: "100 Necessary Measures". The in- itiative was directed at exercisfng greater control over polluting emissions by establishing stricter emission standards for new 1988 cars, and inspection and maintenance requirements (I/M) for 2.7 million vehicles, to be conducted twice a year. In spite of the importance of the initiative, the I/M programme presented serious limitations, owing to irregularities committed during the inspection. In one way or another, practically all vehicles passed inspection. Be that as it may, the programme resulted in vehi- cles whose motors were better tuned and in reason- able condition. Additionally, the programme im- proved over the years and, in 1993, used automated analysers and trained staff who work in the repair shops that form part of the programme.

When the I/M requirements were first put into operation, the authorities estimated they would re- duce carbon monoxide by 20% and hydrocarbons by 15%. A more conservative forecast predicted reduc- tions of 10.7 and 6.1%, respectively, without indicat- ing any specific period of time.

In November 1989, an emergency programme called "A Day Without a Car" was enforced. Although it was only intended for the 1989-90 winter season, 4 it became an ongoing programme. 5 "A Day Without A Car" consisted of prohibiting the operation of each automobile one day a week to reduce emissions. Nevertheless, in the first few months fuel consumption dropped only 6% (Fernan- dez, 1993). Subsequently, it rose by an average annual rate of 4.5% between 1989 and 1993, equal to previous years. To a great extent, this was caused by people making more trips on the days they could drive, greater use of cabs and an increase in new automobiles purchases in order to drive every day. 6 In 1994, "A Day Without A Car" has been in continuous operation for 5 yr, preventing the opera- tion of 600 000 vehicles a day, but it has not pre- vented increases in gasoline consumption and pollu- tion levels. In spite of its practical complexity, the measures to modify by decree the transportation needs of millions of Mexico City inhabitants who must go to their jobs, schools, shopping centres and places of entertainment have been simplistic. The programme is at an impasse, because it has not produced the results expected and it would be completely counterproductive to release 600 000 additional vehicles onto the streets each day.

4This programme is also known as "Hoy no circula" (Don't Drive Today). 5In 1987, a voluntary programme not to drive was promoted but it failed because of a lack of cooperation from drivers. 6In 1990, more than 100 000 new automobiles were sold in Mexico City, an increase of 20% over the previous year.

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In 1990, the Metropolitan Commission began to implement a 5-yr programme at a cost of 3 billion dollars to drastically reduce polluting emissions. It is called the Comprehensive Programme to Fight Air Pollution in the Metropolitan Area of Mexico City (PICCA). PICCA focuses on factors linked to auto- mobile and fuel technology and the development of transport, encouraging the production of unleaded gasoline; a reduction in the sulphur content of diesel (from 5 to 0.1%) and in gasoline (from 4 to 0.8%); expansion of the Metro and trolley-bus transport network; the complete replacement of the fleet of public buses with 3500 new vehicles; the extension of the I/M programme to diesel vehicles; the conver- sion of shipping trucks to gas; the installation of catalytic converters in all taxi cabs and collective transport vehicles; tax and credit incentives for industries to install anti-polluting equipment; the replacement of fuel oil by natural gas in power plants to reduce sulphur emissions; the reforestation of areas surrounding the city to build an extensive green belt; and, finally, the promotion of research on environmental problems. 7 The authorities esti- mate that complete implementation of PICCA in 1995 would reduce 1989 pollutant emissions as fol- lows: carbon monoxide by 36%, sulphur dioxide by 79%, hydrocarbons by 26%, total suspended par- ticulates by 55%, nitrogen dioxides by 5% and lead by 40%.

The most recent initiative in the fight against pollution is the Industrial Inspection Programme, which began in March 1992. Within a year and a half, this programme intended to reduce industrial plant emissions of volatile organic compounds by 90%, suspended particulates by 90% and nitrogen dioxide by 50%. Since then, the Metropolitan Com- mission has followed up on the commitments assumed by the industrial sector. Furthermore, in 1993, the Regulations on Air Pollution Prevention and Control established five official standards that specify the maximum allowable levels of the various pollutants from industrial companies and 23 emis-

7Between 1 December 1992 and 31 March 1993, 11 measures were taken in addition to those normally included in the PICCA: the reformulation of gasolines; the supply of special diesel throughout the country for trucks going to Mexico City; a 25% reduction in the fuel consumption of thermoelectric power plants in Mexico City during the winter season to reduce emissions of nitrogen dioxide; daily air surveillance to detect the burning of materials outside, as well as emissions from ostensibly polluting industries and service establishments; strengthening of the industrial inspec- tion programme (200 monthly inspections were conducted from August to November 1992, and 1100 from December to March 1993); the withdrawal of ostensibly polluting vehicles; moderniza- tion of the vehicle inspection programme; strengthening of mea- sures by the National Health System to identify the effects of pollutants on health through epidemiological monitoring; the extension of the "Day Without A Car" programme to school and worker transportation; and promotion of the use of public transport.

Uncontrolled air pollution in Mexico City: G Garza

sion sampling and rating procedures (Instituteo Nacional de Ecologia, 1993). They also established eight standards for maximum allowable vehicle emissions of hydrocarbons, carbon monoxide and nitrogen oxides.

In spite of the notable efforts involved in this set of programmes, the results obtained have been far from satisfactory for, in our opinion, three main reasons: (1) they fall outside the context of the global dynamics of the metropolis; (2) they have technical limitations; and (3) they have been put into operation too late and too hastily. It is far beyond the scope of this paper to provide a detailed analysis of the above-mentioned shortcomings. But some brief examples are pertinent. In the first place, the growth of stationary and mobile sources, inherent to the dynamics of the city, must be regulated not only in terms of their operations but also in terms of their numbers. The establishment of industrial parks and large commercial centres in the city has consequent- ly been incompatible with the ecological policies. This lack of coordination is evidenced by the fact that the PICCA and other ecological programmes were formulated for the period 1988-94, but no global urban development plan was prepared for one of the most complex cities in the world.

In the second place, lack of knowledge concerning air dynamics, synergic effects and the formation of secondary pollutants resulted in anti-pollution mea- sures that changed air from a reducing agent to an oxidizing agent. In fact, it seems that no considera- tion was given to the direct effects of hydrocarbon reagent emissions resulting from the change in gaso- line formulas and the replacement of fuel oil by natural gas. Lead and sulphur dioxide emissions were indeed reduced, but not nitrogen oxides and hydrocarbons, which changed the air into an oxidiz- ing agent (Bravo, 1992). This explains the growing levels of ozone in the air of Mexico City.

Finally, even though clear warnings on the grow- ing air pollution of Mexico City had been issued since the early 1970s, more than 15 yr passed before any pertinent measures were initiated. The serious- ness of current pollution levels and the lack of a suitable technical and administrative apparatus, ex- plain the failure of these measures and the formula- tion of other economic stimulation policies that are counterproductive from the standpoint of sustain- able metropolitan development.

Megalopolitan concentration: the collapse of ecosystems in a chain reaction? Toward the end of the 20th century, the most advanced urban systems are changing into multi- centred urban fabrics that concentrate more com- plex structures and social relationships, forming integrated subsystems of cities called megalopol ises when they arise from the coalescence of two or more

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Uncontrolled air pollution in Mexico City: G Garza

metropolitan areas. Examples of these conglomer- ates in the world are the Tokaido megalopolis in Japan and the New York megalopolis along the eastern coast of the USA.

Since 1980, the Metropolitan Area of Mexico City has overlapped that of Toluca by incorporating the municipality of Huizquilucan and forming a megalo- politan area that could be called the Megalopolis of Mexico City. In 1990, the Metropolitan Area of Mexico City (MAMC) had 15 million inhabitants distributed in the 16 boroughs of the Federal District and 27 municipalities in the State of Mexico, while the Metropolitan Area of Toluca had 827 000 in- habitants in five municipalities. In 1990, this inci- pient megalopolis already had 16 million people and was beginning a process of major interaction exem- plified by the construction of the Santa Fe shopping mall inaugur~/ted in 1993 and considered the largest in Latin America. It is located to the west of Mexico City and only 20 min to the east of Toluca. By 1994, it is estimated that the MAMC will have 16.8 million inhabitants and the Metropolitan Area of Toluca 1 million, approaching 18 million inhabitants as a megalopolitan complex. 8

In coming decades, this emerging megalopolis will also include Cuernavaca, a city of 450 000 inhabi- tants located 70 km south of Mexico City; Puebla, to the east with 1.8 million inhabitants in 1990; and Quer6taro, to the north with 385 000 inhabitants. In 1994, the metropolitan areas of Mexico City, Tolu- ca, Cuernavaca, Puebla and Quer6taro have a joint population of approx. 21 million people, and esti- mates indicate that they will have at least 25 million by the year 2000 and 34 million by 2010. 9 Apart from the future urban infrastructure and service needs of the megalopolis and the social and political problems they will pose, it is also important to give considera- tion to the environmental deterioration of its ecosy- stems.

The mountains, altitude and prevailing winds of the Valley of Mexico will continue to be factors that exacerbate its air pollution, and the possibility of diminishing it will depend on a significant reduction of stationary and mobile sources of emissions. Dur- ing the first 164 days of 1994, ozone readings in Mexico City exceeded the established standard on 88.5% of days. Under such circumstances, when the current 3 million automobiles increase to at least 6.6

8The MAMC estimate was made by taking the total national population of 91 222 077 people in 1994 and multiplying it by 18.4%, the metropolis' percentage of the total population in 1990. The figure for the 1994 total national population is taken from Camposortega (1994). The population of Toluca was estimated on the basis of the total population in 1990 and the growth rate of 3.3% experienced between 1980 and 1990. 9This estimate was made assuming an annual population growth rate of 3% for the entire megalopolitan complex. The rate is reasonable, since between 1980 and 1990 the population of Puebla grew at an annual rate of 4.6%, Cuernavaca at 4.9% and Quer6taro at 5.9%.

Table 8 Toluca: monthly average of total solid particles, 1988 and 1993 (pg/m 3)

1988 1993

Standard 275.00 275.00

January 328.80 795.80 February 248.70 933.60 March 183.30 1001.50 April 151.60 417.70 May 242.10 1 114.30 June 90.10 549.60 July 94.20 280.10 August 68.70 220.00 September 106.20 207.10 October 107.90 401.00 November 174.30 325.40 December 200.70 508.00 a

Source: Ministry of Social Development. Under-Secretariat for Environmental Protection. Delegation of the State of Mexico. a Since there was no measurement for December 1993, this figure is that of December 1992.

million by the year 2010, daily pollution levels will become so acute that they will seriously jeopardize the self-regeneration capacity of the city's ecosy- stem.

Air quality deterioration in the Valley of Mexico will be accompanied by deterioration in the sur- rounding valleys where the other cities are located: the Valleys of Puebla, Toluca, Cuernavaca and Quer6taro. There will be approx. 1 million vehicles in those areas by the year 2010, and part of them will be continuously coming and going to Mexico City, since they are the principal means of transport that links them to the city.

Mexico City has systematically measured air pollution since 1986, but some of the other cities in its megalopolis did not begin measuring it until 1993. Toluca, however, has information on total sus- pended particulates (TSP) dating back to 1988, which reflects the trend in its air pollution, even though the data are not entirely comparable. In 1988, the TSP standard of 275 ~tg/m ~ was only exceeded in the month of January, with a reading of 328.8 ~tg/m 3. Five years later, in 1993, readings were far above the standard in 10 months of the year and, in May, four times greater than the standard, when the TSP reached the significant figure of 1114.3 ~tg/m 3 (see Table 8).

Series of historical data on pollution are not available for the Valleys of Puebla, Cuernavaca and Quer6taro, but it seems that their air pollution is also increasing significantly, so there is a real possi- bility that a chain reaction of deterioration in the ecosystems of the megalopolis of Mexico City will occur during the first decades of the 21st century.

Conclusions The beginning of accelerated environmental de-

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terioration in the ecosystem of Mexico City was noted well in advance in the 1970s, but the city continued on its path of high economic and popula- tion growth, without concern for its ecological situa- tion until the levels of air pollution became critical in the late 1980s. The pride in building a mega-city that would geographically depict the 1940-80 "Mexican economic miracle" turned to remorse when it be- came the most polluted city in the world. The 1990s have shown that the policies applied are far from solving the problem of extreme environmental pollu- tion in the Valley of Mexico and, what is worse, it is clearly evident that the problem of the environment will be of a megalopolitan magnitude, involving a chain reaction of deterioration in the valleys of the neighbouring cities of Toluca, Cuernavaca, Puebla and Quer6taro.

In spite of the seriousness of that possibility, there is no full awareness of Mexico City's environmental problem which threatens to forever destroy the basic harmony between people and nature. Mexicans have not found a way to prevent the ecological deteriora- tion of our capital city and its megalopolitan region, and our country would be grateful to the internation- al community for its assistance in finding it, bearing in mind that this problem is not isolated from our nation's socio-economic processes and the character of the Mexican political system.

Acknowledgements T h e a u t h o r is g r a t e f u l fo r t h e v a l u a b l e c o m m e n t s o f Adri~in F e r n a n d e z o n t h e p r e l i m i n a r y v e r s i o n o f this p a p e r . H e a lso w i shes to t h a n k G a b r i e l Q u a d r i , V i c t o r H u g o P~iramo a n d G u a d a l u p e de la L u z o f t h e E n v i r o n m e n t a l S t a n d a r d s O f f i c e o f t h e N a t i o n a l I n s t i t u t e o f E c o l o g y fo r t h e i r s u p p o r t in p r o v i d i n g a l a rge p a r t o f t h e i n f o r m a t i o n u sed . F ina l ly , his g r a t i t u d e to N o r a E l v a G o d i n e z fo r h e r e x c e l l e n t w o r d p r o c e s i n g o f th is m a n u s c r i p t .

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