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Air Pollution and Clean Air Policy in Mexico City

by Vicente Sanchez and Margarita Castillejos

In 1900 Mexico City had a population of half a million. Forty years later this figure had increased to 1.7 million. Today there are 15 million people living in the Mexican capital. It is home for 20 per cent of Mexico's population; 44 per cent of the country's gross national product and 52.15 per cent of its industrial goods are produced there. Mexico City accounts for 54.7 per cent of the services provided and 45 per cent of all trade in Mexico.

This growth of the city has placed an increasing burden on the environment and has led to a continuous deterioration in the quality of life of the people living in the Mexico Valley and some of the adjoining areas.

Until the last decade, no records were kept of the quality of the air in the city. The first monitoring network was set up in 1974: it included 15 automatic and 14 manually operated systems. Only five of these have instruments that record hydrocarbons (HC), ozone (03), nitrogen oxides (NOX) and sulphur dioxide (SO2); the other monitors only measure carbon monoxide (CO) and SO2. This network remained in operation until 1976. Since that time recordings have been taken exclusively from the manually operated network. Some of the data resulting from this study are quoted in the following, as well as the estimates made at that time.

 

Stationary sources of air pollution

The data shown in Table 1 indicate that within a five-year period SO2 emissions increased by 50 per cent and NOX emissions by 27 per cent.

This trend of increasing emissions continued until 1980, reaching an estimated total of 370,000 tonnes of sulphur dioxide and 85,000 tonnes of nitrogen oxides.

Table 1: Emissions from stationary installations in Mexico City from 1972 to 1976 (in thousands of tonnes per year).

Year

SO2

NO

1972

201.7

50.8

1973

216.9

52.9

1974

253.7

56.9

1975

278.3

64.4

1976

309.1

69.8

At the beginning of the 1980s, the Mexican government evolved a number of different scenarios of the probable subsequent development and possible ways of stemming emission volumes. It was found that only a full-scale conversion of all power stations from heavy-grade heating oil to natural gas would result in an appreciable reduction, to 280,000 tonnes in the case of sulphur dioxide and 79,000 tonnes in the case of nitrogen oxides. That is, values would be brought back to their 1975 levels.

Only if all energy consumers changed over to natural gas for 75 per cent of their energy needs would pollution levels drop to 1972 levels (approx. 200,000 tonnes of sulphur dioxide).

The Mexican government regards the change-over of industry to different forms of energy as the most effective way of combatting air pollution from stationary sources. However, by 1985 only one of five combined heat and power stations had been converted; the result was merely a slight slowing down in the rate of increase.

 

Mobile sources of air pollution

Measurements showed that emissions from mobile sources during the same period were produced mainly by private vehicles. In 1970, public transport accounted for only 4 per cent of all vehicles. Table 2 shows the increase in the number of cars during the period 1972 to 1976 and the estimates for 1985 and 25 years later, assuming that there are no further changes in the trends.

 

Table 2: Evolution and estimate of number of private cars in Mexico City

Year

No. of cars in 000's

1972

862

1973

950

1974

1,130

1975

1,190

1976

1,310

1985

2,200 (estimated)

2010

10,000 (estimated)

According to these data there was a 70 per cent increase in the number of cars between 1976 and 1985; but this figure is likely to increase still further as time goes on; therefore, the same estimates assume a 500 per cent increase for the following 25 years.

The pollutant emissions from mobile sources are equivalent to 30 per cent of the industrial emissions. The principal emissions from cars are carbon monoxide (CO), hydrocarbons (HC) and nitrogen oxides (NOX). Between 1972 and 1976, emissions of noxious substances comparable to those from stationary sources (sulphur dioxide and nitrogen oxides) increased by 40 per cent. While there was a continuous slowing down of the increase in sulphur dioxide pollution at the end of the 1970s, emissions of carbon monoxide and hydrocarbons increased at a continuously high rate, which was estimated at over 30 per cent between 1980 and 1984. At the beginning of 1980, the plans of the Mexican government assumed that the best that could be achieved would be to stabilize emissions to that year's levels by various measures to reduce them. However, since these measures could not be implemented the increase continued. In comparison with this, the clean air policy with sulphur dioxide is a "success".

 

Pollution levels

Apart from these causes of pollution, there are a number of natural sources of pollution, mainly originating in a semiarid, eroded area in the former Texcoco Basin and in the non-asphalted urban districts and refuse dumps. This type of pollution occurs in particular during the dry season. Prior to 1976, dust clouds, their wind velocity and direction were measured. Since 1976 pollution levels have been determined by aerosol measurement. Only SO2 and aerosols are recorded. Levels of other pollutants, such as S03, have only been measured sporadically.

As regards sulphur dioxide, monthly average pollution levels are so far available from thirteen stations with manual recording and fifteen with automatic recording. By Latin American standards this is quite a dense network, but it can still only indicate individual points of pollutant concentration in the air.

If these data are analyzed on a regional basis it becomes clear that levels and types of pollution differ considerably according to zone. The highest SO2 concentrations are in the north-west and the city centre. This is understandable if one recalls that the north-west is an industrial area with important traffic thoroughfares, and that the city centre is the area where the various winds from all directions collide. Added to this is the density of the traffic in the city centre. Sporadic measurements showed that ozone (03) and HC values in this area were high in comparison with the norms, especially around mid-day, when solar irradiation is at its peak.

In contrast, aerosol levels are highest in the south-east and north-east of the capital, because this is where the Texcoco Basin used to be, and from November to April it is still a source of major whirlwind-like dust clouds. The least-polluted area appears to be the south-west, but this district is mainly a residential area, with plenty of greenery.

The quality of the air in the south has not yet been studied. Although pollution levels here are currently low, the rapid pace of urban development and the chemical industry here could jeopardize this zone as well in the future.

Pollution levels reach a peak in the winter months. In 1982, levels in the city centre exceeded 200 mg/m³, i. e. they were more than twice as high as in summer. For the sake of comparison it should be mentioned that the long term limit in the Federal Republic of Germany is 140 mgSO2/m³. The SO2 levels in the first three years during which records were kept (1980 to 1982) show a clear upward trend.

There was also a clear increase in average aerosol levels during the first three years, from 200 mg/m³ to 400 mg/m³, with a subsequent drop in 1983, though this was not as pronounced as the drop in SO2 levels. There is no obvious explanation for this here, either, although it cannot be ruled out that climatic factors play a part. In contrast to the SO2 data, the aerosol concentration in the city centre is just as low as in the south-west. The pollution caused by these two substances becomes worse during the winter months (October to February), when climatic conditions make thermal inversion more likely.

 

Analysis of pollution

Several conclusions can be drawn by comparing the values measured in Mexico City with the maximum values permitted by the US Environmental Protection Agency (EPA). The standards take into account both the effects on human health as well as the possible effects on plants, animals and buildings; in this way the indirect risk to the population can also be determined.

The mean 24-hour norm for aerosols is 260 mg/m³. In the four years covered by the study the officially recorded values in the north east of the city were 40 times higher than the norm, reaching levels of 653 mg/m³ and 684 mg/m³ respectively in the winters of 1980 and 1982 The studies conducted by the University of Atzcapotzalco, which is in the north-west of the city, revealed that in the winter of 1982, 85 per cent of the daily samples were above the 24-hour values tolerated by the US National Ambient Air Quality norm. According to the results of the study, these values were between 20 and 30 per cent higher than those measured at the same place in January 1977 By 1983, 70 per cent of the indicated values no longer complied with the norm.

The norm for SO2 is 393 mg/m³. According to official information this has never been exceeded, because a very high permitted level was laid down, and possibly because SO2 is quickly converted into sulphates and sulphuric acid.

Acid rain, too, is a phenomenon that Mexicans are now familiar with. In 1980, for example, the rain had a pH of 5.78, but by :1983 this had fallen to 5.14.

Pollution by heavy metals was studied by the Centre for Atmospheric Studies (CEA) and the Department of Environmental Studies of the University of Atzcapotzalco (UAM-A). They agreed that the permissible levels of heavy-metal pollution in the areas they studied were often exceeded. In the winter of 1978, levels exceeded the norm as follows: in Atzcapotzalco the lead pollution limit was exceeded in 75 per cent of the measurements, and the cadmium limit in 85 per cent. Permissible iron levels were exceeded in 90 per cent of the cases, the average being 3.8512 mg/m³ as opposed to a level of 1.50 prescribed by the norm, i.e., the concentration was more than 100 per cent higher. The zinc norm, 11.24 mg/m³, was exceeded by an average of 0.67 mg/m³ in 75 per cent of the cases.

These figures show that there is no doubt that the inhabitants of Mexico City are exposed to a high risk, although so far no epidemiological studies have been conducted which would indicate the extent of this situation.

Unfortunately, the official statistics available are practically useless as a means of determining the causal connections between pollution and health risk. According to investigations in other countries it is the respiratory organs which are most seriously affected by air pollution. In Mexico too, conditions involving the respiratory organs are the principal causes of disease and death. However, in the absence of controlled studies it is difficult to claim that changes in trends have only occurred because of the air pollution. The acute symptoms caused by the pollutants often do not produce a clearly defined no so logical picture, and often they are not so dangerous that it would be necessary to see a doctor. Hence symptoms such as drying out of the mucous membranes, ocular irritation and headaches caused by photochemical oxidants are not reflected in the statistics. On the other hand, the chronic effects are hardly ever seen in connection with the pollution. Even so, it is known from internationally accepted studies that the fact of being constantly exposed to industrial emissions is a major cause of cancer. Mexico is no exception to this.

Results from other countries may initiate research work on this subject. However, it will not be easy to transfer the results to the actual situation in Mexico if the data relate to developed countries. Assistance and studies to provide more detailed information on this problem are indispensable.

 

Politicians aware of environmental problems

Mexico is one of the developing countries in which the public is aware of environmental problems and where governments are intensifying their search for solutions. As early as 1971 an Under-Secretariat for Improvement of the Environment was created, as a part of the Health Ministry. There is a general trend towards modifying legislation in Mexico with environmental protection in mind. For example, the legislative reform covering public buildings takes environmental problems and studies relating to them into account. The country's development planning, for which the Ministry of Planning and the Exchequer is responsible, takes factors relating to environmental problems into account, aiming to prevent potential problems from arising.

In spite of the measures mentioned above, conditions in the Mexican capital have continued to deteriorate, because in practice it is often impossible to satisfy the statutory requirements. Sometimes the institutions themselves are not in a position to do their job. This is not due to any "evil designs", but to the complexity of the problem. In addition, all efforts are hampered by the rapid population growth in the capital. Even the Mexican government's national population policy, which is on the whole successful, is incapable of stemming this, because the city still acts like a magnet and the exodus from the land continues unabated.

As the population increases, so also does the number of motor vehicles, and hence the level of air pollution. There are thousands of old cars that fail to satisfy emission regulations (recently registered cars, however, do meet the requirements). Politically, there is no question of banning these old cars from the roads, because the large underprivileged section of the population would be most affected.

Mexico is a centrally governed country. Most of the decisions are taken in the capital; it is a better place for doing business - as a location for industry and trade. Since this is where half of the country's gross national product is generated it is not hard to understand how it has developed into a "devil's kitchen". Until a short while ago new industries were still being set up in the Mexico Valley, even though it was obvious that the meteorological conditions there would tend to aggravate air pollution.

The intense pressure on government authorities often results in short-term solutions to existing problems, without subsequent effects on the environment being taken into account. This pressure is caused by:

• state-owned and private industry, which would prefer to set up its facilities in the capital to reduce costs;

• the search for better job and training opportunities, and the hope for a better health system. Such opportunities are always better in a big city than in rural areas;

• the need to create jobs in an over-populated city: this makes industrial expansion necessary;

• the International Monetary Fund, which demands an economic policy which will enable the country to repay its foreign debts.

The population's increasing awareness of environmental problems is reflected in particular in the mass media. These report daily on environmental questions and problems but unfortunately do not mention that the population should share the responsibility, or point out possible ways of solving or eliminating the problems. However, a number of imitative groups have now been formed in the population at large which recognize the causative mechanisms of pollution.

It is to be hoped that these groups can apply pressure which will result in the necessary steps being taken to conserve the environment. The political parties in Mexico are also taking these questions up and incorporating them in their manifestos.