Air pollution in Mexico City

Trends in population, vehicular fleet, energy consumption and ozone concentration in the Mexico City Metropolitan Area (1990–2006)

In 1992, the United Nations named Mexico City "the most polluted city on the planet" and "the most dangerous city for children" six years later. From 1950 to 2015, the population in Mexico City increased from three million to twenty million. This population boom occurred mainly because of migrants that were looking for better opportunities, and as a consequence, the industrialization era began. This industrial growth was responsible for emitting over 11,000 tons of waste material into the atmosphere every day.[2] As a result, the economy boomed as did the proliferation of vehicles. In 1980, there were 124 cars and light-duty trucks per 1000 residents. By 2010, there were 267.[3] Population growth, increasing motorization and industrial activities, a constrained basin and intense solar radiation combined to cause intense air-quality problems of both primary and secondary pollutants. The automatic air-quality monitoring network, established in the late 1980s, revealed high concentrations of all criteria pollutants: lead, carbon monoxide, nitrogen dioxide, sulphur dioxide, ozone and particulate matter (PM). Ozone exceeded the air quality standards more than 90 percent of days and peaked above 300 parts per billion (about three times the standard) 40–50 days a year, among the worst in the world.[1]

In 1990, the Mexican government decided to implement some air quality management programs to reduce emissions. Some of the programs included removal of lead from gasoline and the implementation of catalytic converters in automobiles, reduction of sulphur content in diesel transportation fuel, substitution of fuel oil in industry and power plants with natural gas, reformulation of liquified petroleum gas used for heating and cooking.

In 1993, the government mandated the replacement of lead-octane additives with MTBE and PEMEX, the state-run petroleum company, replacing underground storage tanks at all of its petrol stations.

In 2012, ozone and other air pollutants ranked at about the same level as Los Angeles.[4] This improvement in air quality was achieved through the Mexican government's requirement that gasoline be reformulated, that polluting factories be closed or moved, and that drivers be prohibited from using their car one day per week. More recently there has been an expansion of public transportation.[4] Air pollution has been a major issue in Mexico City for decades.

Alongside a doubling of the vehicle fleet in Mexico City from 1992 to 2012, and the slow implementation of low-sulphur standards, the use of fuel ethers contributed greatly to an 86% decrease in CO, a 53% decrease in ozone, and a 32% decrease in particulate matter in that 20-year span.

Air pollution is defined as the presence of substances in the atmosphere that can be harmful to life when they are found in high quantities. In Mexico City's atmosphere, the pollutants are found as primary and secondary pollutants. Primary Pollutants are known as those that are emitted directly to the environment, such as Carbon Monoxide and Sulfur Dioxide. Secondary Pollutants are created in the atmosphere due to chemical reactions, such as Sulfate and Ozone.

In April–May 2016, ozone and suspended matter pollution in Mexico City reached levels that were detrimental to health, though the criterion to signal a pollution alert was lower in 2016 than in the 1980s.[6] The city's population continues to grow, to spread out, which lengthens automobile trips, and the number of autos in the city increases yearly.[7]

The air-quality monitor system (IMECA) is the tool used by the Mexican Authorities to measure air quality. A total of 8 system monitors, located all around Mexico City measure the levels for the 6 main pollutants that are found in the atmosphere of the city. The measured pollutants are: O3, PM10, PM2.5, CO, NO2, SO2. In addition, the stations have a meteorological tower, equipped with sensors that measure the ambient temperature, wind speed and direction, precipitation, solar radiation, relative humidity and barometric pressure.

Indicators for each pollutant allow to assess the state of air quality, with respect to specific pollution events. Indicators of the maximum hourly concentrations and daily mobile averages were used to evaluate the behavior of the concentrations presented by the criteria pollutants and their reference to health protection standards. IMECA is the index used to display the level of pollution and the level of risk that represents to the human health in the greater Mexico City as well as the time of the measurements or the actions recommended for protection.

To report the quality of the air, the IMECA uses 5 index categories:

Imeca Scale for Air Quality

• Good — when the index is between 0 and 50 imecas. The quality of air is considered satisfactory and the air pollution poses minimal or no health risks.
• Fair — when the index is between 51 and 100 imecas. The quality of air is acceptable, however some contaminants could have moderate health effects for a small fraction of the population that has high sensitivity to some contaminants.
• Poor — when the index is between 101 and 150. In this value, some sensitive groups can have health effects. Some people can experience minor symptoms, including people with breathing or heart problems, children and old people. At this stage, there is no significant risk for the general public.
• Harmful — when the index is between 151 and 200. In this situation everybody can experience negative health effects. Members of sensible groups can present serious issues. This interval activates the phases precontingencia (precontingency) and contingencia fase I (1st contingency phase) of the Programa de Contingencias Ambientales Atmosféricas(PCAA) del Valle de México (Atmospheric Environmental Contingencies Program of Mexico City).
• Extremely Harmful — when the index is more than 201. In this condition the general population may experience very serious symptoms and health effects.

Concentration of pollutants during the last decade

Mexico and its Air Pollution

PM10: According to the NOM, the annual limit for PM10 was 50 μg/m3 before 2014 and 40 μg/m3 from 2015 to present day. In Mexico City, the annual concentration of PM10 has been above the limit for the last 10 years .The highest concentration occurred in November 2019, when the PM10 levels were found to be over 110 μg/m3. Failure to comply with this NOM indicates that it is necessary to carry out actions to reduce the concentration of particulate material in the ambient air, given that it is not observed that there is a tendency to decrease in the concentrations registered for this pollutant criterion in recent years.

PM2.5: The values have exceeded the regulated limits (15 μg / m3 before and during 2014 and 12 μg / m3 as of 2015 ) since 2011. In May 2019, the level of PM2.5 reached levels between 150 and 160 μg / m3. This concentrations were reached due to burning of organic matter such as forest fires, firewood, fuel burning, especially diesel and some industrial processes. This concentrations amounts to the equivalent of Mexico City's inhabitants (of all ages) smoking nearly three-and-a-half cigarettes those days.[8]

Smog Mexico City

Ozone (O3): The limits for Ozone according to the NOM were 0.11 Parts Per Million before 2014 and 0.095 ppm from 2015 to present times. Over the past two decades, ozone levels gradually fell below government limits as authorities moved factories out of the capital and tightened regulations on fuel and cars. In 2015, 37% of the total days showed poor air quality due to O3, 2016 brought a small improvement to 35% of the days. And again in 2017, 35% of the days were recorded as "bad" due to O3 levels. At times during the past two years, ozone concentration levels in the city reached such extreme levels that officials issued environmental risk alerts, urging people to stay indoors.[9]

Sulfur dioxide (SO2): The NOM has three different limits to be met for concentrations of SO2: 8 hours (moving average), 24 hours (daily average) and annual average of the hourly data. During the last 10 years, concentrations below the three limits were recorded. However, in all stations the concentrations of this pollutant as a daily average are above the guideline value recommended by the World Health Organization, which is 0.008 ppm. From 2015 to 2017, the % of days with regular amounts of SO2 are minimal (above 5%).

Nitrogen dioxide (NO2): The NOM defines that the maximum allowable limit is 0.210 ppm as an hourly average. To comply with the standard, the established limit must not be exceeded more than once a year. In all monitoring stations in Mexico City where this NOM was evaluated, concentrations below the limit were recorded during 2015 to 2017. However, this NOM has not been updated since 1993 and the World Health Organization recommends as a guide that the limit of the hourly average for NO2: be set to 0.106 ppm. Regarding the distribution of air quality by NO2, monitoring stations dominated the days with good air quality in Mexico City between 2015 and 2017. In each of the years more than 90% of the days presented good air quality.

Carbon monoxide (CO): NOM establishes that the maximum permissible limit for CO is 11 ppm as a moving average of 8 hours. To comply with the standard, the established limit must not be exceeded more than once a year. Since 2011, the limit established in the NOM for CO in stations located within Mexico City has not been exceeded. Good air quality due to carbon monoxide predominate in all monitoring stations where it was possible to evaluate this pollutant.

One strategy to reduce health effects associated with poor air quality is to enhance public awareness and education of air quality and monitoring tools. In 2005, a survey was administered by members of the BMC Public Health with the purpose of finding out the levels of awareness from the general population in Mexico City about air pollution and its consequences. This study consisted of randomly selecting 800 individuals from Mexico City using a polling company named Parametria and asking them some Spanish-translated versions of the United States 2005 Behavioral Risk Factor Surveillance System.

The survey consisted of 17 questions about Demography, geographic, lifestyle and general knowledge about Air Pollution.

The response rate of the survey was 21%, which compares favorably to the average response rate of phone-based surveys administered in the United States. The demographic information recorded by respondents in terms of gender and age distribution mirrors that of publicly available Mexico City census data. Beyond participant demographic information, it was found that 15.5% of respondents either had a respiratory illness or a family member with a respiratory illness living in their household. This was determined through a question asking if a responder, or anyone in their home, had been diagnosed with a respiratory illness such as asthma.

Results indicate that respondents with a respiratory illness themselves or in the home, compared to those who did not, were 14% more likely to be aware of the IMECA index. Respondents with a respiratory illness in the home were more likely to have had a healthcare provider discuss the Air quality index with them. Following the assessment of air quality index awareness, analyses considered whether those who knew of this resource modified their behavior to reduce exposure to air pollution. To gauge this application of IMECA knowledge among respondents, the number of days modified, defined as avoiding exercise or strenuous activity outdoors in response to poor air quality over a 12-month period, was assessed.

Perceived air pollution behavior modification was further separated by air quality index awareness status. It was found that 23.8% of respondents modified their behavior based on perceived poor air quality and were familiar with the index, and only 11.2% of respondents modified their behavior and were unfamiliar with this tool. Finally, 26.2% of respondents modified their behavior in response to an air quality report at least once over the course of 1 year.[11]

In efforts to reduce the pollution, in 1989 the Mexican Government introduced "No-Drive Days", or Hoy No Circula, which prohibited drivers using their vehicles on one weekday per week, and in 2008 this was changed to include Saturdays. There is also large promotions for alternatives for driving, such as bus rapid transit lines and bike-sharing systems.

To address the city's disastrous pollution levels, the city's administration and the Metropolitan Environmental Commission (MEC) have mutually implemented two successive programs: the Comprehensive Programme Against Air Pollution (PICCA), which was launched in 1990, and ProAire, launched in 1995. The purpose of each was to improve the air quality of the metropolitan area of the valle de México (the Metropolitan Area of Mexico City).[12]

ProAire is now into ProAire IV, which addresses eight themes:

• Reduction of energy consumption The first strategy has the purpose of creating a permanent program of ecosystem analysis of Mexico City, as well as the creation of urban sub-centers in the basins of those Metro stations that currently have idle capacity. Another case is the consideration of criteria that minimize the energy consumption of the physical space cycle of solid waste in the city.
• Cleaner and more efficient energy across all sectors This proposal has the purpose of creating a permanent program of ecosystem analysis of Mexico City, as well as the creation of urban sub-centers in the basins of those Metro stations that currently have idle capacity. Another case is the consideration of criteria that minimize the energy consumption of the physical space cycle of solid waste in the city.
• Promoting public transport and regulating fuel consumption. In addition to the requirement increase the supply of medium and high capacity public transport, it is necessary to improve the overall efficiency of the transportation systems operating in Mexico city. It is not enough to improve mobility, it is necessary to improve it by increasing the energy efficiency of motor vehicles and promoting non-polluting means of transport.
• Technology shift and controlling emissions. These include, among other aspects, covering the lags in the applicable regulations to reduce toxic emissions from organic solvents; to reduce emissions of mercury, dioxins and furans; the strengthening of technical capacities of personnel and equipment in the inspection and surveillance activities to improve compliance with the corresponding environmental legislation; to implement adequate schemes that promote and encourage environmental self-regulation in the industry; to regulate, from the environmental point of view, the limits of emissions of gases and particles of water heaters, process and boilers that are not considered in the current regulations and to establish a continuous and real-time monitoring system in the electric power generating plants, among many other measures.
• Environmental education and creation of sustainable culture and citizen participation. The strategy includes the design and development of a metropolitan incentive program for public and private companies to make incremental use of space work and temporarily distributed. Promoting the simplification of procedures in government agencies and private companies, which includes the elimination of the requirement of physical presence and promotion of the intensive use of new telecommunications technologies. And promoting environmental education in the national education system.
• Green areas and reforestation. The city of Mexico is still far from the internationally recommended ratio between green areas and urbanized areas, which contributes not only to the impoverishment of the quality of the urban environment and therefore to the quality of life, but to the proliferation of islands of heat and the resuspension of particles. The strategy promotes actions such as recovery, restoration, conservation and expansion of urban green areas to mitigate particle resuspension. Review and modernization of forest fire prevention and combat programs. And update and modernization of reforestation programs.
• Institutional capacity building and scientific research. The Metropolitan Environmental Commission has historically concentrated its actions on air quality management. However, expanding the search for more solutions to the problem of air pollution requires progress in the implementation of a metropolitan environmental sustainability agenda, incorporating the ecosystem vision proposed in this plan of action as a whole.[13]

• Air quality index
• IMECA—Índice Metropolitano de la Calidad del Aire
• Mexico City metropolitan area topics

• YouTube.com: "Global 3000: Mexico's War on Smog" = 6:32 min. (July 2014)