Mexico City's Air Pollution: A Crisis Unfolding

Mexico City has long been notorious for its poor air quality, having been named the world's most polluted city by the United Nations in 1992. The problem arose from a combination of geographical factors, a population boom, rapid industrialisation, and the proliferation of vehicles. However, the city has since made significant strides in improving its air quality, dropping to the 917th most polluted city in the world in 2021. This improvement is largely due to the Mexican government's efforts to reformulate gasoline, close or relocate polluting factories, prohibit driving one day a week, and expand public transportation. Mexico City has also joined the Breathe Cities initiative, demonstrating its commitment to tackling air pollution and creating a healthier and more sustainable urban environment.

Population growth, industrialisation, and vehicle emissions

Mexico City's air pollution has been a concern for its population and health officials for decades. In the 20th century, Mexico City's population rapidly increased as industrialisation attracted thousands of migrants from across the globe. This population boom, coupled with industrialisation and vehicle emissions, led to the city being declared the most polluted city in the world in 1992 by the United Nations.

Population Growth

From 1950 to 2015, the population in Mexico City increased from three million to twenty million. This population boom was primarily due to migrants seeking better opportunities. As the population grew, so did the demand for housing, infrastructure, and resources, which put pressure on the environment and contributed to air pollution.

Industrialisation

The population boom in Mexico City was closely followed by an era of industrialisation. This industrial growth emitted over 11,000 tons of waste material into the atmosphere daily. Industrial processes, power plants, and stubble burning are significant sources of air pollutants, releasing harmful particles and gases into the atmosphere.

Vehicle Emissions

The economic boom that accompanied industrialisation led to a proliferation of vehicles in Mexico City. The number of cars and light-duty trucks per 1000 residents increased from 124 in 1980 to 267 by 2010. This increase in vehicles contributed to air pollution through the burning of fossil fuels, which release pollutants such as nitrogen oxides (NOx), carbon monoxide (CO), and particulate matter.

While efforts to improve air quality, such as the Mexican government's requirement to reformulate gasoline and restrict vehicle use, have shown some success, air pollution remains a critical issue in Mexico City. The city's ongoing population growth and urban sprawl continue to lengthen automobile trips, and the number of vehicles in the city increases annually.

Poor geographical conditions and topography

Mexico City's poor geographical conditions and topography have significantly contributed to its air pollution problem. The city is located in a valley, surrounded on three sides by mountains, which form a natural basin or bowl-like container. This geographical feature traps contaminants and air pollutants, preventing their dispersion. The city's high altitude of 2,300 meters (7,546 feet) above sea level further exacerbates the issue, as it results in lower oxygen levels and incomplete combustion of carbon-based fuels.

The mountain ranges surrounding Mexico City, such as the Sierra Madre Oriental in the east and the Sierra Madre Occidental in the west, contribute to the trapped air and pollution. Additionally, the city's proximity to the Popocatepetl volcano, which is Mexico's most active volcano, has been linked to increased contamination levels. Volcanic activity, frequent wildfires, and other environmental conditions, such as above-average temperatures, low rainfall, and prolonged droughts, further worsen the air quality.

The geographical location of Mexico City, in relation to the rest of the country, also plays a role. Mexico City is situated in the southern portion of North America, bounded by the United States to the north, the Pacific Ocean to the west and south, the Gulf of Mexico to the east, and neighboring countries such as Belize and Guatemala to the southeast. This diverse geography, including sweeping deserts in the north and dense rainforests in the south and east, contributes to varying climate conditions that can influence air quality.

The basin-like topography of Mexico City, surrounded by mountains, creates a natural barrier that traps pollutants and prevents their dispersion. The combination of geographical features, such as the valley location, surrounding mountains, and high altitude, has made it challenging to maintain good air quality in the city. The climate and weather patterns in Mexico further contribute to the issue. The spring season, with its hot and dry weather, creates favourable conditions for the formation of ground-level ozone, a significant air pollutant in Mexico City.

While Mexico City has made significant improvements in reducing air pollution, geographical conditions continue to play a crucial role in influencing the city's air quality. The interaction of topography, climate, and human activities has resulted in a complex environmental challenge that requires ongoing efforts and interventions to mitigate the impact on public health and the environment.

Government policies and interventions

Mexico City has historically been one of the most polluted cities in the world. In 1992, the United Nations named it "the most polluted city on the planet". While the air quality has improved since then, and Mexico City now ranks as the 917th most polluted city, air pollution remains a significant issue.

To address air pollution in Mexico City, the Mexican government has implemented various policies and interventions over the years. Here are some key examples:

• Vehicle restrictions: The government has introduced rules that prohibit drivers from using their cars one day a week. This is determined by the number of pollutants a car issues. In addition, the government has encouraged the expansion of public transportation and increased bicycle accessibility.
• Fuel reformulation: The government mandated the reformulation of gasoline, which has helped reduce the presence of certain pollutants.
• Factory closures and relocations: Polluting factories have been closed or moved out of the city, reducing emissions from industrial sources.
• Air Quality Index (AQI): Mexico City uses the Air Quality Index (AQI) to measure and communicate the level of air pollution. The AQI provides a numerical value that indicates the amount and severity of atmospheric pollutants. This helps citizens and authorities understand the air quality at a given time.
• IMECA Index: The Índice Metropolitano de la Calidad del Aire (IMECA) is another index used specifically in the Mexico City Metropolitan Area to assess air quality and provide recommendations for protection. It has five categories ranging from "Good" to "Extremely Poor," each indicating the level of health risk associated with the current air quality.
• ACE Programme: The activation of the ACEP (ACE Programme) during the spring season has become a new normal due to the seasonal high levels of air pollution from climate change.
• Reducing Deforestation: Mexico has set a goal of achieving zero deforestation by 2030. Deforestation reduces the city's natural air purification capacity, and strengthening inspection and surveillance systems can help tackle this issue.
• Emissions Standards: While enforcement remains a challenge, the government has implemented emissions standards for vehicles and industries. For example, new rules for cars transiting in CDMX aim to control the number of pollutants issued.
• COVID-19 Lockdowns: During the COVID-19 pandemic, lockdowns and restrictions led to significant reductions in air pollutants, particularly CO, NO2, and PM2.5. These baseline levels during the lockdown can inform policymakers about setting new target limits and developing strategies to improve air quality.

While these interventions have contributed to improving air quality in Mexico City, more aggressive policies may be needed to address the root causes of air pollution effectively. The Mexican government's commitment to tackling environmental issues, including air pollution, is crucial for driving further progress.

Health risks and impacts

Air pollution in Mexico City has been a concern for the city's population and health officials for decades. In 1992, the United Nations declared Mexico City the "most polluted city on the planet". Since then, the city has witnessed a significant improvement in air quality, dropping to the 917th most polluted city in the world in 2021. However, the issue of air pollution and its health risks persist.

The primary sources of air pollution in Mexico City are human activities such as burning fossil fuels in power plants and vehicle emissions. The city's topography, surrounded by mountains and located at a high altitude, also contributes to trapping air pollutants. The main pollutants found in Mexico City's atmosphere are O3, PM10, PM2.5, CO, NO2, and SO2. These pollutants have various health risks associated with them.

Ozone (O3), a secondary pollutant formed from the reaction of organic compounds and nitrogen oxides, can affect the respiratory, cardiovascular, and central nervous systems. Exposure to ozone has been linked to early death, reproductive health issues, and developmental problems. Carbon Monoxide (CO), primarily emitted by vehicles, is a toxic gas that can have detrimental effects on human health, especially in high concentrations.

Particulate matter, such as PM10 and PM2.5, refers to tiny particles suspended in the air. These particles can penetrate deep into the respiratory system and cause or exacerbate respiratory diseases, asthma, and other lung conditions. Nitrogen Dioxide (NO2) and Sulfur Dioxide (SO2) are also harmful pollutants. Nitrogen Dioxide can irritate the respiratory system and aggravate existing respiratory conditions, while Sulfur Dioxide can have negative effects on the respiratory system and overall health, especially for sensitive groups.

The health effects of air pollution in Mexico City are not limited to respiratory issues. Prolonged exposure to polluted air can lead to permanent health consequences such as accelerated ageing of the lungs, reduced lung capacity, and shortened life expectancy. Those with pre-existing conditions, such as heart disease, coronary artery disease, or congestive heart failure, are at an increased risk, along with pregnant women, children, senior citizens, and outdoor workers. Additionally, air pollution has been associated with various types of cancer, particularly lung cancer.

The Mexican government has implemented several measures to combat air pollution and mitigate its health impacts, including vehicular restrictions, the promotion of public transportation, and the expansion of green areas. While these efforts have led to significant improvements, more stringent policies and intergovernmental cooperation are needed to address the root causes of air pollution and protect the health of Mexico City's residents.

Air quality monitoring and indices

One commonly used index is the Air Quality Index (AQI), which provides a way to communicate complex air quality data to the public in a simple and understandable manner. The AQI takes into account multiple pollutants, including particulate matter (PM2.5 and PM10), ozone (O3), nitrogen dioxide (NO2), sulphur dioxide (SO2), and carbon monoxide (CO). Each pollutant has specific health-based standards, and the AQI value represents the highest value among these pollutants, making it a comprehensive indicator of overall air quality.

Mexico City has a network of air quality monitoring stations located across the metropolitan area. These stations continuously measure the concentration of pollutants in the air, providing real-time data that is used to calculate the AQI. The AQI values are typically categorised into several levels, ranging from "Good" to "Hazardous," with corresponding colours to facilitate quick interpretation and understanding of the air quality conditions.

The monitoring stations typically measure pollutant concentrations over a 24-hour period and an average over several days to assess short-term and long-term exposure risks. This data is then used to create air quality maps, which help identify areas with persistently high pollution levels, allowing for targeted interventions and strategies to improve air quality in those specific areas.

In addition to the AQI, other specific indices have been developed to address particular concerns related to air pollution in Mexico City. For instance, the Environmental Pollution Index (IPAS) and the Atmospheric Pollution Index (IA) are employed to assess the environmental and atmospheric conditions, taking into account factors such as land use, population density, and industrial activity. These indices provide a more comprehensive understanding of the unique pollution challenges faced by Mexico City.

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