Mexico City's Pollution: Understanding The Root Causes

Mexico City has long struggled with air pollution, which reached its peak in the 1980s and 1990s. The city's poor air quality is caused by a combination of factors, including emissions from transport and industry, as well as its topography, which traps pollutants in the basin where the city is located. With over 20 million people living in the metropolitan area, rapid population growth and increased human activity have also contributed to the problem. While the Mexican government has implemented various measures to combat pollution since the 1990s, recent years have seen a rise in pollution levels once again, with smoke from wildfires contributing to hazardous air quality.

Industrialisation and urbanisation

Mexico City has long been infamous for its poor air quality, with the problem reaching its peak in the 1980s and 1990s. The air pollution in the city is largely the result of decades of industrialisation and urbanisation.

During the 20th century, Mexico City underwent rapid industrialisation and urbanisation, with the population of the metropolitan area, also known as Greater Mexico City, growing from 3.1 million in the 1950s to 14 million in the 1980s and an estimated 22 million today. This growth in population and human activity has resulted in severely poor air quality.

In the 1980s, there were about 124 cars per 1000 residents. While this may not seem like a high number, these cars were of poor quality and ran on highly toxic fuel. At the same time, unregulated and highly polluting factories, power plants, and oil refineries were operating across the city. The topography of Mexico City, located in a basin, meant that air pollutants were trapped in the metropolitan area.

The combination of vehicle emissions and industrial pollution led to dangerous levels of ground-level ozone, carbon monoxide, sulphur dioxide, nitrogen dioxide, and airborne particles—all major air pollutants. The pollution was so severe that birds were found dead, falling out of the sky, and residents compared living in the city to smoking two packs of cigarettes a day. Even on clear days, it was difficult to see across the street due to the thick layer of smog.

Recognising the severity of the problem, the city's government began implementing policies to reduce air pollution, understanding the threat it posed to public health and economic development. The Management Programme to Improve Air Quality (Proaire) was introduced in 1996, capturing the will of civil society and businesses to tackle air pollution. This programme has successfully brought down pollution levels, with Mexico City dropping to the 917th most polluted city in the world in 2021, according to IQAir.

Poorly regulated factories, power plants, and oil refineries

Mexico City has long been infamous for its poor air quality, with over 21 million people living in its metropolitan area. The city's environmental issues are multifaceted and complex, but one significant contributor is the presence of poorly regulated factories, power plants, and oil refineries.

One notable example is the Tula thermoelectric power plant, located north of Mexico City. Between 2016 and 2019, the plant breached legal limits for sulfur content in the fuel oil it burned, with levels nearly double the cap set by Mexico's energy regulatory commission. This violation resulted in the release of massive amounts of harmful particles and gases into the atmosphere, leading to severe air pollution and potential health risks for residents.

The Tula plant is not an isolated case, as Mexico's refineries produce high-sulfur fuel oil for various purposes, including power generation. The burning of high-sulfur fuel oil without proper contaminant-capturing filters has been linked to the emission of pollutants such as sulfur dioxide, nitrogen oxides, and particulate matter smaller than 2.5 microns (PM2.5). These pollutants have severe health implications, including respiratory diseases and cancers, and can even lead to premature death.

The situation is further exacerbated by the country's oil company, Pemex. Despite receiving significant government support and investments, amounting to USD 3.5 billion in February 2021 and a budget of USD 32 billion in 2023, there are concerns about its environmental impact. Pemex has been accused of benefiting from regulatory changes enacted by the López Obrador administration, which have negatively impacted private sector participants in the oil and gas industry.

To address these issues, the Mexican government has proposed several initiatives to combat air pollution, including incentives for using cleaner fuels and improving environmental infrastructure. However, more stringent enforcement of regulations and a transition to cleaner energy sources are necessary to mitigate the environmental and health consequences of poorly regulated industrial activities.

Poor-quality cars with toxic emissions

Mexico City's air pollution was at its worst in the 1980s and 1990s, with the air quality being described as so severe that birds were found dead from the toxic air. The primary cause of this was the millions of vehicles without pollution controls, running on highly toxic and pollutant-laden fuel. These cars were of poor quality and emitted ground-level ozone, carbon monoxide, sulphur dioxide, nitrogen dioxide, and airborne particles—all major air pollutants. In the 1980s, there were about 124 cars per 1000 residents, and these vehicles were a significant contributor to the city's air pollution crisis.

The topography of Mexico City, located in a basin, meant that air pollutants were trapped in the metropolitan area, exacerbating the problem. The combination of industrialisation, urbanisation, and a rapidly growing population also played a role in the city's poor air quality. The number of vehicles in the city had increased significantly, and these cars were made with outdated technology and inefficient engines, emitting large amounts of harmful pollutants into the atmosphere.

To address the issue of toxic emissions from poor-quality cars, the Mexican government has implemented various innovative proposals and policies over the years. One notable policy is the "Hoy No Circula" or "Cars Don't Circulate" programme introduced in 1989. This programme restricts the use of a fifth of all vehicles on rotating days, depending on the last number of their registration plate. This policy aims to reduce the number of vehicles on the road and, consequently, the amount of toxic emissions.

In addition to the "Hoy No Circula" programme, the government has also introduced emissions testing for all vehicles in Mexico City and the surrounding state. These tests are mandatory every six months, and vehicles are issued a verification rating based on their performance, with "0" for those meeting requirements and "1" or "2" for underperforming vehicles. This initiative encourages residents to drive cleaner cars and take action to improve their vehicle's emissions.

Furthermore, the Management Programme to Improve Air Quality, or ProAire, has been instrumental in reducing air pollution in Mexico City. ProAire has implemented stricter restrictions on fuels, limiting vapor pressure, olefins, aromatics, benzene, and sulphur content. This has led to the development and introduction of cleaner, more efficient fuels by the national petroleum company, PEMEX. ProAire has also focused on improving public transportation, with the Metrobus system providing clean, fuel-efficient buses and reducing the city's carbon footprint by an estimated 80,000 tons annually.

Wildfires

Mexico City's geography, lying in a valley, makes it particularly susceptible to the effects of wildfires. When there is little wind, the air can become stagnant, trapping pollutants and reducing air quality. Wildfires can spread quickly and are often fuelled by strong winds and drought conditions. In the case of the 2019 emergency, smoke from nearby forest fires contributed to the spike in pollution, leading to a smoky haze that blanketed the city.

The impact of wildfires on air quality can be severe, as they release large amounts of smoke and particulate matter into the atmosphere. These fine particles, known as PM2.5, can reach unsafe levels, posing a significant risk to human health. During the 2019 emergency in Mexico City, PM2.5 levels reached more than six times the World Health Organization's daily mean recommended limit.

To combat the effects of wildfires and reduce air pollution, the Mexican government has implemented various initiatives. These include incentives for using cleaner fuels, smog control measures, and restrictions on vehicle usage, such as requiring private car drivers to have catalytic converters or refrain from driving one day a week.

Additionally, Mexico has invested in firefighting resources and equipment, including air tankers to assist in firefighting efforts. Community organizations and local governments have also played a crucial role in fighting wildfires and providing support to affected areas. However, in some cases, such as the 2022 fire in San Lucas Quiaviní, residents have criticized officials for reacting too slowly, highlighting the ongoing challenges in effectively managing wildfires and their impact on air pollution in Mexico City.

Topography and weather patterns that cause pollutants to be trapped in the valley

Mexico City is located in a valley, and its unique topography and weather patterns have a significant impact on the city's air quality. The city's geographic characteristics generally prevent the ventilation of polluted air. The valley is surrounded by higher topography, which acts as a barrier to the dispersion of pollutants.

The interaction between the weather and topography plays a crucial role in trapping pollutants in the valley. Stable atmospheric conditions, calm winds, and temperature inversions can contribute to the accumulation and persistence of pollutants. Calm winds, also known as light horizontal winds, can cause pollutants to become trapped in one area, leading to higher pollution levels. This is further exacerbated by the downward air motion associated with anticyclones, which inhibits vertical mixing and stabilises the top of the boundary layer. The lack of wind prevents the dispersal of pollutants, allowing them to remain concentrated in the valley.

Temperature inversions can also trap pollutants close to the surface. This occurs when a layer of warm air acts as a lid, trapping cold, dense air and pollutants near the ground. The cold air moves slowly, allowing pollutants to accumulate. In addition, the mountains surrounding the valley can block the horizontal transport of smog, preventing it from spreading to other areas. The combination of calm winds and warm temperatures can promote the formation and accumulation of smog in the valley.

The weather patterns that influence the concentration of pollutants in Mexico City have been studied extensively. The dry season, from November to May, experiences more frequent high pollution events due to mid-troposphere high-pressure systems centred around 20°N. La Niña and El Niño years also influence the occurrence of pollution-related weather patterns during the dry season, with La Niña years favouring more pollution. These weather patterns and the city's topography contribute to the persistence and intensification of pollution episodes.

Frequently asked questions