Air pollution in the Mexico City Metropolitan Area (MCMA) is a complex challenge shaped by unique geographical and historical factors. The city is situated in a high mountain basin, approximately 2,240 meters above sea level, which naturally limits atmospheric oxygen content and causes incomplete fuel combustion. The surrounding mountains act as a bowl, trapping emissions and preventing their dispersal, a situation often worsened by thermal inversion layers. Rapid urbanization, with the population soaring to over 20 million today, intensified this problem, leading the United Nations to label Mexico City the world’s most polluted city in 1992. Primary pollutants remain ground-level ozone (\(\text{O}_3\)), nitrogen oxides (\(\text{NO}_{\text{x}}\)), and fine particulate matter (\(\text{PM}_{2.5}\) and \(\text{PM}_{10}\)), mainly stemming from the massive vehicular fleet.
Regulatory Strategies for Vehicle Emissions
The most direct solution implemented to combat air pollution has been the strict regulation of private vehicle use, the largest single source of emissions. The cornerstone of this strategy is the “Hoy No Circula” (No Driving Day) program, first introduced in 1989. This program restricts the circulation of vehicles one day per week, based on the last digit of their license plate number, aiming to reduce daily traffic by 20%.
However, the program’s early effectiveness was hampered by the rebound effect. Citizens, needing mobility, responded by purchasing a second, often older and higher-emitting, vehicle to circumvent the ban. This action negated the intended air quality benefit and resulted in an overall increase in the total number of cars circulating.
To address this, the government strengthened enforcement through the mandatory “Verificación Vehicular” (Vehicle Emissions Testing) program. This requires all internal combustion vehicles to undergo a rigorous emissions inspection every six months. The test outcome determines the vehicle’s environmental hologram, ranging from Double Zero (“00”) for the newest, lowest-emitting cars to Hologram Two (“2”) for the highest-emitting vehicles.
The hologram dictates the vehicle’s circulation privileges; Hologram 00 or 0 vehicles have fewer or no restrictions. Failure to complete the mandatory verification results in a substantial fine. The owner then has 30 days to successfully complete the emissions test and obtain the hologram, enforcing continuous compliance and maintenance.
Fuel Quality and Industrial Sector Reforms
Beyond regulating the existing fleet, shifts in fuel composition and the industrial landscape provided lasting improvements to air quality. One impactful reform was the mandated transition from leaded to unleaded gasoline, which was completely phased out by 1998. This action drastically reduced ambient lead concentrations, addressing severe public health risks.
The initial rollout of unleaded gasoline faced challenges because the new fuel was significantly more expensive. This price difference incentivized some drivers to use the cheaper leaded option, temporarily compromising the effectiveness of newer vehicle technology.
Parallel to the gasoline reform, the government mandated a substantial reduction in the sulfur content of diesel fuel. Cleaner diesel allowed for the effective use of modern emission control technologies in heavy-duty vehicles. Furthermore, a major industrial source of pollution was permanently removed with the closure of the PEMEX Azcapotzalco oil refinery in 1991, which was the single largest industrial polluter.
The government also implemented a voluntary environmental compliance program for stationary industrial sources. This encouraged facilities to adopt cleaner production technologies and promoted switching from high-polluting fuel oil to cleaner alternatives like natural gas. These foundational changes were highly effective in reducing criteria pollutants like sulfur dioxide, carbon monoxide, and lead.
Investment in Sustainable Urban Mobility
Recognizing the limitations of restriction-based policies, Mexico City has invested heavily in developing high-capacity, low-emission public and non-motorized transport alternatives. A major component is the Metrobús, a Bus Rapid Transit (BRT) system launched in 2005. The Metrobús network uses segregated lanes and modern buses, which are effective in reducing localized emissions of carbon monoxide, nitrogen oxides, and particulate matter.
The expansion and modernization of the Metro system, the city’s subway network, remains a central focus, offering a zero-emission alternative for millions of daily commuters. The city has also committed to electrification, introducing over 400 electric trolleybuses to its public transport fleet since 2019. These investments provide viable, efficient alternatives to private car ownership.
Complementing mass transit is the push for non-motorized transport through Ecobici, the public bicycle-sharing system launched in 2010. The city has invested in a growing network of dedicated bike lanes, now spanning over 240 kilometers, to improve safety and access. Ecobici has facilitated millions of kilometers traveled, promoting a healthier, less car-dependent urban culture.
Air Quality Monitoring and Contingency Planning
The effectiveness of all implemented solutions is continuously tracked and managed by the Atmospheric Monitoring System (SIMAT). SIMAT operates the Automatic Air Quality Monitoring Network (RAMA), which provides hourly data on pollutants like ozone and particulate matter across the metropolitan area. This real-time data is used to calculate and communicate the Metropolitan Index of Air Quality (IMECA) to the public, alerting residents to potential health risks.
When pollution concentrations exceed specific health-based thresholds, the government activates the Program for Atmospheric Environmental Contingencies (PCAA). This plan is triggered when IMECA levels reach 150 points for pollutants like ozone or \(\text{PM}_{10}\), signaling a severe air quality event. During a contingency, emergency measures are implemented to achieve a rapid reduction in emissions.
The most prominent reactive measure is the expansion of the driving restriction program, often called “Doble Hoy No Circula.” This action temporarily increases the number of vehicles prohibited from circulating, extending the restriction to vehicles that would normally be exempt. The contingency plan can also mandate the temporary suspension of high-polluting industrial activities or construction projects.