New research from the Guardian and Quinto Elemento Lab reveals Monterrey, Mexico, is suffering a massive air pollution crisis driven by booming industry. Factories run by US, European, Asian, and Mexican companies release toxic heavy metals and carbon dioxide at levels exceeding many US states. Monterrey's 5.3 million people face air with double the fine particles found in Los Angeles and some of the worst pollution worldwide on bad days. Pollution here causes thousands of deaths annually and health issues such as respiratory infections, neurological disorders, and cancer. About 60% of pollutants come from factories and power plants concentrated near populous neighborhoods. Major emitters include steel plants, battery recyclers like US company Clarios, and refineries. One steel plant emitted more lead in a year than all companies in the New York/New Jersey metro area. Heavy metals such as lead, cadmium, and arsenic are released at alarming rates, threatening children's brain development and causing chronic diseases. Mexican officials acknowledge the problem and promise stricter emissions rules and monitoring. President Claudia Sheinbaum is awaiting scientific reports to identify polluters and measures. Residents campaign demanding clean air with protests and signs like “We want to breathe” and “Mexico is not the trash dump for the United States.” They blame the industrial growth and recycling of toxic US waste for the worsening pollution. Environmentalists say Monterrey's rapid industrial expansion has sacrificed health for economic gains. Industry groups argue that the recycling of US materials helps the planet by reducing mining and deforestation. However, local officials and experts call the old pollution standards a “real disgrace” and criticize the lack of enforcement. The governor of Nuevo León warns of “zero tolerance” for polluters. Activists hope that strong government action and industry cooperation will reduce emissions. They stress the urgent need to address the crisis before health costs outweigh economic gains. Monterrey’s fight against pollution highlights the challenges of balancing industrial development with public health.