In 1980 half of all the industrial jobs in Mexico were located in Mexico City. Under relentless growth, the Mexico City government could barely keep up with services. Villagers from the countryside who continued to pour into the city to escape poverty only compounded the city's problems. With no housing available, they took over lands surrounding the city, creating huge shantytowns that extended for many miles. This caused serious air pollution in Mexico City and water pollution problems, as well as subsidence due to overextraction of groundwater. Air and water pollution has been contained and improved in several areas due to government programs, the renovation of vehicles and the modernization of public transportation.

How many of the industrial jobs in the county were based in Mexico City in the late 20th century?