Five of Mexico’s six Pemex oil refineries are among the 25 top polluters in the world for sulfur dioxide (SO2) emissions, monitoring by NASA satellites in 2018 shows.

Globally, the three top SO2 polluting refineries are located in Saudi Arabia, Qatar and Singapore, respectively, but the Mexican refineries in Tula, Hidalgo, and Salina Cruz, Oaxaca, are ranked in fourth and fifth place.

Rounding out the top 25 are the refineries in Cadereyta, Nuevo León, in 18th place; Salamanca, Guanajuato, in 19th place; and Ciudad Madero, Tamaulipas, 23rd.

Alejandro Villalobos Hiriart, an oil refining specialist and consultant, said that Mexican plants lack filtering systems called “scrubbers” which capture SO2 emissions. Other experts say that Mexico’s hydrodesulfurization plants, which use a chemical process to remove sulfur from refined petroleum products, are not adequately maintained.

Pemex’s black footprint poses a problem for the environment and for citizens, particularly due to the location of its refineries. 

“One of the problems is that, for example, the Tula refinery is located about 90 kilometers from Mexico City in the metropolitan area and the prevailing winds cause all this contamination to flow into the Valley of México, causing a tremendous effect on people’s health,” added Villalobos.

SO2 gasses irritate the nose, eyes and lungs and can cause severe respiratory symptoms. SO2 is also a precursor to acid rain. 

The NASA report shows that Mexico’s oil and gas activity, which in addition to crude oil refining includes the operations of oil fields and fuel-fired power plants, was overall the second most polluting in the world in 2018, producing 1,580 kilotons of emissions. 

Topping the list was Saudi Arabia, which produced six times more crude oil than Mexico that year, and emitted 1,783 kilotons of pollutants.

There are approximately 700 oil refineries in the world.