(Bloomberg) -- Environmental authorities ordered Petroleos Mexicanos, the state oil company, to curtail processing at one of the country’s largest refineries due to a deterioration of air quality, undermining the company’s plans to boost fuelmaking. 

The Tula refinery, the closest to Mexico City’s metropolitan area where more than 20 million people live, was ordered to reduce oil processing rates to 75% of the design capacity of 315,000 barrels a day. The power plant associated with it should also cut use of fuel oil for electricity by 30%, according to a notice from the environmental authority. 

High ozone levels can cause respiratory problems and long-term exposure is believed to be one of the causes of asthma. Levels rise when certain fossil compounds, like those produced by burning gasoline or fuel oil, combine with sunlight to produce ozone. Air quality problems prompted a 20% reduction two years ago to 152,000 barrels a day at Tula. 

The facility, in Hidalgo state, is the second-largest in the country. It’s the the top consumer of a medium-light oil variety called Olmeca, of which the refinery consumes about 1 million barrels per month. After Tula curtailed rates, Pemex sold a cargo of the grade to Spanish refiner Cia Espanola de Petroleos SA, known as Cepsa. The ship Aegean Marathon loaded earlier this week and is now sailing to an oil terminal in Huelva, the port of delivery for the Cepsa La Rabida refinery. 

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