Power plants in Iran have have switched from natural gas to heavy fuel oils before the peak winter months, in what could lead to heavy pollution in cities.

The head of electricity generation in Iran’s East Azerbaijan province, Mohsen Mousavi told a meeting in the provincial capital Tabriz that the energy ministry ordered them to begin using mazut instead of liquid gas months before Iranian power plants usually do the switch in high-demand winter months.

Iran has been experiencing natural gas shortages to fuel its power plants in cold and hot months. Last winter and early in summer, power plants switched the mazut, which is a dirty diesel fuel. Heavy smog and pollution covered major cities leading to public outcry.

The early use of mazut will inevitably lead to dangerous levels of pollution in major cities, as the cold season is just beginning in Iran and could last until late February. Mousavi said the power plant in Tabriz belongs to a private owner and the government cannot even install equipment to reduce pollution. The plant produces just 300 megawatts of power and a local official said it is not worth endangering the lives of the city’s residents.