Mexico City is facing the worst air pollution in almost two decades, prompting the authorities to maintain environmental protection measures for the fifth day in a row.

The city’s environmental commission said in a statement that the alarm against the high level of ozone would stay in place on Friday, Xinhua news agency reported.

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The pollution arose from the high-pressure system over the metropolis, which, along with a lack of wind to disperse the contamination, caused intense solar radiation and the formation of ozone, the statement said.

Mexico City has issued traffic restrictions which prevented the circulation of 1.5 million vehicles.

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Industrial plants have been requested to reduce their emissions by 30 to 40 per cent, while 20 per cent of gas stations and liquid petroleum distributors will have to suspend operations.

The last time Mexico City maintained an environmental alert for five days or more was in May 1998.

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In 2016, the Mexico City government passed stringent new laws for vehicular emissions as well as encouraging the use of public transport.