Mexico City officials cancelled classes for millions of students yesterday for a second straight day as smoke from brush fires continued to choke the city of 9 million.

Both lower schools and universities were closed for the pollution alert and Mayor Claudia Sheinbaum said they would remain closed until today.

"We don't expect this situation to change until the weekend," Sheinbaum said.

She said a light, localised rain overnight had done little to cut the pollution. The Government reported that levels of particulate matter of 2.5 micrometres or less, known as PM2.5, had reached 148 micrograms per cubic metre of air. That's about six times the World Health Organisation's daily limit. Ozone levels were also high.

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The city also declared a partial driving ban, but activists of the Citizen Observatory on Air Quality called for officials to limit polluting activities such as truck transportation and construction sites.

The activists said the city should include extremely small particles as a cause for imposing emergency measures. Such particles are frequently found in smoke, diesel exhaust and dust. Emergency measures are currently imposed mainly for ozone levels.

The group said "forest fires are unfortunately going to be an ever more frequent problem as a result of global warming".

Residents are doing what they can to deal with the conditions. Photo / AP
Residents are doing what they can to deal with the conditions. Photo / AP

Sheinbaum said officials would announce changes to the rules for declaring pollution emergencies next week.

Experts said seasonal rains — which usually start around this time of year — could help wash particles out of the air and damp down fires.

But social media users mocked authorities for waiting for Tlaloc — the Aztec rain God — to end the pollution crisis.

Twitter users noted that Sheinbaum herself, when she was in the opposition in 2017 — tweeted that "authorities are waiting for Tlaloc to save them. We need environmental policies in Mexico City".

Federal authorities have also come in for criticism, after the Administration of President Andres Manuel Lopez Obrador cut the budget for the National Forestry Commission, reportedly by about one-third.

Federal Environment Secretary Josefa Gonzalez acknowledged there had been cuts in the budget for the commission, known as Conafor, which co-ordinates federal, state and municipal firefighting efforts.

"It is not just the cuts to Conafor, it is also the heat and the location" of the fires that has made them hard to fight, she said.

- AP