MEXICO CITY (Reuters) - Authorities declared an environmental emergency on Tuesday for metropolitan Mexico City, one of the world’s most populous megalopolises, as smoke from nearby wildfires pushed pollution to levels deemed potentially harmful to human health.

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Environmental authorities advised residents to avoid outdoor activities and exercise and remain indoors with windows and doors shut. It called for especially sensitive groups, including infants, the elderly and sick, to stay at home.

The local government said later that the circulation of vehicles would be restricted for most of Wednesday.

The Mexican capital is home to nearly 9 million people, with more than 21 million in its metropolitan area.

The city’s Environmental Commission of the Megalopolis came under pressure to act after visibility in the city began dropping sharply last week because of ash and smoke in the air. Dry weather has played a role in a spate of fires around the city.

Fine particulate matter known as PM2.5 hit 158 micrograms per cubic meter of air at the Nezahualcoyotl measuring station at 5 a.m. The World Health Organization recommends a daily mean air quality guideline below 25.

Annual averages above that amount are associated with higher long-term mortality risks. The world’s most polluted capital city, New Delhi, has an annual average of 113.5.

Mexico City’s air, once infamously lethal, saw a steady improvement through the late 1990s. In recent years, there have been renewed signs of deterioration.

The environmental authority also asked residents and businesses to do their part in helping reduce emissions, such as using cars less, while authorities fight the numerous blazes raging in Mexico City and surrounding states.

A local soccer league, Liga MX, announced it would push back the semifinals of a tournament because of the poor air quality.

Mexico City Mayor Claudia Sheinbaum told local media that she would consider suspending classes for students later in the week.