Officials in Mexico City declared an environmental emergency after air pollution in the Mexican capital reached levels potentially dangerous to human health.
They urged those at particular risk to stay indoors and restricted the number of cars that can be driven in the city on Wednesday.
Smoke from nearby wildfires has contributed to increased pollution.
The city has been shrouded in a smoky haze for days.
Mexican photographer Santiago Arau tweeted a video taken from a drone showing the extent of the contamination.
Particulate matter measuring 2.5 micrometers or less, known as PM2.5, reached 158 micrograms per cubic meter of air at a gauging station Tuesday morning, more than six times the average daily limit recommended by the World Organization of the Health.
PM2.5 particles are believed to be particularly harmful because they are so small that they can penetrate the deepest parts of the lungs.
More than 21 million people live in the metropolitan area of Mexico City, famous for its poor air quality. Air pollution levels decreased in the late 1990s, but have risen again in recent years.
The city is in a valley and when there is little wind, the air can quickly stagnate.
Mexico City's environmental commission advised residents to avoid outdoor activities and Mexico's first division soccer league postponed a match between León and Club América, to be played in the capital on Wednesday.
Mayor Claudia Sheinbaum said she would consider canceling school classes if the pollution got worse. She said schools were already keeping their students indoors at recess.