Mexico City authorities warned of the risk of air pollution in the capital on Tuesday due to inversion, a meteorological phenomenon that traps contamination.

The Mexico City Atmospheric Monitoring System (SIMAT) said on the X social media platform that inversion at a height of 3,050 meters would remain throughout the day.

According to the United States National Weather Service, a temperature inversion, or thermal inversion, is “a layer in the atmosphere in which air temperature increases with height.”

SIMAT said that inversion “causes an extreme condition of atmospheric stability,” noting that there is “no movement of air” during the phenomenon and that “contaminants accumulate during hours, increasing their concentration and the risk to health.”

According to the Finnish Meteorological Institute, “an inversion can prevent the rise and dispersal of pollutants from the lower layers of the atmosphere, because warm air above cooler air acts like a lid, preventing vertical mixing and trapping the pollution material at the breathing level.”

Despite the presence of inversion in Mexico City, as of midday Tuesday authorities hadn’t issued a formal air quality alert. Three such alerts have been issued this year, all in the first quarter.

The United Nations declared Mexico City to be the most polluted city on the planet in 1992.

While air pollution is still a problem at times, the situation in the capital and surrounding areas has improved significantly over the past three decades.

The explosion of fireworks around Christmas and New Year’s Day has added to air pollution in Mexico City in recent years.

Mexico News Daily