An air quality alert was activated Saturday in Monterrey, Nuevo León, for the second time this week after 12 of the city’s 13 air quality monitoring stations registered high levels of pollutants.

The city’s Comprehensive Environmental Monitoring System (SIMA) have warned residents of poor air quality since Monday.

Photos of the Nuevo León capital showed a dense cloud of gray smog shrouding the urban center, and its iconic backdrop of the Cerro de la Silla mountain skirted in an acrid haze.

Areas most affected were Santa Catarina, San Bernabé and Universidad, authorities said. The Puebla Serena monitoring station was the only one that showed normal air quality.

An environmental alert was issued on Wednesday, prompting officials to urge citizens to minimize activity in the open air, keep their doors and windows closed and not allow vehicles to idle, among other measures. That alert was deactivated on Thursday.

The government also advised the industrial and construction sectors to not engage in activities that increase particle emissions into the air.

However, a number of citizens and businesses disregarded the government’s appeal.

Videos published on social media revealed businesses emitting contaminants, burning trash and tires and generating large amounts of dust that were kicked up into the air.

Although the environmental alert was revoked on Thursday, authorities reinstated it on Saturday after the air quality continued to decline.

Air quality monitoring requirements were increased not only in Nuevo León but throughout Mexico on November 20 when the federal Environment Secretariat issued new regulations. They replace the former Metropolitan Index of Air Quality (Imeca) with the Index of Air Quality and Health Risks (ICARS).

The new regulations require urban areas to check for airborne particulates every 12 hours rather than every 24 hours as was previously required.

Sources: Blog del Regio (sp), El Sol de México (sp), Milenio (sp), Info 7 (sp)