The Mexico City government has declared a pollution alert on Monday, its first in 11 years.

Ozone levels in the city reached almost twice the acceptable limit. On Tuesday, the government required older and more heavily polluting vehicles to stay off the road in Mexico City as a measure to improve air quality.

Officials also advised people to remain indoors and to refrain from performing vigorous exercise outdoors. The alert also limits highly polluting industrial practices.

Causes

Mexico City's environment office blamed the poor air conditions on a high-pressure system and intense sunlight. The city was accustomed to high smog levels in the past before the introduction of a rule that prohibited the use of cars that are more than 8 years old.

That rule, however, was eased by a court order. According to officials and environmental activists, the court order caused more cars to end up on city streets.

Mexico City, which is 7,350 feet (2,240 meters) above sea level, is situated in a high mountain valley surrounded by mountains that can trap pollutants and stop them from dispersing. The smog currently permeating the city contains ozone, which can cause respiratory problems.

Mexico City's last alert for ozone was in 2002 while the last pollution alert for air particles occurred in 2005.

Remedies

In April 2015, Mexico became the first developing nation to declare a formal pledge in cutting global warming pollution. Environment Minister Juan Jose Guerra Abud said that the country expects greenhouse gas emissions to rise by 2026 and then decline.

Part of Mexico's efforts to tackle pollution is the pending standard to regulate emission from heavy duty vehicles, NRDC reported. The standard called NOM-044 is projected to bring economic and health benefits to Mexicans as well as lessen climate-inducing air pollution.

The standard was penned with input from academics, civil society and the industry. However, Mexico's Environment and Natural Resources Secretariat, or SEMARNAT, is currently thinking of easing it due to pressure from a sector of the industry.

The World Health Organization, or WHO, has categorized diesel exhaust and outdoor air pollution as carcinogens. The black carbon element of diesel exhaust is the most powerful climate pollutant, second to carbon dioxide. Mexico's Paris climate commitment aims to reduce black carbon emissions by 51 percent by 2030, and by 70 percent if international aid is given.

With NOM-044, Mexico also expects to avoid more than 55,000 premature deaths. It could also bring more than $120 billion in net benefits to the country.