MEXICO CITY, June 6 (Xinhua) -- Mexico's air-quality monitoring agency on Wednesday issued the first pollution alert of the year for Mexico City and the metropolitan area, after ozone levels rose above acceptable limits.

The Environmental Commission of the Megalopolis (Came) activated a phase-one alert after detecting 162 points of ozone Wednesday afternoon in the south of the city, 12 points above the accepted limit of 150 points, the agency said.

Came blamed the smog on a high pressure system that was preventing high winds from dispersing contaminants, while high temperatures and greater solar radiation generated more ozone.

The warning prevents some cars from circulating on Thursday and also curbs industrial activity in the capital and its surroundings, home to more than 20 million people.

Mexico City already has a day-without-a-car program in place, but the alert keeps more vehicles off the road by selecting those with certain assigned numbers.

Industries with no air-pollution prevention systems in place are expected to reduce emissions by 40 percent.

Came recommended residents stay indoors, especially children, pregnant women, and the elderly.

According to the city government, Mexico City, the most populated city in Latin America, has had only 19 days of "clean air" this year, although pollution levels had not surpassed the limits until now.

Last year, the agency issued two warnings, both in May, typically the city's hottest month before the rainy season in June.?