Mexico City has declared its first air pollution emergency since 2005, and ordered residents to stay indoors because toxic ozone levels hit twice the maximum limit considered acceptable.
The alert also required older and more heavily polluting vehicles to stay off the road, and certain manufacturers to limit production, reports Associated Press. Authorities attributed the "extraordinary increase" in ozone to weather conditions: a high-pressure zone and intense sunlight.
Ozone, a key ingredient of air pollution, is a toxic form of oxygen created by the reaction of sunlight with air containing other pollutants such as hydrocarbons and nitrogen oxide. Heightened ozone levels can cause breathing difficulties and worsen heart disease. Children and the elderly are especially at risk.
Mexico City and its population of 20 million people are particularly vulnerable to pollution. The city is situated in an elevated "bowl" 7,350ft (2,240m) above sea level, and surrounded by mountains that can trap pollutants over the metropolis.
The last time an alert was issued for particle pollution was on New Year's Day 2005. Then, air quality was likely affected by heavy use of firecrackers, according to city officials.
The city often declared dangerous smog days in the early 2000s, but when authorities ordered old, polluting cars off the streets pollution decreased. A recent court order lifting restrictions on many of the cars, however, is beginning to return the city to its smog days, according to officials.