Mexico City's government has advised people to stay indoors and restricted traffic due to air pollution. Ozone levels have been measured at nearly twice the acceptable limit, triggering the first alert in over a decade.
Image: picture-alliance/dpa/A. Cruz
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Authorities said Monday that the alert stemmed from an "extraordinary increase" in ozone concentration due to the presence of a high pressure weather system and intense solar radiation around greater Mexico City. The Mexican capital is home to more than 20 million inhabitants.
An advisory posted by the city authorities recommended that people stay off the streets between 1 p.m. and 7 p.m., refrain from vigorous exercise in the open and curb sporting activities, in order to avoid respiratory problems.
Mexico moves to combat smog
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The last time a a phase one alert was issued for particle pollution was on New Year's Day 2005. Then, air quality was likely affected by heavy use of firecrackers, a city official said. Ozone, which is a key ingredient of air pollution, is a form of oxygen created by the reaction of sunlight with air containing other pollutants such as hydrocarbons and nitrogen oxide.
Altitude
Ozone is a key ingredient of smog. The last alert issued for ozone was January 18, 2002.Image: picture-alliance/dpa
Geography is also a factor. The capital city lies about 7,350 feet (2,240 meters) above sea level in a high mountain valley, where the surrounding mountain range can trap pollutants.
Environmentalists also point to a recent court order that has allowed older cars in the city. Earlier regulations barring vehicles older than eight years old had curbed gross polluters. But the court's order relaxing the rule has allowed more polluting cars back on the streets.
Heightened ozone levels can cause breathing difficulties and worsen heart disease. Children and the elderly are especially at risk.
The big smog: Cities plagued by air pollution
Smog hangs over Ulan Bator - like every winter. This air pollution and intense cold is damaging the lungs of 19-year-old Tsegi. But it’s not only Mongolia’s capital: Here's a look at some of the worst cities for bad air.
Image: picture-alliance/dpa
Ulan Bator, Mongolia
Ulan Bator is not only one of the coldest capitals on earth, it's also a city with massive air pollution. During the winter months, yurts like Tsegi’s are heated with coal and wood which contributes up to 70 percent of the smog in the city. Air pollution in Ulan Bator is seven times higher than what is considered safe by the WHO.
Image: DW/Robert Richter
Beijing, China
The Chinese capital has been suffering from smog so heavy that scientists say the city is almost uninhabitable - although it is home to 20 million people. Models suggest that 3.5 million people die globally every year because of air pollution - almost half of them in China. Having said this, it might be a surprise to learn that smog is an even bigger problem in other cities across the world.
Image: picture alliance/Photoshot
Lahore, Pakistan
Air pollution is one of Pakistan's main environmental concerns. The situation is particularly dramatic in the country's second largest city, Lahore. The smog is caused primarily by the high volume of road traffic, rubbish incineration and dust from the surrounding deserts.
Image: picture-alliance/dpa
New Delhi, India
In the nearly 10 million-strong city of New Delhi, the number of cars has increased from 180,000 to 3.5 million in the last 30 years. Still, it's the city's coal powered plants that are causing the biggest problem. They contribute to around 80 percent of the total air pollution in the city.
Image: picture-alliance/dpa
Riyadh, Saudi Arabia
Sandstorms, like here in Riyadh, can contribute to smog forming because they increase the amount of particles in the air. In a place like Saudi Arabia, the intense ultra-violet rays also transform transport and industry emissions into ozone.
Image: picture-alliance/dpa
Cairo, Egypt
The poor air quality in Cairo causes a number illnesses among city residents, like chronic respiratory problems and lung cancer. The reason for the air pollution is an increase in road traffic and the booming industrial sector.
Image: DW Akademie/J. Rahe
Dhaka, Bangladesh
According to a study by the Max-Planck Institute in Mainz, some 15,000 people die every year in Dhaka due to air pollution. Researchers found the world's highest concentration of sulfur dioxide there.
Image: picture-alliance/dpa
Moscow, Russia
Even if it looks the same the world over, smog is different, depending on the city. Smog in Moscow, for instance, is characterized by high amounts of hydrocarbons. The westerly winds which regularly plow across Moscow mean that the western part of the city generally has better air quality.
Image: picture-alliance/dpa
Mexico City, Mexico
The smog in Mexico City is made worse by the geographical location. The city is surrounded on three sides by mountains. Due to the high levels of sulfur dioxide and hydrocarbons in the air, Mexico City was long considered one of the most polluted cities in the world. The situation is now improving due to new transport policies and certain factories being shut down.
