Government orders for people to stay home to halt the spread of the coronavirushave resulted in a sharp decrease of vehicle traffic around the world.Cities such as Delhi, India, Los Angeles in the United Statesand Seoul in South Korea all have reported better air quality.But that is not the case with Mexico's capital, although its traffic has decreased by 60 percent.The official website of Mexico City says that air quality there is "poor."It warns that exercising at certain times of the day presents a "high" to "very high" health risk.Photographs taken by Reuters last week show a thick haze over the city.Mexico City is about 2240 meters above sea level and is surrounded by mountains.The city and the area around it is home to more than 20 million people.Most are staying inside their homes, but they have increased the use of gas and other fuels possibly for cooking.Methane gas is released from open-air waste holding areas.In addition, a lot of industrial activity continues.All these things have cancelled the reduction in pollution that was expected from putting the economy on hold.Sergio Hernandez oversees air quality in the city.He told Reuters news agency, "We're seeing that it's come down form the peaks,but it's not enough to bring the air quality below the norm."Carlos Alvarez is head of Mexico Communication and Environment, an environmental group.He said that there are 400 open-air waste holding areas, or dumps.They release methane gas.There also are 50,000 industrial generators in the city.The generators provide electricity to hotels, offices and businesses.Most are still operating.The generators use gasoline or diesel fuel, as do the 350,000 trucks and buses that operate in the area.Environmental experts say diesel is a leading cause of harmful pollution.Other causes include the state oil company Pemex's oil processing center in the north,forest fires and ash from the volcano Popocatepetl to the southeast.The air quality in Mexico City is better than it was when the World Health Organization saidit was the world's most polluted city in 1992.But, it is still poor.Bad air quality is linked to high rates of diseases that affect the lungs.Experts warn that COVID-19, the disease caused by the novel coronavirus,could be more dangerous in Mexico City than in other places.Roxana Trejo takes care of patients infected with the coronavirus at the ABC medical center in Mexico City."(Pollution) can be a serious issue once you get the disease," she said.