MEXICO CITY -- In June, the head of the United Nations Environment Program descended through the photochemical cloud that hangs over the world's most polluted city.
The official had chosen Mexico City as the site to celebrate the U.N. Day of the Atmosphere. He couldn't have flown to a better spot.
The day he arrived, the lead content in the air was 40 percent above the permissible level prescribed by the United Nations' World Health Organization.
Every day about 11,000 tons of pollutants are spewed into the air, leaving the city's population coughing and gasping. Others are sickened by 1 billion pounds of fecal dust that wafts across the city each year from the 2.7 million people who lack bathrooms. Last year a U.N. study found the number of microorganisms in the air "to be uncountable," among them streptococcus, salmonella and staphylococcus.
So far this year, Mexico City's air pollution has exceeded by astronomical amounts the safety standards for ozone set by the WHO, which recommends that human beings not be exposed to ozone of more than 0.11 parts per million for more than onehour a year. Mexico City residents have been exposed to excessive amounts 380 hours between January and June. The city's level of carbon monoxide has exceeded WHO safety levels 200 days this year.
But despite such alarming figures, the U.N. environmental official had come to mark Mexico City's decided progress in cleaning up the air.
Through a combination of twice-a-year emission inspections, cleaner fuels, fortuitous weather and a ban that prohibits citizens from using their cars one workday a week, the city and its 17 neighboring municipalities have reduced carbon monoxide by at least 7 percent.
And the rate of increase for four other pollutants is lessening or staying the same, say government officials and private ecologists. Besides carbon monoxide and ozone, the other pollutants are airborne lead, nitrogen oxide and sulfur dioxide.
The government is soon expected to announce stiffer restrictions that will require an estimated 200,000 public vehicles -- from taxis to police cars -- to have catalytic converters (costing $600 to $1,000) to enable them to use lead-free gas.
Moreover, it is expected to announce a five-year program to plant about 200 million trees in the windward approaches of the capital and 5 million more in the city itself. The trees will reoxygenate the air in a region that has only 31 percent of the required green space recommended by the WHO.
Other plans call for adding 37 miles to the city's 87 miles of subway by 1994 and establishing 17 special routes to woo middle-class car drivers with deluxe air-conditioned buses at fares of about $1 -- more than the average person can afford.
The initial phase of the fight is expected to cost about $2.8 billion, with technical assistance and soft loans from the World Bank, Japan, Germany, France and the United States. Many experts predict that it will take at least 10 years to bring pollution down to a bearable level, with the cost exceeding $50 billion.
"I believe the support and the money is there -- that is, if the government doesn't steal it," said ecologist Luis Manuel Guerra, noting Mexico's endemic corruption.
City officials are also pleased by recent preliminary figures from the 1990 census showing that the metropolitan area is not as large as previously believed, 18 million inhabitants as opposed to 20 million.
Moreover, Mexico's liberalized trade regulations should lead to further decentralization as export industries move to ports or to areas along the border with the United States, said Ismael Aguilar Barajas, a planning specialist at the Colegio de Mexico.
Still, the anti-pollution task remains enormous, given metropolitan Mexico City's 18 million people and its setting in a 7,349-foot-high valley where the polluted air from 30,000 industries and 2.7 million motor vehicles is trapped by thermal inversions, especially in winter.
Compounding the problem is that the metropolitan area has 23 percent less oxygen than at sea level, leading to incomplete engine combustion.
The environment has become so bad that some embassies forbid diplomats from bringing their children or strongly advise against it. Environmental hazardous duty pay, weekend retreat houses, free air filters and medical checkups are commonplace for diplomats.
"Frankly, I don't like living here," said a coughing Japanese diplomat whose embassy flies its employees to Los Angeles for a breath of fresh air.
By far the biggest attack has been waged against motor vehicles, which cause 83 percent of the pollution, said Fernando Menendez Garza, coordinator of environmental programs in the capital.
The government has increased gas prices and slapped a 12 percent "polluter's tax" on fuel sales, hoping to cut down on car use, he said.
The day-without-a-car policy, which also includes commercial vehicles, is blasted by businessmen who say it hurts the economy and is subject to widespread cheating.