In a city so desperate officials once sprayed liquid detergent from airplanes in an effort to clean the air, stiff restrictions on vehicles take effect Monday to contain the mushrooming brown cloud that marks Santiago as one of the world's dirtiest cities.

The restrictions will take 20 percent of vehicles out of circulation once a week. But critics charge more needs to be done to clean the brown air that now blocks the towering Andes Mountains from view on the city's eastern border.Santiago follows Mexico City and Sao Paulo, Brazil, among cities with the most the polluted air, according to a government expert. Santiago has fewer people, 4.5 million, but is cursed with a geography that puts it between two mountain ranges and with little wind or rain.

Some 425,000 cars, 30,000 taxis and 10,000 belching diesel buses clog Santiago's streets daily. Compounding the problem, wood is a major source of heat for homes and energy for businesses, and huge areas of the city have no vegetation to keep down dust.

The pollution has caused various health problems for nearly all residents, according to a study by the opposition Party for Democracy.

"Bronchial pneumonia and pneumonia have become the No. 1 cause of infant mortality," the study charged.

Last year, the government tried a smog control program in which liquid detergent was sprayed from an airplane over the city. The effort failed and there are no plans to repeat the process.

Starting Monday, the government of President Augusto Pinochet will impose restrictions on vehicle use - once a week, a car owner will not be able to drive downtown, and each bus also must be kept off the streets once a week.

Mechanical inspections also have been tightened for buses, the main cause of particulate pollution. Owners will be forced to retire buses that are more than 20 years old.

Humberto Jorquera, head of the Transportation and Telecommunications Ministry's pollution control program, said the restrictions and tighter inspections should slightly improve air quality during the winter months of May to August.

"Monday is not going to be a magic day," Jorquera said. "But there will be less congestion and less pollution."

But ecologist Manuel Baquedano charged the government effort "will halt the pollution growth but will not solve it."

The government has done little to control bus routes or impose serious measures to control private cars, the chief source of carbon monoxide, Baquedano said.

He traces the rapid growth of the pollution back 15 years when Pinochet took power in a military coup and imposed a laissez faire economic system.

Private car ownership has been growing about 10 percent a year, burdening the city's streets and filling the enclosed valley with smog, especially in winter when an atmospheric inversion sits over the area.

Also, privately owned buses have concentrated on more lucrative routes, most of them running through the downtown.

But Baquedano said the real problem in Santiago - like Mexico City and Sao Paulo - is there are too many people in one place to ensure a healthy environment.

Jorquera said long-term measures under study include requiring buses to burn cleaner fuel, installation of electrical trolley buses, construction of urban railroad system and more subway lines.