MEXICO CITY, Mexico (AP) -- Mexico City marked its first year in a decade
                  without a smog alert, although new studies warn that the relatively fresh air
                  won't last.

                  A Massachusetts Institute of Technology study released Monday suggested that
                  Mexico City's air pollution plays a role in thousands of deaths every year, and
                  pointed to a new villain: emissions from old and poorly maintained diesel freight
                  trucks.

                  And a Mexican federal government study warned that rapidly growing numbers
                  of private cars, a decline in subway and bus use, and urban sprawl threaten to
                  take the metropolis of 18 million back to the worst days of pollution in the early
                  1990s.

                  It all came on a day when the majestic volcanos ringing the city stood out against
                  a nearly blue sky, an increasingly frequent sight. The metropolis once viewed as
                  a laboratory for smog problems is now grappling with the question of whether to
                  celebrate victory, or redouble anti-pollution efforts.

                  City authorities are clearly proud of the work that got them this far: tougher
                  vehicle inspections and standards, driving bans and factory improvements -- all
                  aided by unusually strong winds that have temporarily blown some smog out of
                  the valley.

                  "After a year without a smog alert, I think we can take some steps forward,"
                  said Aaron Mastache, Mexico City's environment secretary. The last alert -- such
                  warnings were frequent in the 1990s -- was declared Oct. 15, 1999.

                  One obvious step, which Mastache's office has proposed, is to lower the
                  threshold for smog alerts, which Mexico City currently calls only when pollution
                  reaches 2.4 times the acceptable levels.

                  Most U.S. cities declare alerts at much lower smog concentrations. Mexico
                  City's rules force motorists to use their cars less during alerts.

                  But the main enemy, ozone, still exceeded acceptable limits on about 80 percent
                  of days in 1999. Ozone is a colorless, secondary pollutant produced by the
                  interaction of sunlight and vehicle exhaust.

                  M.I.T. researcher Mario Molina, who won the Nobel Prize in Chemistry in 1995
                  for his work on the ozone layer, suggested that nitrogen dioxide -- the kind of
                  emission that old freight trucks spew in large amounts -- may be key to ozone
                  formation.

                  The United States had previously refused to allow Mexican trucks to drive U.S.
                  highways, precisely because of concerns about their poor maintenance and age.

                  "More attention must be given than in the past to policy on freight trucks,"
                  Molina wrote.

                  In the meantime, Mexico City residents appear to be paying the price for the
                  poor air quality: Citing previous studies, Molina said that as many as 1,000 deaths
                  could be prevented each year by reducing suspended particles -- also produced
                  by diesel engines -- by just 10 percent.

                  Given the influence of the business community, it's uncertain how far proposals
                  such as reducing alert thresholds will go.

                  Once considered one of the world's most polluted cities, Mexico City has
                  probably been surpassed in smog by Beijing, New Delhi and others.

                  But the National Ecology Institute noted that the number of private cars is
                  growing by 6 percent annually, which could bring a total of 5 million autos onto
                  Mexico City streets by 2005.

                  Subway and bus routes should be improved, and vehicles converted to natural
                  gas, the institute recommended.

                  "Long-term measures need to be taken now," the report said.

                  Copyright 2000 The Associated Press.