Molina said much has been achieved in phasing out these harmful chemicals.
But he added that there's still no clear evidence the ozone layer -- or the ozone hole over the Antarctic -- is recovering.
"We need a big signal before we can tell unambiguously that the ozone layer is recovering," Molina said. "We had a very big signal in the 1980s that something was happening to the ozone layer and that it was clear that it was the result of the emissions of CFCs, halons [a common substance used in fire extinguishers] and other ozone-depleting substances. But we don't expect the ozone hole to disappear for decades. We might be able to tell before that that the ozone layer is recovering, but that's not the situation yet."
Under the 1987 Montreal Protocol, countries must gradually phase out CFCs and other harmful chemicals that were once common in refrigeration, aerosols, and fire extinguishers.
Developed countries have mostly stopped using them; developing ones have a longer deadline to phase them out.
Though emissions have dropped sharply, scientists at the symposium said the ozone layer is currently in a vulnerable state.
That's because the harmful chemicals released in previous years are still rising into the stratosphere.
And now there's another danger -- some rich countries are now asking to be allowed to go on using one of the chemicals that destroy ozone.
"It is the consumption of methyl bromide, an ozone-depleting substance, which unfortunately is still a matter of debate," said, Shafgat Kakahel, deputy executive director of the UN Environmental Program. "We are aware of the recent debate at the meetings of the parties and the applications by certain countries to exempt quantities of methyl bromide, for critical uses, from the 1 January 2005 total phase-out date. This has caused great controversy."
Methyl bromide, used for fumigating soil and controlling pests, should be phased out completely by the beginning of 2005.
But the protocol allows countries to use limited quantities of this chemical for "essential" or "critical" uses, for which no alternatives have yet been found.
The group of industrialized countries wants to use about 15,000 tons of this chemical in 2005, and around the same amount again the following year.
A decision on these exemptions is expected at this week's Prague meeting.
Molina said it's causing scientists some concern: "We are worried about the situation with respect to methyl bromide. From the point of view [of] the science, which does show that it contributes to ozone depletion -- if we pursue the objective of restoring the ozone layer, then it is important that we limit the emissions of methyl bromide."
Molina argued that it should be feasible to find alternatives to methyl bromide. After all, he said, this was harder to do with CFCs, but science and industry quickly developed alternatives.