The Montreal Protocol, which entered into force in 1989, banned the use of chemicals that destroy the ozone layer, which helps protect the planet from damage caused by solar radiation.
It has been signed by 197 parties and is considered to be the most successful environmental agreement in the world.
Repairing an ozone layer, redirecting wind flows, according to a new study
But Canadian researchers have found that the chemicals used to replace banned products create their own problems.
Heidi Pickard of Harvard University says that replacements break down in the atmosphere into new contaminants.
They appear in distant Arctic ice cores and have increased sevenfold since 1990.
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Pickard says little is known about the new contaminants, although some have been shown to be very toxic to plants.
She says the discovery is a warning that solutions to current problems can have unexpected consequences down the line.
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