The Crown Research Institute NIWA is celebrating 25 years of the Montreal Protocol today.
The protocol was adopted by UN countries, to phase out substances which damage the ozone layer.
It's recognised as the most successful international environmental protection treaty.
NIWA's Tom Clarkson says protecting the ozone layer has been a very long term goal and the Protocol has done its job.
"The thing about the protocol that I think I would emphasise is how successful it has been as an international treaty, because as far as I know every single country in the United Nations has signed it. This one is quite remarkable."