The recovery of the ozone layer in Earth's stratosphere could be delayed by several years, according to an international study led by Swiss research institution Empa which included contributions from University of Bristol researchers. The cause is persistent emissions of so-called feedstock chemicals, which are still permitted as raw materials in industry. These ozone-depleting substances have so far been excluded from international agreements because, according to the current study, their emissions and use have been significantly underestimated.