What is the primary factor used by the Montreal Protocol to categorize substances?

The Montreal Protocol primarily categorizes substances based on their ozone depletion potential (ODP), which directly measures a substance's capacity to deplete the ozone layer. ODP is an essential metric because the agreements established under the protocol aim to protect the stratospheric ozone layer by controlling and phasing out substances that exhibit a high potential to cause ozone depletion.

When the protocol was formed in 1987, it recognized that certain chemicals, particularly chlorofluorocarbons (CFCs) and halons, have significant adverse effects on the ozone layer. By categorizing substances according to their ODP, the protocol ensures that action can be taken to reduce or eliminate those with the most damaging impacts on atmospheric ozone.

While factors like chemical composition, atmospheric lifetime, and global warming potential are also important in understanding the broader environmental impacts of various substances, the primary focus of the Montreal Protocol is specifically on how substances affect ozone depletion. Therefore, the emphasis on ODP is crucial for the goals of the agreement and its ongoing success in protecting the ozone layer.