OZONE DEPLETING SUBSTANCES
Some chemicals can damage the earth's protective ozone layer. This layer is composed of ozone molecules in the stratosphere, which extends from about 15 to 50 kilometers above the Earth's surface. Stratospheric ozone is a naturally-occurring gas that filters the sun's ultraviolet (UV) radiation. Consequently, any thinning of the ozone layer allows more radiation to reach the Earth's surface. In terms of human health, overexposure to UV rays can lead to skin cancer, cataracts, and weakened immune systems. In terms of ecological health, increased UV can lead to reduced crop yield and disruptions in the marine food chain.
Ozone depleting substances (ODS), including chlorofluorocarbons (CFCs), halons, and several other chemicals, are responsible for thinning the stratospheric ozone layer. When these substances reach the stratosphere, UV radiation from the sun breaks them apart to release chlorine or bromine atoms which react with ozone, starting chemical cycles of ozone destruction that deplete the ozone layer. One chlorine atom can break apart more than 100,000 ozone molecules, while a bromine atom can destroy about 4,000,000 ozone molecules.
OZONE DEPLETING POTENTIAL
Ozone depleting substances vary in their capacity to destroy ozone molecules, so scientists have developed a method for characterizing the relative depletion caused by different ODS. Ozone Depleting Potential (ODP) is the ratio of calculated ozone column change for each mass unit of a gas emitted into the atmosphere relative to the calculated depletion for the reference gas CFC 11 (ODP = 1.0). This allows different chemicals to be compared using a common unit (CFC 11-equivalents). For example, a chemical such as Halon 2402 (ODP = 6) is six times more detrimental to the stratospheric ozone layer than CFC 11.
In Scorecard's ranking sections, air releases of ozone depleting substances are weighted by chemical-specific ODPs to indicate the amount of ozone depletion they can cause.