Which class of human-made chemicals is responsible for reducing ozone levels?

• A. Hydrofluorocarbons
• B. Chlorofluorocarbons (CFCs)
• C. Nitrous oxides
• D. Sulfur hexafluoride

Answer: (B)

Ozone depletion in the stratosphere is driven by chlorine released from certain human-made chemicals. These compounds are very stable and can travel up to the upper atmosphere intact. When ultraviolet light hits them, they break apart and release chlorine atoms. Those chlorine atoms then enter catalytic cycles that destroy ozone molecules, converting them into regular oxygen and regenerating the chlorine to keep the process going. That’s why chlorofluorocarbons are the classic culprits—they contain chlorine and persist long enough to reach the stratosphere, enabling widespread ozone loss.

Other substances listed don’t fit as well. Hydrofluorocarbons lack chlorine, so they don’t deplete ozone in the same way. Nitrous oxides do influence ozone chemistry but aren’t the primary driver of the major depletion associated with human-made pollutants. Sulfur hexafluoride is a potent greenhouse gas with negligible impact on stratospheric ozone.