Which of the following is not considered a carcinogen?

• A. Tar
• B. Benzene
• C. Oxygen
• D. Formaldehyde

Answer: (C)

Understanding what a carcinogen is helps distinguish substances that increase cancer risk from those that do not. Tar, benzene, and formaldehyde have well-established links to cancer in humans or animals. Tar, a component of tobacco smoke, contains carcinogenic compounds that can initiate cell changes leading to cancer. Benzene exposure is linked to leukemia, reflecting its carcinogenic potential. Formaldehyde is classified as a human carcinogen based on evidence from occupational studies and animal data.

Oxygen, by contrast, is essential for life and, by itself, does not have proven cancer-causing effects at normal exposure levels. While reactive oxygen species that can form during metabolism can contribute to DNA damage under certain stress conditions, oxygen isn’t designated as a carcinogen.