What do substances classified as mutagens primarily cause?

Substances classified as mutagens primarily cause changes in DNA that lead to genetic mutations. Mutagens can interact with the genetic material in cells, resulting in alterations to the DNA sequence. These changes can affect the organism's genetic information and may lead to various consequences such as increased cancer risk, heritable mutations, or developmental issues. Understanding the role of mutagens is crucial in toxicology and genetics because it highlights how certain chemicals or environmental factors can influence genetic stability and overall health.

In this context, the other options do not accurately reflect the primary action of mutagens. Adverse health effects unrelated to genetics would encompass a broader range of toxicological issues that do not specifically focus on genetic changes. Physical harm to living organisms might refer to injuries or irritations caused by different types of toxins without a genetic component. Lastly, increased lifespan in cells does not pertain to mutagens, as these substances are often associated with harmful outcomes rather than beneficial effects such as cellular longevity.