Here's why:

* Luminous materials don't emit ionizing radiation: The glow comes from a process called phosphorescence, where materials absorb energy (like light) and release it slowly as visible light. This doesn't involve the harmful radiation that can cause cancer (like X-rays or gamma rays).

* Cancer is caused by DNA damage: Ionizing radiation can damage DNA, leading to mutations that can cause cancer. Phosphorescent materials don't emit this type of radiation.

However, there are some things to consider:

* Certain luminous materials might contain toxic chemicals: While not directly cancerous, some older glow-in-the-dark products might contain substances like radium, which is radioactive. However, these are rarely used today.

* Always follow safety instructions: If you're using any luminous product, it's important to read the manufacturer's instructions and handle it safely.

In general, glowing in the dark is a safe phenomenon and does not pose a cancer risk.