Naphthalene balls are considered toxic when used in the following situations:

1. Ingestion:
   If ingested, naphthalene balls can cause serious health problems, including nausea, vomiting, abdominal pain, and even liver damage or failure.
2. Inhalation:
   Inhaling naphthalene vapors can cause respiratory problems, such as coughing, wheezing, and shortness of breath.
3. Prolonged exposure:
   Prolonged exposure to naphthalene balls, either through inhalation or skin contact, can lead to health issues like headaches, dizziness, and nausea.
4. High concentrations:
   Using naphthalene balls in small, poorly ventilated areas can lead to high concentrations of the chemical, increasing the risk of toxicity.
5. Children and pets:
   Naphthalene balls are particularly dangerous for children and pets, as they may ingest them or be exposed to high concentrations of the chemical.
6. Pregnancy and breastfeeding:
   Exposure to naphthalene balls during pregnancy or breastfeeding can potentially harm the developing fetus or baby.

To minimise risks, always follow these guidelines when using naphthalene balls:

- Use them in well-ventilated areas.

- Keep them out of reach of children and pets.

- Avoid inhaling vapors or ingesting the chemical.

- Follow the manufacturer's instructions and recommended usage.

- Wear gloves and protective clothing when handling naphthalene balls.

Remember, it's always better to err on the side of caution when using potentially toxic substances like naphthalene balls. If you have concerns, consider alternative methods or consult a healthcare professional.