At what concentration ozone is harmful?

Explanation

**Step-by-Step Solution:**

1. **Understanding Ozone**: Ozone (O₃) is a molecule composed of three oxygen atoms. It exists in different states: as a pale blue gas, a dark blue liquid, and a violet-black solid.

2. **Concentration Measurement**: Ozone concentration is often measured in parts per million (ppm). This unit indicates how many parts of ozone are present in one million parts of air.

3. **Identifying Harmful Levels**: Research indicates that when the concentration of ozone exceeds 100 ppm, it can start to have adverse effects on human health.

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