What should be the tolerable limit of fluoride ions in drinking water ? What happens if it is higher than 10 ppm ?

Explanation

### Step-by-Step Solution:

1. **Identify the Tolerable Limit of Fluoride Ions:**
- The tolerable limit of fluoride ions in drinking water is generally considered to be **1 ppm (parts per million)**. This level is safe and beneficial for dental health as it helps in the formation of stronger teeth.

2. **Effects of Higher Concentrations:**
- When the concentration of fluoride ions exceeds **2 ppm**, it may lead to dental fluorosis, which is characterized by the brown mottling of teeth. This condition occurs due to excessive fluoride intake during the developmental stages of teeth.

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